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US8121687B2 - Cardiac motion characterization by strain measurement - Google PatentsCardiac motion characterization by strain measurement Download PDF
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- A61N1/00—Electrotherapy; Circuits therefor
- A61N1/02—Details
- A61N1/04—Electrodes
- A61N1/05—Electrodes for implantation or insertion into the body, e.g. heart electrode
- A61N1/056—Transvascular endocardial electrode systems
- A61B5/00—Detecting, measuring or recording for diagnostic purposes; Identification of persons
- A61B5/103—Detecting, measuring or recording devices for testing the shape, pattern, colour, size or movement of the body or parts thereof, for diagnostic purposes
- A61B5/11—Measuring movement of the entire body or parts thereof, e.g. head or hand tremor, mobility of a limb
- A61B5/1107—Measuring contraction of parts of the body, e.g. organ, muscle
- A61B5/00—Detecting, measuring or recording for diagnostic purposes; Identification of persons
- A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
- A61B5/6846—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive
- A61B5/6847—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive mounted on an invasive device
- A61B5/6851—Guide wires
- A61B5/00—Detecting, measuring or recording for diagnostic purposes; Identification of persons
- A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
- A61B5/6846—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive
- A61B5/6847—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive mounted on an invasive device
- A61B5/6852—Catheters
- A61B5/6857—Catheters with a distal pigtail shape
- A61B5/00—Detecting, measuring or recording for diagnostic purposes; Identification of persons
- A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
- A61B5/6846—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive
- A61B5/6847—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive mounted on an invasive device
- A61B5/6852—Catheters
- A61B5/6858—Catheters with a distal basket, e.g. expandable basket
- A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
- A61B2562/02—Details of sensors specially adapted for in-vivo measurements
- A61B2562/0261—Strain gauges
- A61B2562/0266—Optical strain gauges
- A61B5/00—Detecting, measuring or recording for diagnostic purposes; Identification of persons
- A61B5/68—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient
- A61B5/6846—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive
- A61B5/6847—Arrangements of detecting, measuring or recording means, e.g. sensors, in relation to patient specially adapted to be brought in contact with an internal body part, i.e. invasive mounted on an invasive device
- A61B5/686—Permanently implanted devices, e.g. pacemakers, other stimulators, biochips
- A61N1/00—Electrotherapy; Circuits therefor
- A61N1/02—Details
- A61N1/08—Arrangements or circuits for monitoring, protecting, controlling or indicating
- A61N1/00—Electrotherapy; Circuits therefor
- A61N1/18—Applying electric currents by contact electrodes
- A61N1/32—Applying electric currents by contact electrodes alternating or intermittent currents
- A61N1/36—Applying electric currents by contact electrodes alternating or intermittent currents for stimulation
- A61N1/362—Heart stimulators
- A61N1/3627—Heart stimulators for treating a mechanical deficiency of the heart, e.g. congestive heart failure or cardiomyopathy
Pursuant to 35 U.S.C. §119 (e), this application claims priority to the filing date of: U.S. Provisional Patent Application Ser. No. 60/624,427 filed Nov. 1, 2004; U.S. Provisional Patent Application Ser. No. 60/638,247 filed Dec. 20, 2004; U.S. Provisional Patent Application Ser. No. 60/638,947 filed Dec. 22, 2004; and U.S. Provisional Patent Application Ser. No. 60/667,749 filed Mar. 31, 2005; the disclosures of which are herein incorporated by reference.
In a diverse array of applications, the evaluation of tissue motion is desirable, e.g., for diagnostic or therapeutic purposes. An example of where evaluation of tissue motion is desirable is cardiac resynchronization therapy (CRT), where evaluation of cardiac tissue motion as observed by traditional ultrasound techniques is employed for diagnostic and therapeutic purposes.
CRT is an important new medical intervention for patients suffering from heart failure, e.g., congestive heart failure (CHF). When congestive heart failure occurs, symptoms develop due to the heart's inability to function sufficiently. Congestive heart failure is characterized by gradual decline in cardiac function punctuated by severe exacerbations leading eventually to death. It is estimated that over five million patients in the United States suffer from this malady.
The aim of resynchronization pacing is to induce the interventricular septum and the left ventricular free wall to contract at approximately the same time.
Resynchronization therapy seeks to provide a contraction time sequence that will most effectively produce maximal cardiac output with minimal total energy expenditure by the heart. The optimal timing is calculated by reference to hemodynamic parameters such as dP/dt, the first time-derivative of the pressure waveform in the left ventricle. The dP/dt parameter is a well-documented proxy for left ventricular contractility.
In current practice, external ultrasound measurements are used to calculate dP/dt. Such external ultrasound is used to observe wall motion directly. Most commonly, the ultrasound operator uses the ultrasound system in a tissue Doppler mode, a feature known as tissue Doppler imaging (TDI), to evaluate the time course of displacement of the septum relative to the left ventricle free wall. The current view of clinicians is that ultrasonographic evaluation using TDI or a similar approach may become an important part of qualifying patients for CRT therapy.
As currently delivered, CRT therapy is effective in about half to two-thirds of patients implanted with a resynchronization device. In approximately one-third of these patients, this therapy provides a two-class improvement in patient symptoms as measured by the New York Heart Association scale. In about one-third of these patients, a one-class improvement in cardiovascular symptoms is accomplished. In the remaining third of patients, there is no improvement or, in a small minority, a deterioration in cardiac performance. This group of patients is referred to as non-responders. It is possible that the one-class New York Heart Association responders are actually marginal or partial responders to the therapy, given the dramatic results seen in a minority.
The synchronization therapy, in order to be optimal, targets the cardiac wall segment point of maximal delay, and advances the timing to synchronize contraction with an earlier contracting region of the heart, typically the septum. However, the current placement technique for CRT devices is usually empiric. A physician will cannulate a vein that appears to be in the region described by the literature as most effective. The device is then positioned, stimulation is carried out, and the lack of extra-cardiac stimulation, such as diaphragmatic pacing, is confirmed. With the currently available techniques, rarely is there time or means for optimizing cardiac performance.
When attempted today, clinical CRT optimization must be preformed by laborious manual method of an ultrasonographer evaluating cardiac wall motion at different lead positions and different interventricular delay (IVD) settings. The IVD is the ability of pacemakers to be set up with different timing on the pacing pulse that goes to the right ventricle versus the left ventricle. In addition, all pacemakers have the ability to vary the atrio-ventricular delay, which is the delay between stimulation of the atria and the ventricle or ventricles themselves. These settings can be important in addition to the location of the left ventricular stimulating electrode itself in resynchronizing the patient.
Current use of Doppler to localize elements in the heart have been limited to wall position determination via external ultrasonography, typically for purposes of measuring valve function, cardiac output, or rarely, synchronization index.
There is currently no useful clinically available means of determining optimal CRT settings on a substantially automatic or a real-time, machine readable basis. It would be an important advancement in cardiology to have an implantable means of monitoring the mechanical performance of the heart in real time, an immediate application being in setting the functions of cardiac resynchronization therapy pacemakers, with further application to the pharmacologic management of heart failure patients, arrhythmia detection and ischemia detection, etc.
Methods for evaluating motion of a cardiac tissue location, e.g., heart wall, are provided. In the subject methods, timing of a signal obtain from a strain gauge stably associated with the tissue location of interest is employed to evaluate movement of the cardiac tissue location. Also provided are systems, devices and related compositions for practicing the subject methods. The subject methods and devices find use in a variety of different applications, including cardiac resynchronization therapy.
Methods for evaluating motion of a cardiac tissue location, e.g., heart wall, are provided. In the subject methods, timing of a signal obtain from a strain gauge stably associated with the tissue location of interest is employed to evaluate movement of the cardiac tissue location. Also provided are systems, devices and related compositions for practicing the subject methods. The subject methods and devices find use in a variety of different applications, including cardiac resynchronization therapy.
Before the present invention is described in greater detail, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, representative illustrative methods and materials are now described.
All publications and patents cited in this specification are herein incorporated by reference as if each individual publication or patent were specifically and individually indicated to be incorporated by reference and are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The citation of any publication is for its disclosure prior to the filing date and should not be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates which may need to be independently confirmed.
It is noted that, as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as "solely," "only" and the like in connection with the recitation of claim elements, or use of a "negative" limitation.
As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present invention. Any recited method can be carried out in the order of events recited or in any other order which is logically possible.
In further describing the subject invention, a general review of the subject methods and systems as well as various aspects thereof is first provided. Next, a more detailed review of specific embodiments with reference to various figures is provided. Following this section, representative applications in which the subject invention finds use are described, as well as other aspects of the invention, such as computer related embodiments and kits that find use in practicing the invention.
Overview of Cardiac Motion Characterization by Strain Measurement
As summarized above, the subject invention provides methods for evaluating movement of a cardiac tissue location of interest by strain measurement. In the subject methods, data obtained by a strain sensing element stably associated with the cardiac tissue location of interest is employed. A feature of the present invention is that the data which is employed at least includes, and sometimes solely includes, the timing of a signal obtained from a strain gauge. Accordingly, an aspect of embodiments of the invention is to ignore the total deflection of the element, but instead to use the timing of the motion to evaluate, e.g., determine, when the tissue or organ moves. The advantage of a strain gauge over other motion sensors, such as an accelerometer or other transducer, is that the strain gauge is orders of magnitude less complicated, and thereby more robust and reliable. An embodiment of the present strain measurement innovation is configured to provide critical information on wall motion for cardiac resynchronization therapy, such as for the treatment of congestive heart failure. Aspects of the invention provide strain gauges positioned in or on the heart for determining cardiac wall contraction synchrony and/or disynchrony.
The inventive use of strain gauges to determine cardiac wall motion timing allows a minimally invasive, highly miniaturized means for the direct measurement in real-time of the wall motion occurring in the beating heart. The measurements provided by the present invention provided the amplitude and timing of the strain and/or shortening of the muscle fibers of the heart.
The inventive strain gauge sensors enjoy special qualities not available to other sensors. By example, these prior methods typically indicate general motion that is occurring in the heart, using, for example, accelerometers and Doppler effect transducers. Cardiac accelerometers will measure any movement as a person walks around a room, travels in a vibrating vehicle, or similar daily activities. Doppler approaches will measure relative motion between different points. However, the signal is relatively small and thus sophisticated data filtering is needed. The advantages of the present strain measurement invention allow it special qualities in cardiac sensing, which, in combination with simplicity and minute size requirements, in many cases proves synergistic with other sensors, as reviewed in greater detail below.
In contrast to many other cardiac sensors, the strain gauge approach of the present invention measures the timing of motion of the cardiac wall. This characteristic provides unique advantages in providing critical information to the clinician in providing and optimizing cardiac synchrony therapy. Important goals of research in this area have been to measure the relative movement of the right ventricular free wall, the left ventricular free wall, and the septum (between the two ventricles). These goals are met by the innovative use of the strain gauges in the means and methods of the present invention.
The present inventive use of strain gauges provides information based on timing and deflection of the sensors. The measurement is a simple signal, measuring whether or not strain is occurring; and therefore indicating whether or not the muscle is contracting. By way of example, when the inventive strain gauge is placed in a cardiac vein that runs along the endocardial aspect of wall of the left ventricle, data is provided to the clinician as to the timing of left ventricular wall movement. These data can be obtained using a single strain gauge or multiple strain gauges, depending on the specific application. In contrast to currently employed methods using pressures and/or Tissue Doppler imaging, the use of the present strain gauge invention allows the clinician to directly measure from just this sort of on/off motion sensor, whether or not the two particular cardiac chambers are contracting at the same time. More important is the determination of whether the septal and free wall contractive element of the left ventricle are also contracting at the most optimal time for continuing to cardiac output. These particular representative applications are reviewed below in greater detail.
As such, the inventive cardiac timing device is a medical tool using strain gauges as timing devices for measurement and control of local organ motion, such as various cardiac features. The new and unobvious use for strain gauges is to use the sensor primarily as an event timing measurement device. In this new use, the total magnitude of the change in strain is of secondary concern. This strain signal would be filtered to indicate motion of tissue through appropriate filters, but then would provide positive signal as to when the tissue started to move. In addition, through calibration of the device, the direction of motion could also be determined.
As is known in the art, a stain gauge is traditionally used in an application where the deflection (strain) is calibrated for certain force (stress). Strain gauges are used most often in tensile testing machines. In the majority of these applications the strain gauge is only used to determine the force applied to a test specimen when it experiences a force at a known rate.
The new and novel use of a strain gauge described herein is to ignore the total stress measurement that this sensor would normally provide. Instead only the timing of motion is the required output of the device. The force (stress) can also be measured but it is of only second order concern in this application. The advantage of a strain gauge over other motion sensors, such as an accelerometer or other transducer, is that it is orders of magnitude less complicated, and thereby more robust and reliable. Additional advantages over such devices as accelerometry are the reduction of confounding signals. An accelerometer will detect motion of the body that the device is used in, walking for example. An accelerometer will also detect motion from adjacent organs, including motion from respiration. All these extraneous signals will need to be filtered out to avoid interference with the data that the accelerometer is trying to measure. With the strain gauge, motion timing from a specific local tissue area is much easier to measure.
Aspects of the strain gauges employed in the subject methods are the presence of one or more transducers that generate a signal, e.g., electrical, optical etc., in response to an applied stimulus, e.g., deflection, bending, stretching, etc. In representative embodiments, the strain gauge is incorporated into a medical device at or under surface of the device. The strain gauge provides a signal when the tissue location of interest moves, since the strain gauge is stably associated with the tissue location of interest.
As summarized above, the subject invention provides methods of evaluating movement of a tissue location. "Evaluating" is used herein to refer to any type of detecting, assessing or analyzing, and may be qualitative or quantitative. In representative embodiments, movement is determined relative to another tissue location, such that the methods are employed to determine movement of two or more tissue locations relative to each other.
The tissue location(s) is generally a defined location or portion of a body, i.e., subject, where in representative embodiments it is a defined location or portion (i.e., domain or region) of a body structure, such as an organ, where in representative embodiments the body structure is an internal body structure, such as an internal organ, e.g., heart, kidney, stomach, lung, etc. In representative embodiments, the tissue location is a cardiac location. As such and for ease of further description, the various aspects of the invention are now reviewed in terms of evaluating motion of a cardiac location. The cardiac location may be either endocardial or epicardial, as desired, and may be an atrial or ventricular location. Where the tissue location is a cardiac location, in representative embodiments, the cardiac location is a heart wall location, e.g., a chamber wall, such as a ventricular wall, a septal wall, etc. Although the invention is now further described in terms of cardiac motion evaluation embodiments, the invention is not so limited, the invention being readily adaptable to evaluation of movement of a wide variety of different tissue locations.
In practicing embodiments of the invention, following implantation of any required elements in a subject (e.g., using known surgical techniques), the a signal (representing data) from a strain gauge sensing element that is stably associated with the tissue location of interest is then detected to evaluate movement of the tissue location. As reviewed above, the timing of the signal obtained from the sensing element is employed, e.g., in characterizing movement of the tissue location of interest.
By "stably associated with" is meant that the sensing element is substantially if not completely fixed relative to the tissue location of interest such that when the tissue location of interest moves, the sensing element also moves. As the employed strain gauge sensing element is stably associated with the tissue location, its movement is the same as (or at least a proxy for such that it serves as a suitable indicator on the movement of the tissue location to which it is stably associated. The sensing element may be stably associated with the tissue location using any convenient approach, such as by attaching the sensing element to the tissue location by using an attachment element, such as a hook, etc., by having the sensing element on a structure that compresses the sensing element against the tissue location such that the two are stably associated, etc.
In a given embodiment, the sensing element can provide output in an interval fashion or continuous fashion for a given duration of time, as desired.
In certain embodiments, a single sensing element, e.g., strain gauge, is employed. In such methods, evaluation may include monitoring movement of the tissue location over a given period of time. In certain embodiments, two or more distinct sensing elements are employed to evaluate movement of two or more distinct tissue locations. The number of different sensing elements that are employed in a given embodiment may vary greatly, where in certain embodiments the number employed is 2 or more, such as 3 or more, 4 or more, 5 or more, 8 or more, 10 or more, etc. In such multi-sensor embodiments, the methods may include evaluating movement of the two or more distinct locations relative to each other.
In certain embodiments, the subject methods include providing a system that includes a strain gauge sensing element that is stably associated with the tissue location of interest. This providing step may include implanting one or more new elements into a body. This step, if employed, may be carried out using any convenient protocol, where a variety of protocols are well known to those of skill in the art.
The subject methods may be used in a variety of different kinds of animals, where the animals are typically "mammals" or "mammalian," where these terms are used broadly to describe organisms which are within the class mammalia, including the orders carnivore (e.g., dogs and cats), rodentia (e.g., mice, guinea pigs, and rats), lagomorpha (e.g., rabbits) and primates (e.g., humans, chimpanzees, and monkeys). In many embodiments, the subjects or patients will be humans.
The tissue movement evaluation data obtained using the subject methods may be employed in a variety of different applications, including but not limited to monitoring applications, treatment applications, etc. Representative applications in which the data obtained from the subject methods finds use are further reviewed in greater detail below.
General Design Considerations
The following section reviews a number of different features of different aspects of representative embodiments of the invention.
A wide variety of materials are appropriately employed in the fabrication of the strain gauges of the present innovative cardiac timing device. These materials are well know to the artisan, and can be varied with the particular intended use and placement of the device.
In representative embodiments, the strain gauges of the present invention are fabricated from a metal or metal alloy. In certain embodiments, the alloy is selected with high resistance characteristic. High resistance alloys appropriate for use in the strain gauges of the inventive cardiac timing device have at least one high resistance material component. By high resistance is meant a resistance of at least about 1000 ohms, such as at least about 2000 ohms or higher, e.g., 5000 ohms or higher. Examples of such materials are Constantan (copper-nickel alloy), Nichrome V (nickel-chrome alloy), platinum alloys (e.g., alloys with chromium, nickel, tungsten, etc.), Isoelastic (nickel-iron alloy), or Karma-type alloy wires (nickel-chrome alloy), foils, or semiconductor materials, e.g., poly-crystalline silicon or similar materials. In representative embodiments, the strain gauges are fabricated from copper-nickel alloys and nickel-chromium alloys, and can be employed in the present invention.
Platinum alloys are the materials employed in representative embodiments for construction of the strain gauges of the present cardiac timing device. Platinum alloys are particularly suitable for use in the present invention because they can often be used in direct contact with the body with limited ill effects, such as corrosion or toxic seepage into the body. Thus, platinum alloy materials of the present invention can effectively allow some contact, and in some cases, direct contact of the sensors with the body and body fluids.
Sensor Not Requiring Hermeticity Seal
The embodiments of the present invention that allow for direct contact of the sensor with body fluids provides a number of advantages. Direct contact with the cardiac features to be assessed provides an increase in sensitivity. Obviation of the requirement of a hermeticity layer allows an even greater miniaturization of the sensing device, providing the opportunity to position sensors in previously unavailable areas, and to provide many multiple sensors in a small space. This simpler design also provides a much simplified fabrication process.
The ability of the strain gauges of certain embodiments of the present cardiac timing device to function successfully without the requirement of hermetic sealing provides an important functional design advantage. An advantage of the direct body fluid contact inventive configuration is the concomitant increase in sensitivity of strain measurements which occurs when the sensors are not isolated from the cardiac features to be measured. The capacity to function without a hermetical seal is particularly advantageous in the context of the timing of the present invention as the direct ascertainment of the time of strain allows the most accurate assessment of the timed event.
The capability of the present strain gauges to tolerate direct contact with body fluids allows an even greater miniaturization of the present device, a key advantage of the present invention over other sensing devices. Thus, the strain gauges of the present invention can go beyond their uniquely small dimension to an even smaller profile using platinum alloys or other design approaches which circumvent the requirement of a hermetic seal to survive in the body environment.
The elegantly simple design approach of the innovative cardiac timing device eliminates numerous manufacturing steps which would otherwise be required for the assembly of the device. Fabrication processes are much simplified, limiting quality control problems, speeding production, and decreasing fabrication expenses.
Fatigue Considerations
The fatigue life of the material used in the strain gauge components of the present cardiac timing device also needs to be considered in selection of an optimal material for a particular embodiment of the invention. The fatigue life aspect of material selection is more critical when the cardiac timing device is planned for use in implantable applications, but less so for temporary use applications.
In long-term implantable applications of the inventive cardiac timing device, a material for the strain gauge component is typically one that has a fairly long fatigue life. Such materials have a relatively high yield point, and can be selected from a range of alloys. While modified for other design considerations, longevity of the device is preferably selected from strain gauge materials that have the highest yield point practicable. Platinum alloys, particularly those hardened with tungsten Pt-8W, Pt-10Ir or Pt-20Ir typically provide a high fatigue life to the strain gauge component.
Sensor Substrate
The strain gauge transducers may be bonded on any of a number of flexible or compliant substrates in the strain gauge sensor element. In certain embodiments, the substrate is a compliant substrate. Examples of suitable materials for the substrate are polyimide, polyetherimide, polyetherketone (PEEK), polyphenylene sulfide, polyethylene terephthalate, fluoroethylene-propylene (FEP), among others. Certain of these substrate materials are known to biocompatible, and thus and can be directly exposed to blood or tissue. PEEK is one such substrate.
The key to material selection for the strain gauge substrate is assuring that the thickness of the material and its bending stiffness is matched with the flexibility of the medical device that it is incorporated into. The substrate material also can be so designed to inhibit the motion of the strain gauge in unintended directions. Using this approach, a flexible construction is provided which allows the strain gauge to experience forces to which it is subject in its implanted position, such as within a cardiac lead.
Sensors Not Requiring Substrate
In a particularly minimalist embodiment of the inventive cardiac timing device, the strain gauge components are fabricated to be incorporated into the medical device without a flexible substrate. The strain gauge metallic member is molded into lead body itself. This inventive design approach limits or even eliminates the flexible backing component of the embodiments described above.
Optical Strain Gauges
One embodiment of the present invention provides an inventive cardiac motion detection system that includes optical strain gauges. By detecting changes in optical signals instead of electrical signals, and by combining fiberoptics and micro-machining technologies, the inventive cardiac motion detection system presents a powerful new tool in a physician's armamentarium. This new tool performs accurate, real-time monitoring of cardiac motions, and thus facilitates more precise CRT treatment. In these embodiments, an optically conductive element is employed to convey timing signals from the transducers of the sensing elements, as opposed an electrically conductive element, as reviewed in connection with other embodiments elsewhere in this application.
The use of optical sensing immunizes the present inventive cardiac motion detection system of these embodiments from external electrical or magnetic interferences. One embodiment of the present invention uses optical sensing and detection techniques to identify cardiac motion. Optical signals, when transmitted in a confined medium such as optical fiber, are known to be immune from external electrical or magnetic interferences. This feature allows the present detection system to detect much weaker signals which cannot be distinguished by prior motion detection systems. As a result, these embodiments have superb sensitivity and dynamic range.
In addition, the present system allows simultaneous operation of a large number of strain gauges. The present system enjoys this unique advantage because multiple wavelengths can be transmitted through a single optical fiber without interfering with each other. In one embodiment of the present invention, a cardiac motion detection system multiplexes many wavelengths into one optical fiber and operates each fiberoptic strain gauge at a different wavelength. In this way, the present inventive system can simultaneously detect the cardiac motion at a number of locations without occupying additional space within a catheter.
By using fiberoptic strain gauges, the present inventive cardiac motion detection system allows unprecedented reduction of the size of a motion sensor. The inventive combination of fiber optics and micro-machined structures provides a strain gauge with a size comparable to the outer diameter of a regular optical fiber, which is typically 125 μm. Such fiberoptic strain gauges enable a physician to place them in optimal positions within the heart and to limit implantation procedure time. Moreover, the implantation procedure is made much less invasive to the patient. As a result, the risk associated with the implantation procedure and the subsequent monitoring of cardiac motions is considerably lower.
Multiple Stain Gauges
In representative embodiments, a given strain gauge sensing element includes two or more individual transducer elements. As such, in a range of embodiments of the present cardiac timing device, multiple strain gauge transducer elements can be incorporated within a specific location, e.g., on the same and/or opposing surfaces of a substrate, in a given sensing element. This configuration of the inventive cardiac timing device allows the subtraction of motion timing components in directions which are not of interest.
In certain embodiments of the inventive cardiac timing device, axial strain is a force component that is a potential source of interfering background signal. A second strain gauge transducer forming a half bridge and located on the opposite side of the member, e.g., compliant substrate, can allow for the elimination of this motion component. More complex configurations with additional transducers are also employed in certain embodiments. Representative configurations of interest include, but are not limited to: two pairs of transducers, where each member of the pair is on an opposite surface of the compliant member, where the pairs can be parallel to each other, at right angles to each other, etc.
In some inventive cardiac timing device designs, timing can be determined from axial strain as well as bending strain. Proper selection and orientation of two strain gauges forming a half bridge limits or eliminates the bending strain component of the signal. Additional orientations of the strain gauges can be planar. Alternatively, the gauges can be inserted into the cardiac timing device in a curved shape. Other configurations of individual or groups of strain gauges provide the best sensing for timing of the motion of the heart which is of greatest interest in a particular application. Referencing multiple different sensors with different orientation and/or designs allows a range of information to be available to the clinician. A full bridge strain gauge configuration can be employed that, when wrapped around the outside body of the lead device, provides orientation data independent of the measure of deflection amplitude.
Multiple strain gauges in the proper orientation allows for the elimination of potential confounding changes in the resistance of the strain gauge due to temperature. Resistance changes can occur due to fluctuations in body temperature, such as when a patient is exposed to cold causing hypothermia or exposure to heat or fever from infection causing hyperthermia.
Post Processing Steps
The analysis of the strain gauge data may benefit from post processing to determine the timing of the motion. For people skilled in the art of signal processing it is well known how to process the signal to provide the most accurate data for clinical interpretation.
In certain embodiments of the device, signal processing is performed. For example, a strain signal from the device drives an auditory device that relays timing signals to the clinician as a series of tones. The clinician, prompted by these tones, then measures and adjusts the timing of the cardiac muscle by listening to different frequency tones that converged to one frequency as the timing of the heart was improved.
A visual signal can also be derived from the strain timing information. A series of lights is produced from the strain gauge signal that would change from red to green as the change in timing of the cardiac muscle was improved.
Electrical Design Considerations
The electrical connections which are employed in the inventive cardiac timing device incorporating the strain gauge are, in some cases, provided with appropriate diodes and electrical components. These components are arranged in a manner which limits or eliminates any conduction of stray electrical signals which could otherwise prove disadvantageous. This design consideration is particularly relevant in the combined effecting-sensing embodiments of the present cardiac timing device. A typical example would be a cardiac sensor/pacing system. Additionally, if a patient is subjected to cardiac defibrillation therapy, the inventive cardiac timing device is so designed to avoid compromise of function.
The strain gauges selected or developed for inventive cardiac timing may be constructed using any convenient fabrication method. In representative embodiments, a vapor deposition process is an appropriate fabrication method for strain gauges in the inventive cardiac timing device. In this fabrication, a photo resistive mask is employed, similar to a MEMS processing approach.
An additional fabrication method for strain gauges in the inventive cardiac timing device employs photo etching of the strain gauges. Alternatively, a laser cutting approach can be utilized. For the latter, the strain gauges are cut from thin sheets of the alloy of choice. A pattern for the strain gauge is drawn with an electronic CAD program. The data file for that pattern is exported to a laser cutting machine where the CAD file is translated to a machine readable file. That file is used to drive the cutting of the desired shape. The shape can also be laser cut or photo etched into tubing of the appropriate alloy selection.
Positioning of Strain Gauges in Device, and Device in Body
The location of the strain gauge in the body of the cardiac timing device directly impacts the significance of signal which it transmits. During insertion of the cardiac timing device, it is possible to fine tune the location of the sensor elements. The optimal positioning of strain gauge sensors within the device is also pivotal in providing the best possible cardiac timing information from the device.
Positioning of the strain gauges both within the device and within the body provides for optimum motion signal and motion timing for detection of heart wall motion. In preferred embodiments, the positioning of the strain gauge within the device or positioning means will be off the neutral axis. The positioning can also be provided at the outside edge around the diameter of the device. Optimal location of the strain gauge components within the cardiac timing device are determined as a function of the bending stiffness of the material of which the lead or other device body components, such as the catheter device are composed.
If the material of the cardiac timing device body components are selected from within a relatively soft range, there is the opportunity for the strain gauge component to be implanted more deeply into the device without disadvantageously compromising sensing capabilities. An example of such materials are silicone and its derivatives. Conversely, if the material of the cardiac timing device body is more rigid, the strain gauge component will typically implanted less deep into the device. A typical example of such harder materials is polyurethane.
Where multiple strain gauges are used, it is advantageous to separate them from the mechanical strain from adjacent sections of the body of the device. This can be accomplished by providing softer material in areas of the body of the device between locations where the strain gauge components are embedded. This approach serves to mechanically isolate the strain gauge components from each other. This approach can ameliorate or obviate a potential risk of strain from one section of the device being transmitted through the device to another section. If such transmitted strain is recorded by the more distant strain gauges, these background strains can obscure or compromise the readings of ambient physical strains of the areas where the strain sensors have been positioned.
Temporary Cardiac Devices
For temporary cardiac sensing applications of the strain gauge, the strain gauge attachment and communication means for the inventive cardiac timing device can be accomplished using a wide range of methods. Because the placement is temporary, miniaturization and long term survivability do not represent the serious challenges to design which they do for the development of permanent devices.
One highly simplified fabrication approach well suited to construction of temporary cardiac timing devices is to simply run small gauge conductive wires down the center of a catheter. Once in position, these wires are soldered or welded to the desired strain gauges.
In the case of temporary cardiac timing device, a multiplicity of wires can be provided down the catheter device as dimension is a much less critical component than in the case of permanent devices. This multiwire configuration allows for the gleaning of data from a number of strain gauges at different locations or at the same location, each having a unique wire connection. Such multiple wire configurations are more practical in temporary devices, as the denier size of the device can be larger.
A more compact interaction of the above multiplicity of wire design for the inventive cardiac timing device is accomplished by running conductors down to each strain gauge in a cable configuration. The cable is provided in a wound configuration, where each conductor wire is individually insulated. At the location where the wires are to be attached to the strain gauge, these wires are stripped. At these stripped points, the strain gauges are either welded or soldered to provide permanent contact with the appropriate wires.
Permanent Cardiac Devices
A particularly advantageous design for semi-permanent and permanent embodiments of the inventive cardiac timing device is the approach of using one common connection and a single wire running to each individual connection. This is a bus type configuration. In this design of the innovative cardiac timing device, the opportunity is provided for a long string of strain gauges implanted along the length of the device.
In contrast to the temporary configurations described above, however, a bus configuration provides a single wire or conductor which serves all of the strain gauge components for one side of the electrical connection. This bus configuration allows an extremely small denier size, which can be pivotal in providing the cardiac timing device in an acceptable form for more permanent uses. This bus configuration also plays on the strength of the very small dimensions available for the strain gauge components, as compared to other motion sensors such as accelerometers.
In the permanent implant embodiment of the inventive cardiac timing device, conductors are selected to each strain gauge which have a relatively high fatigue life. The capacity to survive 400 million cycles prior to failure is the typical requirement for long term implant cardiac devices. For the construction of devices meeting these requirements, several design approaches are particularly suitable.
Additionally, strain gauges for permanent implant cardiac timing devices are advantageously connected to a central set of wound conductor cables. These wound conductor cables would typically be fabricated of stainless steel with a silver core. This construct is then coated with an electrically isolating coating of polyimide, and wound together to form a cable. This cable configuration provides the capacity to run a multiplicity of conductors down to multiple strain gauge locations.
Methods Overview
A simple application of the inventive strain sensor is to place catheters, with strain sensors attached, on both the left ventricular free wall, the right ventricular free wall, and potentially directly on the septum. This placement of the inventive device produces a very simple signal indicating when each of these cardiac features is contracting, the amplitude and time course of the contraction, and in some configurations their direction of movement. In order to obtain data on direction of movement, the strain gauges are oriented appropriately with a steerable implantation device. When combined with pressure readings from other sensors, the present invention allows the comparison of contraction timing and the pressure changes in the ventricles.
A prior art data point for CRT therapy has been pressure-pressure loops. In the general case it is thought that in the healthy heart, both ventricles contract at the same time. In that case, peak pressure would be achieved in both ventricles simultaneously. This test has been employed as a measure of potential synchrony. In dyssynchronous hearts, the pressure peak typically occurs at different times, suggesting that the muscle is contracting at different times. This difference in contraction can be directly measured with the present inventive strain gauges. Comparison to RV and/or LV pressures will add additional global data over strain gauges alone.
In an additional embodiment of the present invention, the strain gauge is attached to the ventricular septum, the wall between the two ventricles. Cardiologists often characterize the septum moving much like a sail. If pressure peaks are occurring at different times in the ventricles, the septum bows into whichever ventricle is at the lowest pressure. If the left ventricle compresses later than the right ventricle, the right ventricle will be in a relaxation phase during the LV contraction. In this case, the septum is likely to deflect into the right ventricle. By measuring the direction and/or timing of the deflection of the septum related to pressure changes, the clinician can determine, without even measuring a left ventricular pressure, when the pressure peak is occurring in the left ventricle.
An additional use of the present strain gauges is to determine apex movement. In this case, a strain gauge is placed near or at the tip of a device at the apex of one of the ventricles, left or right, preferably the right ventricle. These devices provide a measure of when the apex is moving. A number of parameters can be measured by this device placement. By example, the timing of apical contraction can be determined, as well as changes in the amplitude of the contraction. This sensing capacity can be provided by one of the following features.
In general, two types of data are collected in representative embodiments of the invention. In the simplest case, the timing of the motion is collected by simply analyzing when the contractions occur. In this case the amount of strain is not important and the strain measuring elements do not need calibration. This case could also be used as a method to measure the difference between contractions that occur before, during and after CRT optimization. The actual amplitude of the contractions is not important, only the difference between the contractions before and after an optimization procedure.
In a more complex case, the device assembly with the strain gauges is calibrated. With this calibrated device, the amount of strain, or deflection of the device, and therefore the tissue in question is measured accurately. This approach is employed for more accurately measuring the real motion or strain that is occurring. For example, the motion of the apex alone shows, in certain embodiments, whether a heart is beating synchronously or not. If the apex moves from side to side due to dyssynchronous contraction of the ventricles (interventricular dyssynchrony) or within the left ventricle (intraventricular dyssynchrony), a reduction in the side to side and a shift to more apical to basal motion of the apex, or other changes in motion, is employed as an indication of improving synchrony. A combination of these measures also proves clinically useful.
An additional application of the inventive system is to place catheters, with strain sensors attached, on the left ventricular free wall, the right ventricular free wall, and potentially directly on the septum. Such a configuration produces direct, motion-derived signals indicating when each of these cardiac features is contracting, and the respective amplitude and time course of the contractions. In other configurations, a physician measures their direction of movement using the present detection system. In order to obtain data on direction of movement, the strain gauges are ideally oriented appropriately with a steerable implantation device. When combined with additional pressure readings, one embodiment of the present invention allows the comparison of contraction timing and the pressure changes in the ventricles.
Additional Features Found in Representative Systems
Embodiments of the subjects systems incorporate other physiologic sensors in order to improve the clinical utility of wall-motion data provided by the present invention. For example, an integrated pressure sensor could provide a self-optimizing cardiac resynchronization pacing system with an important verification means, since wall motion optimization in the face of declining systemic pressure would be an indication of improper pacing, component failure or other underlying physiologically deleterious condition (e.g., hemorrhagic shock). One or more pressure sensors could also provide important information used in the diagnosis of malignant arrhythmias requiring electrical intervention (e.g., ventricular fibrillation).
Incorporation of other sensors is also envisioned. For example, the prior work by some of the present inventors describes the use of dimension sensors for resynchronization. This system is described in part in currently pending patent applications U.S. patent application Ser. No. 10/764,429 entitled "Method and Apparatus for Enhancing Cardiac Pacing", U.S. patent application Ser. No. 10/764,127 entitled "Methods and Systems for Measuring Cardiac Parameters", and U.S. patent application Ser. No. 10/764,125 entitled "Method and System for Remote Hemodynamic Monitoring", all filed Jan. 23, 2004, U.S. patent application Ser. No. 10/734,490 entitled "Method and System for Monitoring and Treating Hemodynamic Parameters" filed Dec. 11, 2003, U.S. Provisional Patent Application 60/638,692 entitled "High Fatigue Life Semiconductor Electrodes" filed Dec. 22, 2004, and U.S. Provisional Patent Application 60/638,928 entitled "Methods and Systems for Programming and Controlling a Cardiac Pacing Device" filed Dec. 23, 2004. These applications are herein incorporated into the present application by reference in their entirety.
Some of the present inventors have developed Doppler, pressure sensors, additional wall motion, and other cardiac parameter sensing devices. Some of these are embodied in currently filed provisional applications; "One Wire Medical Monitoring and Treating Devices", U.S. Provisional Patent Application No. 60/607,280 filed Sep. 2, 2004, "Stable Micromachined Sensors" U.S. Provisional Patent Application 60/615,117 filed Sep. 30, 2004, "Amplified Complaint Force Pressure Sensors" U.S. Provisional Patent Application No. 60/616,706 filed Oct. 6, 2004, "Implantable Doppler Tomography System" U.S. Provisional Patent Application No. 60/617,618 filed Oct. 8, 2004, and PCT Patent Application PCT/US2004/041430 entitled "Implantable Pressure Sensors" filed Dec. 10, 2004, "Shaped Computer Chips with Electrodes for Medical Devices" U.S. Provisional Patent Application 60/655,609 filed Feb. 22, 2005. These applications are incorporated in their entirety by reference herein.
In the implantable embodiments of this invention, as desired wall motion, pressure and other physiologic data can be recorded by an implantable computer. Such data can be periodically uploaded to computer systems and computer networks, including the Internet, for automated or manual analysis.
Uplink and downlink telemetry capabilities may be provided in a given implantable system to enable communication with either a remotely located external medical device or a more proximal medical device on the patient's body or another multi-chamber monitor/therapy delivery system in the patient's body. The stored physiologic data of the types described above as well as real-time generated physiologic data and non-physiologic data can be transmitted by uplink RF telemetry from the system to the external programmer or other remote medical device in response to a downlink telemetry transmitted interrogation command. The real-time physiologic data typically includes real time sampled signal levels, e.g., intracardiac electrocardiogram amplitude values, and sensor output signals including dimension signals developed in accordance with the invention. The non-physiologic patient data includes currently programmed device operating modes and parameter values, battery condition, device ID, patient ID, implantation dates, device programming history, real time event markers, and the like. In the context of implantable pacemakers and ICDs, such patient data includes programmed sense amplifier sensitivity, pacing or cardioversion pulse amplitude, energy, and pulse width, pacing or cardioversion lead impedance, and accumulated statistics related to device performance, e.g., data related to detected arrhythmia episodes and applied therapies. The multi-chamber monitor/therapy delivery system thus develops a variety of such real-time or stored, physiologic or non-physiologic, data, and such developed data is collectively referred to herein as "patient data".
Specific Representative Embodiments and Systems
In further describing aspects of the subject invention, various representative systems and methods for their use are reviewed in terms of figures.
Guide Wire Applications
A key advantage of embodiments of the inventive cardiac timing devices employed in the subject methods is the unusually small dimensions available for the strain gauge sensor, even as compared to other micro-miniature cardiac sensors. Smaller devices which will inventively incorporate strain gauges are typically termed guide wire size applications. For guide wire size applications of the inventive cardiac timing device, devices are provided with a design to accommodate strain gauges that are on the order of 0.014 inches in diameter. Strain gauges are commercially available which are of a dimension sufficiently small to fit within currently available guide wires.
As shown in
A longitudinal cross section of the spiral configuration of the device shown in
In this configuration, the conductor wires 5 are wound all the way to the tip of the guide wire core 4. In this position, the conductor is split out, stripped of its insulation, and then attached either by welding or soldering to strain gauges 6 located anywhere from half a centimeter from tip to 20 centimeters from distal tip of the device. A multiplicity of strain gauges 6 may be placed along the length of the device. Strain gauges 6 can be attached directly to the guide wire core 4 with adhesive. Alternatively, strain gauges 6 may be restrained in a specific orientation when the guidewire was over-molded and encapsulated with plastic material as part of the manufacturing process.
The guide wire 4 distal tip may have strain gauges 6 located 0.5 cm to 4.0 cm from the distal tip. Additional strain gauges 6 are located anywhere from 4.0 cm to 20.0 cm from the distal tip. Additional strain gauges could optionally be located in additional locations, the only constraint being the number of conductors that are available to connect with each individual strain gauge.
As shown in
The inventive cardiac timing device described above has the capacity to provide motion timing and electrical pacing and sensing on device enjoying a uniquely small profile device. This inventive cardiac timing device can be used in the coronary venous or arterial system to record motion, pace and sensing electrical signals. The inventive cardiac timing device can be used in the manner of a typical guide wire as a rail that a catheter would then follow to the location where it is permanently implanted. The device embodiment provides an opportunity to identify the optimum location for the permanently implantable device without the added work of placing a larger and less steerable implant device.
An additional embodiment for guide wire strain gauge device incorporates an optical fiber material guide wire 4 which is situated through the center of the device running from the proximal end shown in
The distal construction is located inside a long metallic coil, such as made of stainless steel, with a distal 2 cm section of Pt-10Ir wire 10A. The outside diameter of this coil is 0.0135 inches. The whole construction can optionally be coated with PTFE or it can be over molded with a polymeric material 10B. Alternate embodiments can eliminate the coil. The device can be over molded with a polymeric material to make the final diameter of the device uniform.
For cardiac motion sensing, strain gauges at multiple wall locations can provide a higher degree of granularity for detecting non-uniform wall motions. In addition, non-synchronous motion of the left ventricle can be detected by strain gauges in contact with the inter-ventricular septum.
Compression, tensile and shear stresses in the tissue wall would produce a timing signal to be registered by the strain gauges. This provides a specific, very localized measurement of tissue motion deeper into the tissue. A multiplicity of strain gauges 21 provide a reading indicating the direction of that motion and the magnitude of that motion, in addition to timing. This device configuration, in combination with a pacing electrode the outside of the corkscrew device may be constructed similar to a screw tip pacing electrode that was made out of platinum iridium and then coated with coatings that are know in the industry, iridium oxide.
In addition, this device may be provided with a pressure sensor located at the tip, and the device screwed through the septal wall to measure left side pressures as an implant device. Strain gauge 21 is located in the center of the coil, which is then provided with filling 22, such as epoxy or other suitable material. This filling 22 provided within coil 20, which can be formed of such materials as platinum iridium, producing a fine coil of similar dimensions to pacing tips. At the tip, strain gauges 21 can be provided in a multiplicity of locations. Strain gauges 21 may be fabricated on a flex circuit that is typical construction for strain gauges. A pressure sensor 23 of
From the physicians perspective this device would appear exactly the same as screw tip pacing leads that are in common use. It would provide additional strain or timing data for the pacing tip in addition to providing electrical signals at the screw tip.
As such, designs for localized measurement of cardiac timing is described in
This device has obvious uses beyond cardiac timing measurement. It can be used for measuring contractions in the stomach to provide a feedback loop for stomach pacing devices. Stomach contractions could be measured by this device and pacing for control of obesity could be turned on at the appropriate time.
This device could also be used to measure diaphragm motion for devices that are used to pace the diaphragm. This device of similar construction could be used to measure contractions during labor. Additionally, the inventive device could be used in other parts of the body where an electrical signal may not be available but timing could be determined from the sensing of motion. In yet another application, this device when implanted in the bladder can detect the extension of the bladder to allow for timing of electrical signals to the sphincter to allow for control of incontinence.
An alternate embodiment of the invention incorporates strain measurements into the body of a stent or stent graft, as is shown graphically in
The strain gauge can also be provided as a film applied to the surface of the device separated from the body of the device by a polymeric or ceramic insulator. The polymeric insulator would typically be polyimide, PEEK. The ceramic insulator would typically be SiN or SiO2. The strain gauge can be sputtered or plated Pt, Pt-10Ir, Ni—Cr, Pt-8W or other suitable metal. The strain measurements can be taken over the length of the device or in multiple locations along the length or around the circumference.
This device provides measurements of blood flow timing and blood pressure. In addition, this device can provide information on aneurisms. After an aneurism is stented or has a stent graft is implanted the strain measurements provides such clinically important information, such as if the aneurism continues to expand.
Strain gauge lead 30 is placed in the heart in a procedure similar to the typical placement procedures for cardiac right ventricle leads. Strain gauge lead 30 is placed in the heart using the standard cardiac lead devices which include introducers, guide catheters, guidewires, and/or stylets. Strain gauge lead 30 is inserted into the clavicle vein, through the superior vena cava, through the right atrium and down into the right ventricle. Strain gauge lead 30 is positioned under fluoroscopy into the location the clinician has determined is clinically optimal and logistically practical for fixating the strain gauge lead 30 and obtaining motion timing information for the cardiac feature area surrounding the attachment site. Under fluoroscopy, the active fixation helix 38 is advanced and screwed into the cardiac tissue to secure strain gauge lead 30 onto the septum.
Once the strain gauge lead 30 is fixed on the septum, strain gauge lead 30 provides timing data for the regional strains and/or deformation of the septum. The strain gauges 32 which are located more proximally along strain gauge lead 30 provide timing data on the regional strains in those areas of the heart. By example, a strain gauge 32 situated near the AV valve, which spans the right atrium in the right ventricle, provides timing data regarding the closing and opening of the valve. Furthermore, strain gauges 32 situated along other portions of the lead provide local blood flow data. Such information is important in provided cardiac timing determinations.
In a preferred embodiment, strain gauge lead 30 is constructed with the standard materials for a cardiac lead such as silicone or polyurethane for the lead body, and MP35N for the coiled or stranded conductors connected to the strain gauges 32. Alternatively, these device components can be connected by a multiplex system, as describe above, to the proximal end of strain gauge lead 30. The proximal end of strain gauge lead 30 connects to a bridge completion module for strain gauges 32, an amplifier, and other such display units to provide timing data from the various strain gauges 32.
The strain gauge lead 30 is typically fabricated of a soft flexible lead with the capacity to conform to the shape of the heart chamber. The only fixation point in this embodiment of the present cardiac timing device is the active fixation helix 38 which is attaching the strain gauge lead 30 to the cardiac septum.
The clinical motivation for these fixation alternatives is to provide cardiac timing information via strain gauge leads 36 and 30 about the regional strains or motions of the cardiac tissue where they are fixated. In
The inventive embodiment described in
U-shape strain gauge lead 54 is deployed using a guide catheter which would be placed into the right ventricle. The straightened U-shape strain gauge lead 54 is then slowly advanced out of the guide catheter. As it exits the guide catheter, U-shape strain gauge lead 54 assumes its intrinsic U-shape within the right ventricle. Alternatively, a straight stylet placed within U-shape strain gauge lead 54 can be used to hold the lead in a straight position during initial right ventricle placement. Once the lead is placed in the right ventricle, the stylet is removed and U-shape strain gauge lead 54 assumes its intrinsic U-shape.
The fabrication of the U-shape can be accomplished through a number of known methods. By example, the silicone lead body can be molded as a U-shape during the processing. Alternatively, the metal conductor coils or strands within the lead body can be shape set into a U-shape using various heat treatment methods.
U-shaped strain gauge lead 54 may optionally include an active fixation helix (not shown) along the length of the lead to fixate it as shown in the figures above. However, such additional fixation need only be provided when there is an unusually demanding cardiac feature target area preferred for fixation, or the point of attachment needs to be highly precise. In a representative embodiment, U-shaped strain gauge lead 54 does not require an active fixation, but by the nature of its U-shape holds this position within the ventricle chamber of the heart. In operation of certain embodiments, during systole and diastole of the heart, the U-shaped strain gauge lead 54 flexes back and fourth and shifts slowly up and down. One of the advantages of the U-shape is that it gives a direct correlation of timing and strain with the contraction timing and magnitude the septal and right ventricle walls.
In the inventive configurations of
The guide catheters 60 and 62 are used as part of the delivery system for cardiac leads in the right ventricle or in the right atrium, as well as in the coronary sinus and cardiac vein. During such procedures, it is advantageous for the clinician to monitor regional timing and strains of cardiac contractions along guide catheters 60 and 62 in the right atrium, the coronary sinus, the cardiac vein, and the right ventricle.
The main construction of a strain gauge guide catheter can be accomplished using well known techniques for guide catheters. Such standard fabrication methods typically involve a triple layer construction, as depicted in
The guide catheter which is placed into the coronary sinus can also involve a smaller strain gauge guide catheter which can then be advanced much further beyond the coronary sinus and into one of the cardiac veins. This would allow measurements of timing and strain of regional contractions near the cardiac vein and left side of the heart. Such measurement would ideally be provided by sensors situated along lateral wall of the left ventricle. The strain gauges placed on the guide catheter introduced into the right ventricle provide information about contractile timing of the lateral wall of the right ventricle and the septal wall. During the delivery of the cardiac leads, the strain gauge guide catheters are used for measuring interventricular and intraventricular dyssynchrony and thereby used to optimize cardiac resynchronization therapy variables such as location of pacing leads and pacing parameters.
Other embodiments of the inventive strain gauge guide catheter involve a strain gauge introducer, a strain gauge sheath, or strain gauge sleeve, all of which can make part of the delivery system of cardiac leads. One advantage of these configurations is the simplicity of integrating strain gauges into these catheter type devices. Another advantage is that these strain gauge catheters are compatible with already existing implantable cardiac pacing systems.
In another embodiment where there is a need for a permanently implanted strain gauge, a strain gauge sleeve is provided which is slid over a standard cardiac lead for permanent implantation.
In another embodiment of the present cardiac timing device, the same display unit 84 is connected to electrical connectors on the proximal end of the strain gauge guide catheter, strain gauge introducer, strain gauge sheath, or strain gauge sleeve shown in
Optical Strain Gauge Embodiments
Device installation is accomplished using a standard lead standard guide wire. After installation is completed, the standard lead guide wire may be withdrawn and replaced with a guide wire instrumented with strain gauges of a variety of types, augmented as appropriate with other sensors.
In an additional embodiment, a guide wire that is already instrumented is used to cannulate the coronary sinus and the cardiac vein. The lead is then slipped over that guide wire, a typical current approach for other procedures, and left in place. The guide wire thus positioned is available to acutely re-synchronize the heart in the initial catheter. It can then be withdrawn or left in the heart for a long period of time as an implant. In the latter case, the device is available for and re-synchronize the heart for years afterward. The design described above could be used for both acute or long term use.
For the purposes of measuring the bending of the axis of lead 90, it is not critical that the sensors be centered or off-axis, so long as the sensing element is measuring bending forces and not stretching forces. If the sensing element is measuring stretching, being at the center of lead 90, it is a potential disadvantageous design. The confounding factor is that the center line typically does not stretch unless the ends are pinned and, as a result, change the radius of curvature. An important consideration in such designs is that if guide wire 94 slides within the lead, it will not stretch. If guide wire 94 is pinned at the ends, it will undergo a stretching force between the places where it is pinned. These approaches represent an improvement upon the basic initial design of a guide wire that does slide within a lead. An additional design embodiment is to crimp or otherwise restrain the guide wire 94 within lead 90 after it is installed in order to generate a stress within the guide wire 94.
In the inventive embodiment employing optical sensors shown in
If birefringent fiber is employed for fiber 101 in the entire length of lead 90, the birefringent fiber is sensitive to bending forces throughout its length. In this configuration, bending anywhere along lead 90 is measured and reported, potentially confounding the desired data signal. Therefore, in some inventive configurations, it is advantageous to employ an optical fiber that is made of two parts. In the two-part optical fiber configuration of the inventive cardiac timing device, the first part is constructed of a coupling fiber that is not sensitive to bending in any way. The second section of the optical fiber is constructed of a birefringent fiber. While there will be, at times, some reflections at that interface between the coupling fiber and the birefringent fiber of this construct, these reflections can be filtered out by means well know to the skilled artisan.
The cardiac timing data is converted through the signal received from the optical sensor device. The light passes through the birefringent fiber, and returns through the same fiber. In this way only the region of the fiber that is birefringent is sensitive to bending in that area, and can provide clear data regarding the cardiac features to which it is adjacent.
In another embodiment of the inventing cardiac timing device, a single optical fiber is a starting material. By preferentially doping certain segments of that single fiber, characteristics are selectively changed to produce birefringence. An important benefit of this fabrication approach is reflection, which can occur when two fibers are welded together.
For some applications, the above fabrication approach produces a cardiac timing device which provides measurement of bending at different locations within lead 90, allowing data generation from discreet cardiac features, and areas on those features. In an additional iteration, multiple fibers are provided, each with a birefringent region. These fibers are then bundled together. In this configuration, each of the fibers measures the bending at a different location down the length of the lead 90.
In another inventive cardiac timing device configuration that allows the measurement of bending, stressor strain is detected in a fiber by the use of fiber Bragg gratings as strain gauge features. In this implementation of the present invention, the fiber is selected from those that, when exposed to an intense amount of light, and chosen for that fiber, will change its index of refraction. During fabrication, if a mask is used that has lines in a certain pitch to expose alternating regions of that fiber, a sequence of regions that are varying in the index of refraction is created. This is called a fiber Bragg grating (FBG).
In practice, a protective material is provided for one length of fiber, all with the same pitch. Once the mask fabrication has been accomplished, the fiber reflects colors that have a wavelength related to that pitch and determined by the index of refraction of the exposed region and the unexposed region. While somewhat complex, a well known formula describes this phenomena, and would be employed to select the preferred wave length reflectance for this application.
Measuring strain with a fiber Bragg grating derives from the characteristic of such fibers that, if they are stretched or bent, the pitch of the grating will change. If the fiber is stretched, the pitch becomes longer, which changes the wavelength directly. If the fiber is bent, then some areas of the fiber get a little longer and some areas get a little narrower. In this manner, forces on the fiber tend to change the reflectivity at a given wavelength, and broaden the reflective signal.
Using the above approach, linear strain is measured by ascertaining a shift in the reflective wavelength. Typically, the band width of the reflected light will not change and the center frequency shifts with linear strain. In this manner, the inventive cardiac timing device measures bending at the same time by looking at the changes in the width of the reflected signal. In the case of pure bending, center frequency does not shift, but the bandwidth increases. At a lower frequency, some wavelengths at a lower wavelength would be reflected and some wavelengths at a higher wavelength would be reflected. The amplitude at the center frequency would be reduced.
In this embodiment, a broader signal is provided. Bending is measured by the width at half height.
If fiber 101 stretches, the wavelength that is reflected shifts because the pitch of the various fiber Bragg gratings changes. If the fiber 101 bends, the width in wavelength terms of the reflected light reflected broadens. This phenomena is shown in
By providing different grading periodicities in the inventive cardiac timing device, different wavelengths are caused to be reflected at different locations on fiber 101. Those locations are sensitive to bending or stretching. This configuration allows the use of a single fiber to measure the strain or bending at various locations along the fiber's length. In practice, a light generator is provided with the capacity to send out appropriate inquiring light signals, and other features are also provided to accomplish excitation and signal recovery through circuitry.
Beam splitter 105 functions to modify the light beam so that much of the light goes through the fiber optic. When this is accomplished in the region, light is sent in the region of one of the fiber Bragg grating positions 98, 99 and 100. Thus, in position 99 where blue is being queried, wavelengths are sent at a little bit less and a little bit more than blue. The returning light is then measured while the emitted light is scanned for the light through the blue region. A similar approach is used in some cases for infra-red region for example. Various other common color communication types of wavelength are also used.
The light reflected from the fiber Bragg grating returns to beam splitter 105. The beam splitter 105 functions to return half of the light to a pin detector, 107. Pin detectors are standard devices in the telecommunications industry. In the case, the pin detector measures the amplitude of the reflected light. VCSEL 103 generates light of different wavelengths which are sent as pulses down the fiber optic. Some of this light is reflected back by the various fiber Bragg gratings at positions 98, 99 and 100. The amplitude of these light signals is recorded on pin detector 107.
The data thus generated by the inventive cardiac timing device provides, essentially, a drawing as in
In some iterations of this embodiment of the inventive cardiac timing device, MEMS filter 109, such as a Fabry-Perot cavity, are provided where the distance between plates is modulated to select for a given wavelength of light. This programmable filter 109 is then scanned over the wavelengths of interest. The light flows through programmable optical filter 109, and impinges upon a diode detector 111 which measures how much amplitude returns. In this case, a broadband light source 103 and a programmable light filter 109 checks how much reflection is at each of the wavelengths.
In both cases, these different designs produce a system such as shown schematically in
One embodiment of the present invention provides a cardiac motion detection system which includes a number of fiberoptic strain gauges coupled with each other through a single optical fiber.
Right ventricular lead 302 emerges from communication device in communication means 301, and travels from the preferentially subcutaneous location of communication means 301 via the subclavian venous access through the superior vena cava through the right atrium and then through the tricuspid valve to a position along the right ventricle. This location is preferentially located along its distal portion in close association with the intraventricular septum terminating distally with fixation in the right ventricular apex.
Particular to distal aspect of right ventricular lead 302 are fiberoptic strain gauges 303 and 304. In other embodiments of the present invention, an additional number or smaller number of fiberoptic strain gauges may be employed.
Additionally emerging at the proximal aspect of communication means 301 is left ventricular lead 305. Left ventricular lead 305 starts by following the same route as right ventricular lead 302 via subclavian vein through the superior vena cava into the right atrium. At this point, left ventricular lead 305 is placed via the coronary sinus around the posterior aspect of the heart and thence into cardiac vein draining into said sinus.
Right ventricular lead 302 may optionally be provided with pressure sensor 308 which is located in the right ventricle. Pressure sensor 308 provides a pressure signal which can also simultaneously be obtained with wall motion data. Note that adding active devices to said lead such as pressure sensor 308 is facilitated through the use of a multiplexing system
Principle of operation of fiberoptic strain gauges 303, 304, 306, and 307 is that communication means 301 will communicate with each of the sensors. Cardiac movement signals detected by each gauge are then timed.
The cardiac motion data together with an optional pressure signal or signals is used to optimize cardiac resynchronization therapy where the goal is to maximize the contractility of the left ventricle. This goal is obtained by encouraging effectively simultaneous contraction of the bulk of the muscle of the left ventricle. In many congestive heart failure patients, such contractility is impaired with dyskinetic contraction, which typically occurs with a septal contraction against a relaxed left ventricle followed by ventricular contraction against a relaxing septum. The result is inefficiency with regards to the blood being moved around the ventricle rather than constrictively expelled from the ventricle as in a normal case.
Current systems provide biventricular pacing on an empiric basis only where optimization is attempted. It is generally a time-consuming process based on external cardiac ultrasound. A physician typically attempts to visualize wall motion. Conventionally, with the aid of tools in an ultrasonic machine, the physician calculates a synchronicity index based on the wall motion just described. In contrast, the present cardiac motion detection system provides such data in real time and in a numeric format useable by both the implanting physician and by an automated pacing system such as a CRT device.
A fiberoptic strain gauge provided in one embodiment of the present invention detects abnormalities in wall motion associated with ischemia or arrhythmia and other cardiac abnormalities including progression of underlying disease states such as congestive heart failure. If connected to an external or fully implanted real time monitor, such abnormalities can trigger an alarm. This alarm alerts the patient or a physician of the advent of these abnormalities.
When a fiberoptic strain gauge is in a normal, unbent state, the light signals entering the input fiber of the strain gauge are transmitted and coupled into the reflection/pass-though fiber. The reflection/pass-through fiber contains a fiber Bragg grating, which reflects light signals at a given wavelength (i.e., the Bragg wavelength) back to the input fiber and allows light signals at other wavelengths to pass through. When the strain gauge is bent, the amount of reflected light at the Bragg wavelength varies. In addition, the amount of pass-through light also varies due to the coupling loss. In this way, by transmitting a number of wavelengths into fiber 3104, and by detecting the amount of reflected light at different Bragg wavelengths corresponding to each strain gauge, control module 3102 can determine the amount of bending incurred at each strain gauge. Furthermore, control module 3102 can monitor the timing of cardiac contractions based on the timing of variations of the reflected light. Note that the system illustrated in
In representative embodiments, the strain gauges include fiber Bragg gratins, as indicated above. Fiber Bragg gratings can be manufactured in a number of ways. One type of fiber Bragg gratings contain gratings external to the core, which are placed in the cladding as described in the article "A single mode fiber evanescent grating reflector" by Sorin and Shaw in IEEE Journal of Lightwave Technology, vol. 3, pp. 1041-1045 (1985), and in U.S. patents by Schmadel (U.S. Pat. No. 4,268,116, issued May 19, 1981), Ishikawa (U.S. Pat. No. 4,622,663, issued Nov. 11, 1986), and by Sorin (U.S. Pat. No. 4,986,624, issued Jan. 22, 1991). These designs involve periodic gratings which operate in the evanescent field within the cladding layer proximal to the core of the fiber.
A second class of fiber gratings involves internal gratings fabricated within the core. One technique involves modulating the core refractive index. In-core fiber gratings were disclosed by Hill et al. in "Photosensitivity in optical fiber waveguides: Application to reflected filter fabrication" in Applied Physics Letters, vol. 32, pp. 647-649 (1978). These gratings are written internally by interfering two counter propagating electromagnetic waves within the fiber core, one of which is produced from the reflection of the other from the fiber end face. Meltz et al. further showed how to write gratings externally by the split-interferometer method involving side-illumination of the fiber core by two interfering beams produced by a laser, as described in "Formation of Bragg gratings in optical fibers by a transverse holographic method" in Optics Letters, vol. 14, pp. 823-825 (1989).
If the refractive index within the fiber grating's core is plotted as a function of distance that is measured along the core's length, the resulting curve would have a periodic variation. Correspondingly, the core has a nominal or average index of refraction, and the core's actual refractive index varies from this average value.
Mathematically, the Bragg wavelength λB of a fiber grating is described as:
where P is the grating period and n is the nominal or average refractive index of the core.
As shown by the above equation, the Bragg wavelength λB of a fiber grating is shifted by a change in the grating period P, and/or by a change in the average refractive index n of the core. In general, the grating period and the average index of refraction change when the fiber grating is subjected to a change in strain (e.g., the fiber grating is stretched lengthwise), a change in length that is caused by a temperature change, or to a change in length that is caused by pressure, all of which can be collectively defined as photoelastic and thermo-optic effects.
Both the peak reflectivity and the spectral bandwidth of reflected light at a Bragg wavelength are functions of the fiber grating's length and the amount of refractive index variation that is present at each periodic index variation. Conventionally, as a result of these above properties, a fiber grating can be used as a strain sensor, a temperature sensor, and a pressure sensor. In each case, the strain, temperature, and/or pressure is measured by determining a shift in the fiber grating's Bragg wavelength from a calibration point, or by measuring the absolute value of the Bragg wavelength.
The fiberoptic strain gauge provided in one embodiment of the present invention detects strain or bending imposed upon the gauge based on the amount of light reflected by a fiber Bragg grating, instead of the shift of Bragg wavelength. This design significantly increases the sensitivity and dynamic range of the strain gauge.
As shown in
MEMS folded spring structure 3201 maintains a gap between fiber 3203 and grating 3202. Because of the flexibility of the folded springs, the strain gauge enjoys certain degree of freedom when there is a lateral strain. When such a lateral strain is present, the alignment between fiber 3203 and grating 3202 is disturbed. Consequently, the amount of reflected light at grating 3202's Bragg wavelength is reduced. Therefore, by detecting the amount of reflected light at grating 3202's Bragg wavelength, the system can easily determine when the strain gauge is bent. Furthermore, the system can also determine the amount of bending based on the coupling loss. Note that because the bending also increases the coupling loss of light at other wavelengths, the amount of pass-through light is reduced when the strain gauge is bent. One advantage of this strain gauge design is that, unlike previous designs which require detection of optical signals at an output fiber, the detection of reflected signals occurs at the input fiber. This one-port operation significantly reduces the complexity of the cardiac motion detection system. In addition, because there is no looped double-fiber configuration, the entire system can be constructed with a considerably reduced form factor.
Also shown in
Note that when an upstream strain gauge is bent, the pass-through light coupled to the downstream strain gauges suffers from the coupling loss caused by the bend at the upstream gauge. When light is reflected by a downstream strain gauge, the reflected light must go through the same upstream gauge and hence experience once again the coupling loss. These coupling losses dictate that the absolute amount of loss of reflected light at different Bragg wavelengths may not be equal.
However, the absolute amount of losses at different Bragg wavelengths is of less significance to the present cardiac motion detection system. The present system detects cardiac motions based on the relative variation of losses at each Bragg wavelength instead of the absolute values of these losses. Hence, as long as the system uses detectors with a sufficiently large dynamic range, the bending of different strain gauges can be successfully detected. In one embodiment, the dynamic range of a detector used by the system is larger than about 30 dB, such as larger than about 50 dB, including larger than about 70 dB.
An alternative approach to reduce the cumulative coupling losses experienced by different strain gauges and to accommodate detectors with smaller dynamic range is to adjust the power of the input light at each Bragg wavelength. For example, the power of the input light at the Bragg wavelength corresponding to the most upstream strain gauge can be set at the lowest level, while the power of the input light at. Bragg wavelengths corresponding to down stream strain gauges can be set at progressively higher levels. In this way, detectors with smaller dynamic range can still detect loss variations for all the Bragg wavelengths. Note that this configuration would require individual power adjustments at different wavelengths. A broadband light source typically does not provide such individual power adjustability at specific wavelengths.
In most cases, it is beneficial to minimize the unnecessary coupling and reflection losses between the input fiber and the Bragg grating fiber. Excessive coupling loss occurs when a light beam exiting the input fiber diverges quickly due to the refractive index difference between the fiber core and the air. Such quick divergence prevents efficient coupling of light into the core of the reflection/pass-through fiber. Moreover, the fiber-air index difference also causes light to reflect back at a fiber's end face. A reduction of such losses helps improve the sensitivity of the strain gauge. One approach to reduce these unnecessary losses is to fill the gap between two fibers with an index-matching material, such as a polymer. Ideally, the index of this material matches the index of the fiber core.
Ideally, the range of bending of a strain gauge is sufficiently large so that a loss caused by cardiac motion can be detected. At the same time, the bending of a gauge is limited so that the gauge is not over-bent and that the light is not completely lost for downstream strain gauges. The range of bending can be controlled through the catheter in which the strain gauge resides, or through controlling the stiffness of the MEMS folded spring structure.
Although in previous examples, the MEMS folded spring structure includes folded torsion beams perpendicular to the axis of the fiber, a large variety of folded spring designs can be used in different embodiments of the present invention.
In one embodiment of the present invention, the input fiber and the reflection/pass-through fiber are held and aligned by a pair of V-grooves.
The structure shown in
During bulk micromachining of the V-groove, first a mask defining the area of the top opening for the V-groove is laid. Next, the silicon wafer is etched anisotropically. Both dry and wet anisotropic etching techniques can be used. These techniques include potassium hydroxide (KOH) etching, tetramethyl ammonium hydroxide (TMAH) etching, ammonia/hydrogen peroxide/water (AHW) etching, ethylenediamine/pyrocathecol/water (EDP) etching, chemical vapor etching, and plasma etching, among others. In one embodiment of the present invention, a V-groove is fabricated on a [100]-silicon wafer with KOH etching.
Once a V-groove is fabricated on a silicon wafer, a gap needs to be cut in the middle of the V-groove and folded springs needs to form on both sides of the gap. This can be achieved by etching through the entire wafer with surface micromachining techniques, such as deep reactive ion etching (DRIE). The etching can be performed either from the front side (the side with the V-groove) or from the back side. In one preferred embodiment of the present invention, the patterning and etching of the gap and folded springs are performed on the back side of the silicon wafer.
As shown in
In one embodiment of the present invention, the thickness of the torsion beams for the folded springs is the same as that of the wafer. This design allows the strain gauge to bend in one lateral direction (i.e., left and right as shown in
The two-axis design allows a strain gauge to be more sensitive to cardiac motions. However, the system cannot tell around which axis is the strain gauge bent. Sometimes it may be desirable to limit the bending of a gauge to only one direction. If this is the case, the single-axis design shown in
Note that it is also possible to extend the region of folded springs to the space above and below the gap without interfering the light coupling between fibers. Doing so may help further reduce the size of the MEMS holding structure by providing more spring tension using less material for the holding structure.
Because of the unprecedented small sizes of the fiberoptic strain gauges, it is possible to place the present cardiac motion detection system within a metal coil which is typically part of a cardiac guidewire.
In another embodiment, instead of multiple individual light sources, one or more tunable lasers can be used. In this case, a tunable laser can be tuned to the Bragg wavelength of a given fiberoptic strain gauge so that its reflected light signals can be monitored. Alternatively, a broadband light source can be used. The use of a broadband light source eliminates the need for a multiplexer. Because each strain gauge reflects only at its Bragg wavelength, the spectrum analyzer can detect these peaks and can therefore easily distinguish the signals corresponding to different strain gauges. Note that a broadband light source does not typically allow adjustment of the output power at specific wavelengths. However, the total output power of the broadband light source can be adjusted for signal-reading of each individual strain gauge to compensate for its accumulated losses.
If individual light sources are used for different Bragg wavelengths, it is possible to automatically adjust the power input at these wavelengths to compensate for the accumulated coupling losses and to fully utilize the dynamic range of the detectors. In one embodiment, the spectrum analyzer is also interfaced with a control module which monitors the absolute power of each of the Bragg wavelengths. If the power at a specific Bragg wavelength is too low, the control module can automatically adjust the power of the light source at that wavelength. As a result, the light detector's dynamic range can be better utilized and a physician can more easily observe the signal variation at that wavelength.
On the signal-detection end, the system alternatively can use a tunable filter coupled with a broadband light detector, such as a PIN diode. This configuration significantly reduces the system cost by avoiding using an expensive spectrum analyzer. During operation, a physician can tune the filter to the Bragg wavelength associated with a specific strain gauge. In another embodiment, the system can also use a demultiplexer and a number of light detectors for each Bragg wavelength.
Following regular fiber section 710 is an unguided section 712. In this section, the index of the fiber is changed and the core can no longer fully constrain the power of the propagating wave. The power profile of the light spreads out as light travels forward. At the end of unguided section 712 is a light-capture section 714, which recaptures the spread light. The amount of recaptured light typically depends on the bending incurred on unguided section 712. The more the fiber is bent, the less light can be recaptured.
Following light-capture section 714 is a light reflection section 716. Reflection section 716 contains a fiber Bragg grating which reflects the light at its Bragg wavelength. In this way, by detecting the amount of signal loss at a given Bragg wavelength, the system can detect the amount of bending incurred on the fiber. Note that, in a degenerate design, light-capture section 714 can also be incorporated into reflection section 716 wherein the Bragg grating region also serves to recapture the spread light.
The unguided section can be fabricated using an exposure process similar to that used for fabricating Bragg gratings using the same mask. Alternatively, the exposure process for the unguided region can be a separate process using a different mask. In general, the distance between the Bragg grating and the unguided section is not critical. Therefore, the separate-exposure approach can also enjoy a larger tolerance.
Following regular fiber section 740 is an expanded-core section 742. In this section, the core is expanded and the power of the propagating light spreads out accordingly. This tapered-core structure can be obtained by exposing the fiber to a high-intensity laser. The temperature change induced by the exposure in the targeted region typically expands the core to a tapered shape as shown in
Resonant Marker Band Bending Sensor
Since electrically conductive platinum marker bands are inherently integral components of a catheter-based system, these bands make ideal candidates for elements of a simple strain measurement device. In one embodiment of the present invention, a marker band 200 is created that contains two metal rings 201 and 202 separated by either a compliant insulator or a piezoelectric material 203 as shown in
The marker band is then driven into resonance by either electrostatic attraction between the metal components or by piezoelectric actuation of the piezoelectric material. The resonant frequency of the band shifts as the catheter is bent and the shape of the compliant insulator/piezoelectric material changes, as shown in
Piezoelectric Resonant Bending Sensor and Other Piezoelectric Embodiments
In two additional embodiments of the present invention, a piezoelectric approach is provided. In the first approach, strain is sensed by monitoring the shift in the resonant frequency of a piezoelectric crystal 210 mounted in the catheter/guidewire 212, as shown in
For the first piezoelectric approach, the piezoelectric crystal is driven into resonance by an AC voltage source. Bending the crystal will change its resonant frequency, regardless of position in the device. The resonant frequency may be measured using a frequency counter.
A second piezoelectric approach utilizes a bulk piece of piezoelectric material. When a mechanical stress is applied to a piezoceramic element in the longitudinal direction (parallel to polarization), a voltage is generated which tries to return the piece to its original thickness. That voltage may be measured by an AND after it has been amplified and filtered.
Both approaches promise a greater signal to noise ratio which means the crystal does not need to be mounted off of the neutral axis (something that must be done for other strain gauges). More common piezoresistive elements should be located at the place of maximum strain, such as the outer edge of the device. Positioning a strain sensor in the outer wall of a catheter or guidewire may prove difficult as this location is typically used for wire coils or other catheter/guidewire components. The sensor element could be mounted closer to the neutral axis of bending in the lumen of a catheter or guidewire and still produce a measurable response.
Both approaches also promise simpler sensor construction. Unlike resistive strain gauges, there is not a need to etch precision resistors. Both approaches require relatively simple measurement so the practitioner's choice between the two ideas depends on material cost, power requirements and signal to noise ratio.
Capacitive Marker Band Bending Sensor
Since electrically conductive platinum marker bands are already integral components of a catheter-based system, the bands make ideal candidates for elements of a simple strain measurement device. Two marker bands 221 and 222 could be placed in close proximity on a catheter as shown
Coupling Strain Gauge Timing Measurements with Mechanical Measurements at a Second Location
In certain embodiments, the subject methods and system include employing strain gauges to determining timing of tissue movement at a first location or series of locations, and then employing a second movement characterization element, such as a mechanical characterization element, at a second tissue location. For example, certain embodiments including using strain gauges to detecting timing of tissue movement a left ventricle wall location and an inductive sensing element at a septal wall location, such as is described in U.S. Provisional Patent Application Ser. No. 60/638,947 filed Dec. 22, 2004 titled "Septal Dynamics Measurement Device." In these embodiments, a magnetic coil septal dynamic measurement apparatus provides data which is used synergistically with cardiac wall motion data retrieved from a string of strain gauges.
The magnetic coil septal dynamic measurement apparatus includes a transmitter coil and a receiver coil in an inductive coupling relationship. The transmitter coil is positioned on one surface of the septal wall, e.g., the right ventricular surface. In representative embodiments, the transmitter coil has 0.1 to 1 cm, such as 0.5 cm diameter. The receiver coil has a spiral configuration, and is threaded through the septal wall from a location close the right ventricular surface to a location close to the left ventricular surface, such that the overall length of the spiral receiver coil changes as a function of septal wall thickness. As such, when the septal wall is relaxed, the spiral receiver coil has an extended configuration such that all of the turns in the receiver coil can pick up magnetic flux from the transmitter coil. However, in the contracted state, only a fraction of the turns, e.g., half of the turns, are available to pick up magnetic flux. In representative embodiments, the number of coils ranges from about 5 to about 15, such as 10. As such, the voltage in the receiver coil varies detectably between the relaxed and contracted states, and detection of this variation is readily employed to determine timing of contraction and relaxation of the septal wall. Any convenient approach may be employed to detect the change in voltage, such as a sliding contact connection, where the coil ends to be contacted are insulated but for the distal end, which has a sufficiently large diameter exposed tip to provide for current flow across any resistive medium, e.g., tissue, separating the ends. For implantation of the coils, biodegradeable coatings that provide for desired stiffness during delivery but then degrade to leave a flexible coil may be employed.
In a first embodiment of such a system, the septal dynamic measurement apparatus provides data which is used synergistically with data retrieved from a string of strain gauges inserted into the venous system of the left ventricle outer wall. The strain gauge date is leveraged and informed by the inductive contraction measuring device. The inductive contraction measuring device is inserted into the septum by a catheter, in some cases passing through the vena cava.
When the heart goes from diastole to systole, the heart muscles contract producing a thickening of the ventricular wall and the septum. The contraction reduces the overall size of the heart introducing therefore increased curvature into the outer walls of the heart. The string of strain gauges in the left ventricular venous system in the present invention measure their curvature. When that curvature is followed over time, this data allows observation of the timing of the contraction of the ventricular wall and, in particular, the observation of the timing of the maximum and minimum contractions.
To measure left ventricular efficiency, a second measurement is made at a second site, e.g., a site on the cardiac septum. The inductive muscle contraction monitor is inserted into the second site, e.g., the septal site. Septal wall motion contraction is then monitored. From this data, the time course of the contraction of the septum can be easily determined on an on-going real time basis.
The signals from the two sensing devices, i.e., the strain gauges and the septal wall monitor, can be compared and their degree of synchrony established. When the resynchronization theory is optimal, ideally the contraction of the ventricular wall and the septum will be synchronous, or a least enjoy a clinically useful increase in synchrony.
Based on the data obtained with these two sensing systems, a biventricular stimulation system will have appropriate reference points where the relative timing of the left and right ventricular stimuli can be varied until maximum synchrony is obtained.
An alternative embodiment of the present inventive cardiac wall timing system would have an inductive muscle contraction monitor inserted into the outer wall of the right ventricle. In certain embodiments, muscle contraction monitors are provided in both the septum and the right ventricular wall. This inventive orientation allows comparison of contractility of the right ventricular outer wall with the contractions of the left ventricular wall as measured by the string of strain gauges.
The timing of the contractions is the important parameter to be extracted from the observations of the signals from the strain gauges and the muscle contraction monitors. These observations can be used in turn to optimize the stimulus parameters.
In a representative embodiment, a string of strain gauges is positioned along the left ventricular wall such that they extend down the wall of the left ventricle. A muscle contract monitoring device, e.g., as described above, is inserted in the septal wall. In a variation of this configuration, an additional muscle contraction monitor is inserted into the right ventricular wall
The subject methods of evaluating tissue location movement find use in a variety of different applications. As indicated above, an important application of the subject invention is for use in cardiac resynchronization, or CRT, also termed biventricular pacing. As is known in the art, CRT remedies the delayed left ventricular mechanics of heart failure patients. In a desynchronized heart, the interventricular septum will often contract ahead of portions of the free wall of the left ventricle. In such a situation, where the time course of ventricular contraction is prolonged, the aggregate amount of work performed by the left ventricle against the intraventricular pressure is substantial. However, the actual work delivered on the body in the form of stroke volume and effective cardiac output is lower than would otherwise be expected. Using the subject approach, the electromechanical delay of the left lateral ventricle can be evaluated and the resultant data employed in CRT, e.g., using the approaches reviewed above and/or known in the art and reviewed at Col. 22, lines 5 to Col. 24, lines 34 of U.S. Pat. No. 6,795,732, the disclosure of which is herein incorporated by reference.
In a fully implantable system the location of the pacing electrodes on multi electrode leads and pacing timing parameters are continuously optimized by the pacemaker. The pacemaker frequently determines the location and parameters which minimizes intraventricular dyssynchrony, interventricular dyssynchrony, or electromechanical delay of the left ventricle lateral wall in order to optimize CRT. This cardiac wall motion sensing system can also be used during the placement procedure of the cardiac leads in order to optimize CRT. An external controller could be connected to the cardiac leads and a skin patch electrode during placement of the leads. The skin patch acts as the reference electrode until the pacemaker is connected to the leads. In this scenario, for example, the optimal left ventricle cardiac vein location for CRT is determined by acutely measuring intraventricular dyssynchrony.
The subject methods and devices can be used to adjust a resynchronization pacemaker either acutely in an open loop fashion or on a nearly continuous basis in a closed loop fashion.
Other uses for this system could be as an ischemia detector. It is well understood that in the event of acute ischemic events one of the first indications of such ischemia is akinesis, i.e., decreased wall motion of the ischemic tissue as the muscle becomes stiffened. A Wall motion system would be a very sensitive indicator of an ischemic process, by ratio metrically comparing the local wall motion to a global parameter such as pressure; this has been previously described in another Proteus patent. One can derive important information about unmonitored wall segments and their potential ischemia. For example, if an unmonitored section became ischemic, the monitored segment would have to work harder and have relatively greater motion in order to maintain systemic pressure and therefore ratio metric analysis would reveal that fact.
Another application of such position indicators that record wall motion would be as a potentially superior arrhythmia detection circuit. Current arrhythmia detection circuits rely on electrical activity within the heart. Such algorithms are therefore susceptible to confusing electrical noise for an arrhythmia. There is also the potential for misidentifying or mischaracterizing arrhythmia based on electrical events when mechanical analysis would reveal a different underlying physiologic process. Therefore the current invention could also be adapted to develop a superior arrhythmia detection and categorization algorithm.
Additional applications in which the subject invention finds use include, but are not limited to: the detection of electromechanical dissociation during pacing or arrhythmias, differentiation of hemodynamically significant and insignificant ventricular tachycardias, monitoring of cardiac output, mechanical confirmation of capture or loss of capture for autocapture algorithms, optimization of multi-site pacing for heart failure, rate responsive pacing based on myocardial contractility, detection of syncope, detection or classification of atrial and ventricular tachyarrhythmias, automatic adjustment of sense amplifier sensitivity based on detection of mechanical events, determination of pacemaker mode switching, determining the need for fast and aggressive versus slower and less aggressive anti-tachyarrhythmia therapies, or determining the need to compensate for a weakly beating heart after therapy delivery (where these representative applications are reviewed in greater detail in U.S. Pat. No. 6,795,732, the disclosure of which is herein incorporated by reference), and the like.
In certain embodiments, the subject invention is employed to overcome barriers to advances in the pharmacologic management of CHF, which advances are slowed by the inability to physiologically stratify patients and individually evaluate response to variations in therapy. It is widely accepted that optimal medical therapy for CHF involves the simultaneous administration of several pharmacologic agents. Progress in adding new agents or adjusting the relative doses of existing agents is slowed by the need to rely solely on time-consuming and expensive long-term morbidity and mortality trials. In addition, the presumed homogeneity of clinical trial patient populations may often be erroneous since patients in similar symptomatic categories are often assumed to be physiologically similar. It is desirable to provide implantable systems designed to capture important cardiac performance and patient compliance data so that acute effects of medication regimen variation may be accurately quantified. This may lead to surrogate endpoints valuable in designing improved drug treatment regimens for eventual testing in longer-term randomized morbidity and mortality studies. In addition, quantitative hemodynamic analysis may permit better segregation of drug responders from non-responders thereby allowing therapies with promising effects to be detected, appropriately evaluated and eventually approved for marketing. The present invention allows for the above. In certain embodiments, the present invention is used in conjunction with the Pharma-informatics system, as described in U.S. Provisional Application Ser. No. 60/676,145 filed on Apr. 28, 2005 and U.S. Provisional Application Ser. No. 60/694,078; the disclosures of which are herein incorporated by reference.
Non-cardiac applications will be readily apparent to the skilled artisan, such as, by example, measuring the congestion in the lungs, determining how much fluid is in the brain, assessing distention of the urinary bladder. Other applications also include assessing variable characteristics of many organs of the body such as the stomach. In that case, after someone has taken a meal, the present invention allows measurement of the stomach to determine that this has occurred. Because of the inherently numeric nature of the data from the present invention, these patients can be automatically stimulated to stop eating, in the case of overeating, or encouraged to eat, in the case of anorexia. The present inventive system can also be employed to measure the fluid fill of a patient's legs to assess edema, or other various clinical applications.
Embodiments of the present invention may also be used for orthopedic procedures, wherein the strain gauges can detect how bones are bending or stretching. For example, the present inventive fiber-optical strain gauges can be used in FES or stimulation of muscles. The advantage herein is that a physician can place a single strand of fiber that runs along the length of a leg and measures various parameters. In other applications, the present inventive strain gauges can be placed in facial bones for plastic surgical purposes which allow observation of changes in dimension over time.
In other applications, the present inventive strain gauges can be used for spinal surgery to assist examination of fused parts of the spine and to detect residual motion between fused parts in which case the fusing has to be redone. Additional applications of embodiments of the present invention include use of the fiber-optical strain gauges within or in the vicinity of organs such as bladders, lungs, and stomachs. By placing multiple strain gauges around the periphery of these organs, a physician can determine the change in their sizes. Embodiments of the present invention allow measurement at multiple points with a single strand of fiber.
Computer Readable Medium
One or more aspects of the subject invention may be in the form of computer readable media having programming stored thereon for implementing the subject methods. The computer readable media may be, for example, in the form of a computer disk or CD, a floppy disc, a magnetic "hard card", a server, or any other computer readable media capable of containing data or the like, stored electronically, magnetically, optically or by other means. Accordingly, stored programming embodying steps for carrying-out the subject methods may be transferred or communicated to a processor, e.g., by using a computer network, server, or other interface connection, e.g., the Internet, or other relay means.
More specifically, computer readable medium may include stored programming embodying an algorithm for carrying out the subject methods. Accordingly, such a stored algorithm is configured to, or is otherwise capable of, practicing the subject methods, e.g., by operating an implantable medical device to perform the subject methods. The subject algorithm and associated processor may also be capable of implementing the appropriate adjustment(s).
Of particular interest in certain embodiments are systems loaded with such computer readable mediums such that the systems are configured to practice the subject methods.
As summarized above, also provided are kits for use in practicing the subject methods. In certain embodiments, the kits at least include a computer readable medium, as described above. The computer readable medium may be a component of other devices or systems, or components thereof, in the kit, such as an adaptor module, a pacemaker, etc. The kits and systems may also include a number of optional components that find use including but not limited to, implantation devices, etc. In addition, the kits may include various systems or components thereof, as described above.
In certain embodiments of the subject kits, the kits will further include instructions for using the subject devices or elements for obtaining the same (e.g., a website URL directing the user to a webpage which provides the instructions), where these instructions are typically printed on a substrate, which substrate may be one or more of: a package insert, the packaging, reagent containers and the like. In the subject kits, the one or more components are present in the same or different containers, as may be convenient or desirable.
The following examples are offered by way of illustration and not by way of limitation.
The goal of the strain gauge studies described herein was to demonstrate that strain gauge leads placed in various locations of the heart can provide direct mechanical contractile timing information of the heart. This information is very valuable for the optimization of cardiac resynchronization therapy for heart failure patients.
In a first lead that was used in the following experiments, two strain gauges were placed at the distal end of the strain gauge lead, 3 or 5 cm apart. The first strain gauge was a 120 ohms resistive strain gauge. The strain gauge was constructed by embedding a foil measuring element into the carrier. The overall thickness of the strain gauge was approximately 30 μm. The carrier dimension was 4×1.5 mm and the foil grid was 1.0×0.7 mm. The strain gauge was attached to two conductor wires. The entire resulting construct was inserted into the lumen of an extruded polyurethane tube.
A second strain gauge with conductor wires was also inserted into the polyurethane tubing such that there was spacing of 3 or 5 cm between the strain gauges. A flexible adhesive was injected into the lumen of the polyurethane tubing in order to hold the strain gauges fixed in place. The polyurethane tubing was equivalent to a 5 French catheter which has outer diameter of 1.7 mm. On the most distal tip of the strain gauge lead was a radio opaque marker which was a platinum iridium band adhered into place using a flexible adhesive.
The distal tip of the strain gauge lead was a rounded cap of soft UV stable adhesive. With this construction any deflection of the lead caused the strain gauges, which were embedded within the lumen of the lead, to deflect and display a strain signal.
Another set of strain gauge leads was constructed using Oscor permanent pacing leads with active fixation or screw in tips. These leads were modified by integrating two strain gauges as described above onto the distal portion of the Oscor permanent pacing lead. These leads are referred to as screw in strain gauge leads.
Porcine animal models were used for these studies in a catheterization laboratory with the animals anesthetized. Pressure sensor catheters were placed in both the left ventricle and right ventricle via the femoral artery and the femoral vein. The catheter in the left ventricle was also able to measure volume.
Under fluoroscopy, the radioopaque marker was shown to be placed in the right vectricle. The placement of the strain gauge lead in the right ventricle provides mechanical contractile timing information of the right ventricle free wall and septum.
The screw in strain gauge lead was observed under fluoroscopy to be located in the right ventricle and was fixated on the right ventricle free wall. This placement was set to isolate the mechanical contractile timing information of the right ventricle free wall. A screw in strain gauge lead was also placed on the septum for isolating the mechanical contractile timing information of the septum. A guide catheter was placed into the coronary sinus to provide the placement of a strain gauge lead into one of the cardiac veins of the left ventricle. This placement gives information about contractile activity occurring on the left ventricle free wall.
Once all the pressure and volume sensing catheters and strain gauge leads were placed in the heart, they were connected to a data acquisition system which included Lab View Software and National Instrument equipment. Data was sampled at 500 Hertz. The strain gauge signals were filtered using a 100 hertz low pass filter. Strain, pressure, volume, and EKG were all recorded simultaneously.
1. The left ventricle distal strain gauge signal.
2. The left ventricle proximal strain gauge signal.
3. The right ventricle distal strain gauge signal.
4. The right ventricle proximal strain gauge signal.
5. The right ventricle distal screw in strain gauge signal.
6. The right ventricle proximal screw in strain gauge signal.
7. The left ventricle volume (LVV).
8. The right ventricle pressure (RVP) and the left ventricle pressure (LVP).
9. The EKG signal.
The results show that all the strain gauges show very repeatable deflections with each cardiac cycle and the periodicity of all the strain gauge signals are clearly tied to the periodicity of the EKG, pressure, and volume signals. Overlaid on top of these curves is the respiration of the pig, which is set at 5 second period by the respirator and shows up as a slight change in amplitude of the strain signals every 5 seconds. This direct correspondence of the periodicity of the strain gauge signals and the heart contractions was also observed in a fluoroscopy video. The strain gauges in the right ventricle and strain ventricle cardiac vein were clearly seen to move and deflect in concert with the cardiac contractions.
Before reaching the maxima of the strain signal there is a slight pick-up 142 as visible and appears to coincide with the aortic valve opening. The strain signal reaches its maxima 144 or main peak at the moment when the AV valve opens and the rapid filling of the heart ventricle commences. The strain signal then continues to decrease until it reaches the sharp valley or minima which defines the beginning of the next cardiac cycle.
These results demonstrate that the strain signals provide valuable information, not only about when each cardiac cycle starts or ends, but also as to the various phases within the cardiac cycle which involve opening and closing of heart valves. This data will also elucidate the various phases of the cardiac cycle such as iso-volumetric contraction, rapid ejection, reduced ejection, iso-volumetric relaxation, rapid filling, reduced filling and atrialy systole.
In this experimental testing, the signature shape of the strain gauge signals appears to vary from one strain gauge to another depending on is location. Indeed, a strain gauge lead placed in the cardiac vein on the outside surface of the heart may experience different deflection profiles during the cardiac cycle than a strain gauge lead placed inside or against the endocardiac surface of the right ventricle. It is therefore important to initially study the different strain gauge signal profiles based on their location.
1. the left ventricle strain gauge signal in dark blue,
2. the left ventricle proximal strain gauge signal in light blue,
3. the left ventricle volume measurement in grey,
4. the left ventricle and
5. right ventricle pressures in dark pink and light pink, and
6. at the bottom, the EKG signal in black.
Around the 28 second time mark 150, the strain gauge was repositioned by pulling it back 10 mm within the cardiac vein of the left ventricle. A clear change is visible between the left ventricle distal strain signal 152 before repositioning and the strain signal after repositioning 154. A clear change is also visible to the left ventricle proximal strain gauge in comparing the strain signal before repositioning 156 and after repositioning 158. This sensitivity to location of the strain gauge suggests that having a strain gauge lead with a multiplicity of strain gauges along its length provides for the capture of a comprehensive picture of the strain signals occurring across the heart.
1. the left ventricle distal strain gauge,
2. the left ventricle proximal strain gauge,
3. the right ventricle distal strain gauge,
4. the right ventricle proximal strain gauge,
5. the left ventricle volume,
6. the left and right ventricle pressures, and
7. at the bottom, the EKG signal.
Pacing was set at 130 beats per minute and initiated at the 32 second time mark 160. Before pacing was initiated regular strain signals are visible in all the strain gauges. The regularity and shape of the strain gauge signals is disrupted once pacing has been initiated in the right ventricle screw in lead. However it is still possible to observe that the periodicity of the strain gauges has increased from the 98 beats per minute of the base line heart to the 130 beats per minute of the paced heart.
The above data shows a clear timing link between strain gauge signals and the global cardiac contractions of the heart. Current analysis and studies involve investigating the link between the strain gauges and not only global, but regional, mechanical contractile timing of the septum, right and left ventricles. Comparing the strain signals from various signals in the heart allows measurement of dis-synchronous mechanical contractions across the heart of the congestive heart failure patient.
The signal analysis of the strain signals are performed using signal processing techniques such as peak and valley detector algorithms and fast Foley transformations.
As is evident from the above results and discussion, the subject invention provides numerous advantages. Advantages of various embodiments of the subject invention include, but are not limited to: low power consumption; real time discrimination of multiple lines of position possible (one or more); and noise tolerance, since the indicators are relative and mainly of interest in the time domain. A further advantage of this approach is that there is no need for additional catheters or electrodes for determining position. Rather the existing electrodes already used for pacing and defibrillation can be used to inject AC impulses at one or more frequencies designed not to interfere with the body or pacing apparatus. As such, the subject invention represents a significant contribution to the art.
Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.
Accordingly, the preceding merely illustrates the principles of the invention. It will be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure. The scope of the present invention, therefore, is not intended to be limited to the exemplary embodiments shown and described herein. Rather, the scope and spirit of present invention is embodied by the appended claims.
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