critical care medicine
Monitoring and Testing the Critical Care Patient
Noninvasive Cardiac Output Assessment
Other methods of determining cardiac output, such as point-of-care ultrasonography, esophageal Doppler monitoring, and thoracic bioimpedance, can be used to avoid the complications of pulmonary artery catheters (PACs). Although these methods are potentially useful, none is yet as reliable as a PAC. Point-of-care ultrasonography has become indispensable in critical care for rapid diagnosis of functional as well as anatomic abnormalities. Hand-held ultrasound devices are portable and thus save time and obviate the need to move the patient. The quality of information obtained from point-of-care ultrasonography sometimes matches or exceeds that provided by more expensive and labor-intensive imaging techniques. Judicious use of ultrasonography decreases exposure to ionizing radiation. In acute care, point-of-care ultrasonography is particularly useful for evaluating the abdomen, thorax, and heart. It can sometimes be used to diagnose deep vein thrombosis. can be used to identify free (extravascular) fluid, typically as part of Testing (FAST—usually done during trauma evaluation and resuscitation). Free fluid in a hypotensive trauma patient is likely to be blood, an indication for surgical intervention. Other abdominal organs can also be evaluated. is essential for evaluating anatomy as well as hemodynamic status by assessing chamber size, wall motion, contractility, and ejection fraction. A focused rapid echocardiographic evaluation (FREE) is one example of a structured ultrasound assessment. FREE is done using the 4 standard echocardiographic windows: the parasternal long axis, parasternal short axis, apical, and subxyphoid windows. FREE evaluates left ventricular ejection fraction (EF), stroke volume (SV), cardiac output (CO), and cardiac index (CI) and allows for the evaluation of patients with hypotension ( Noninvasive cardiac output assessment reference ). In evaluating patients with hypotension, ultrasonography is indispensable in confirming the following: Hypovolemia: Even if the inferior vena cava looks full (as can occur in a hypovolemic ventilated patient), hypovolemia is suggested by a hyperdynamic left ventricle with almost no blood at the end of systole and little at the end of diastole. Left ventricular dysfunction: Left ventricular dysfunction is suggested by wall motion abnormalities and decreased ejection fraction that is either measured or estimated (by an experienced operator who assesses the overall size and apparent contractility and inward movement and thickening of the various segments of the left ventricular wall). Right ventricular failure: The right ventricle should be 60% the size of the left ventricle, triangular, and have a rough inside surface. Right ventricular failure may suggest pulmonary embolism. Pericardial effusions and tamponade can be used to identify pleural fluid and pneumothorax with higher sensitivity and negative predictive value than with plain x-rays. For example, loss of lung sliding in an area spanning over 3 intercostal spaces and A-lines (horizontal artifacts) are each nearly 100% sensitive, and, when combined, appear highly specific. The echogenicity of pleural fluid and changes in the pleura and adjacent lung parenchyma also help determine the etiology of pleural fluid. (See Point-of-care ultrasonography ) Point-of-care ultrasonography is also useful for looking for deep venous thrombosis and evaluating intra-abdominal organs. This device is a soft 6-mm catheter that is passed nasopharyngeally into the esophagus and positioned behind the heart. A Doppler flow probe at its tip allows continuous monitoring of cardiac output and stroke volume. Unlike the invasive PAC, the esophageal Doppler monitor (EDM) does not cause pneumothorax, arrhythmia, or infection. An EDM may actually be more accurate than a PAC in patients with cardiac valvular lesions, septal defects, arrhythmias, or pulmonary hypertension. However, the EDM may lose its waveform with only a slight positional change and produce dampened, inaccurate readings. These systems use topical electrodes on the anterior chest and neck to measure electrical impedance of the thorax. This value varies with beat-to-beat changes in thoracic blood volume and hence can estimate cardiac output. The system is harmless and provides values quickly (within 2 to 5 minutes); however, the technique is very sensitive to alteration of the electrode contact with the patient. Thoracic bioimpedance is more valuable in recognizing changes in cardiac output in a given patient than in precisely measuring its absolute value. 1.Murthi SB, Hess JR, Hess A, et al: Focused rapid echocardiographic evaluation versus vascular catheter-based assessment of cardiac output and function in critically ill trauma patients.J Trauma Acute Care Surg72 (5):1158–1164, 2012. doi: 10.1097/TA.0b013e31824d1112 