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Can clinical monitors be used as scientific instruments?

Jeffrey M. Feldman, MD, MSE For many decades, the Grass polygraph was the workhorse of physiologic investigation. The instrument gave the investigator precise control over the manner in which transduced signals were filtered and amplified. In exchange for ink-stained fingers, the investigator could document the calibration, gain, and filtering used during signal processing. This assured that experimental results obtained would be repeatable. The Grass polygraph is another casualty of the digital age. Data collection no longer requires cumbersome laboratory equipment. Even the clinical monitors of today are equipped with interface ports through which they willingly share processed waveforms and collected data. Investigators need only connect a computer to one of these monitors to acquire signals and data without “getting their hands dirty.” In this issue of Anesthesia & Analgesia, Natalini et al. (1) investigate the utility of the photoplethysmogram (PPG) derived from a pulse oximeter to assess fluid responsiveness in patients during mechanical ventilation. The PPG waveform was acquired by connecting a standard clinical pulse oximeter to a computer. The arterial pressure waveform was collected simultaneously with the PPG from 57 mechanically ventilated patients in both the operating room and intensive care units. During mechanical ventilation, indices of respiratory-induced variations derived from the arterial pressure waveform have been related to the potential for a patient to increase arterial blood pressure (cardiac output) in response to fluid administration. Since the PPG appears analogous to the arterial waveform, and is easily obtained noninvasively, it is intriguing to investigate whether it can be used as a reliable surrogate for the arterial waveform to assess fluid responsiveness. In their study, Natalini et al. derived indices from the PPG that are analogous to those typically derived from the arterial waveform to quantify respiratory-induced variation. Of the indices studied, the percentage change in pulse pressure and systolic pressure during mechanical ventilation were found to be similar from both sources. Using data from previous studies, the authors defined a critical value of pulse pressure variation more than 13% to indicate the potential to increase cardiac output in response to fluid administration. On the basis of that definition, the authors applied ROC analysis to determine that a change in the pulse pressure from the PPG more than 9% correctly identified hypotensive patients likely to respond to fluid administration. Can we now use this information to make clinical decisions about fluid administration based upon interpreting changes in the PPG? Certainly it is not possible with the current generation of pulse oximeters to calibrate changes in the PPG directly to arterial pressure values. Yet, Natalini et al. attempt to quantitatively relate changes in the PPG to quantitative changes in the arterial pressure waveform. Several strategies were used to facilitate the analysis, given the quantitative limitations of the PPG. The auto gain function on the pulse oximeter was disabled, providing constant amplification of the PPG signal. Percentage changes in the arterial waveform and PPG were compared, rather than absolute changes, and so the results should be independent of scale. The authors assert that many factors influence the PPG, but that if all of those factors remain constant during the From the Children’s Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania. Address correspondence and reprint requests to Jeffrey M. Feldman, MD, MSE, Department of Anesthesiology and Critical Care Medicine, Children’s Hospital of Philadelphia, 34th and Civic Center Blvd., Philadelphia, PA 19129. Address e-mail to feldmanj@email.chop.edu. Copyright © 2006 International Anesthesia Research Society DOI: 10.1213/01.ane.0000247882.20257.b6