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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{at}email.chop.edu.
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 measurement period then only ventilation should cause PPG amplitude variation.
Without complete knowledge of the manner in which the PPG signal is processed by the oximeter, the impact of the study by Natalini et al. is limited (as observed by the authors as well). Natalini et al. have clearly demonstrated that changes in blood volume and vascular tone can influence the PPG of the monitor in their study. However, we cannot know how reproducible this result will be with the same monitor used in another setting, nor can we know how these results will compare to PPG data from other oximeters.
Contemporary clinical monitoring devices are compact, reliable, sophisticated signal processing tools. They guide clinical decisions at the bedside, and are attractive research tools, given the ease with which data can be collected. However, whether the results can be applied to similar devices from other manufacturers depends on the details of the signal processing, and signal processing algorithms are typically neither standardized nor reported by the manufacturer. Lacking signal processing standardization, each monitor, and perhaps each revision of each monitor, will need to be evaluated on its own.
Investigators no longer need to use the messy inkwells of the Grass polygraph. However, until device manufacturers publish algorithmic details, and standards emerge, either by default or by consensus agreement, the "generalized knowledge" derived from using clinical monitors as scientific instruments will not be generalizable.
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This article has been cited by other articles:
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  A. Perel Automated Assessment of Fluid Responsiveness in Mechanically Ventilated Patients Anesth. Analg., April 1, 2008; 106(4): 1031 - 1033. [Full Text] [PDF]
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K. H. Shelley Photoplethysmography: Beyond the Calculation of Arterial Oxygen Saturation and Heart Rate Anesth. Analg., December 1, 2007; 105(6S_Suppl): S31 - S36. [Abstract] [Full Text] [PDF]
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M. Cannesson, O. Desebbe, and J.-J. Lehot Fluid Responsiveness Assessment Using the Pulse Oxymeter Waveform: Not Yet Ready for Prime Time Anesth. Analg., June 1, 2007; 104(6): 1598 - 1599. [Full Text] [PDF]
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B. Tavernier, M.-S. Destandau, and B. Vallet Variations in Photoplethysmographic Waveform During Mechanical Ventilation Anesth. Analg., June 1, 2007; 104(6): 1599 - 1599. [Full Text] [PDF]
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