We read with interest the letter from Shelley et al. commenting on our editorial.1 

Interestingly, although Shelley et al. disagree with our “editorial’s implication that there is something inherently limiting about the photophlethysmography signal that would prevent its use during high-risk surgery,” we actually strongly feel that this letter echoes what we pointed out in our editorial. Our point—and Dr. Shelley’s point—was that the photophlethysmography signal is complex and requires sophisticated processing to obtain a meaningful information. That does not imply that the photophlethysmography signal cannot be used during high-risk surgery. It only means that a simple analysis is limited. As stated in our editorial, we are convinced that more sophisticated signal analysis will help to better define the use of the photophlethysmography signal during high-risk surgeries, and we feel that Dr. Shelley’s technique of using both frequency domain and time domain approaches is promising.2  We also believe that any study assessing the ability of the photophlethysmography signal to predict fluid responsiveness during surgery should actually test the predictive value of the photophlethysmography signal (by performing volume expansion and testing the ability of the photophlethysmography signal to predict responders and nonresponders) and not just compare the photophlethysmography signal to the arterial pressure waveform.3 

Ironically, in the interval between our editorial was published and the present exchange of letters to the editor, another article assessing the ability of the photophlethysmography signal to predict fluid responsiveness during major surgery was released in the British Journal of Anaesthesia.4  In this study, Vos et al. showed that the photophlethysmography signal is as accurate as the arterial pressure waveform for the prediction of fluid responsiveness in this setting. It is important to note that this study used a rigorous fluid responsiveness prediction methodology and a complex digital signal processing.

There is nothing inherently limiting about the photophlethysmography signal that would prevent its use during major surgery. We just need to identify the correct way to analyze this complex signal to extract the relevant information in the appropriate setting.

References

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Cannesson
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Noninvasive hemodynamic monitoring: No high heels on the farm; no clogs to the opera.
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Shelley
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Using time-frequency analysis of the photoplethysmographic waveform to detect the withdrawal of 900mL of blood.
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Comparison of arterial pressure and plethysmographic waveform-based dynamic preload variables in assessing fluid responsiveness and dynamic arterial tone in patients undergoing major hepatic resection.
Br J Anaesth
2013
[Epub ahead of print]