First of all, we would like to thank Shelley et al. for the positive critique and discussion of our recent study in Anesthesiology.1 The letter of Shelley et al. pointed out the role of the venous blood pool variation in the generation of the photophlethysmography signal. Indeed, by study design, this component of the photophlethysmography signal was neglected in our article. Our purpose was to evaluate the correlation between ventilation-induced variations of signals acquired by arterial pressure transducer and by pulse oximetry by using commercially available monitors. Operating this way, we used the same devices as previous teams who compared time-point measurements.2,3 We found a weak correlation between both signals acquired all along the anesthetic procedure. The numerous explanations noted in our article and the letter of Shelley et al. for this discrepancy have a two-fold source. The signal processing on one hand and the complex physiologic components of the photophlethysmography signal on the other hand. The latter involve stroke volume, sympathetic activity, and ventilatory-induced arterial and venous pressure variations. Extracting the last component could be an elegant manner to gain information on blood volume variation before the cardiac output being affected. However, this extraction requires sophisticated signal processing involving frequency domain analysis, and the way to a clear indicator is not that simple. Several steps toward a reliable monitor remain to be carried out.4 But whatever the future signal, it will have to prove efficiency in low- and high-risk surgery patients. Our feeling is that no one would be confident in a monitor providing reliable indications in low-risk patients, but failing if this patient becomes at risk for whatever intraoperative event, or of no use in high-risk patients.
We believe that the future of the photophlethysmography signal use is the development of a more advanced signal processing. Dr. Shelley’s letter adds further interest in the photophlethysmography signal suggesting extraction of hidden information included in the venous modulation.