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Intensive Care Unit; Department Of Anesthesia, Intensive Care and Emergency; Poliambulanza Foundation Hospital; Brescia, Italy; natalini-giuseppe{at}poliambulanza.it
In Response:
I appreciate the opportunity to respond to some of the questions raised by Cannesson et al. about our paper (1). First, analysis of ventilation-induced photoplethysmographic (and arterial) variation can be clinically relevant if it works in any type of mechanically ventilated patient without arrhythmias. The prediction of the impact of volume expansion on cardiac output is useful in patients with and without vasopressors, during anesthesia and during shock (regardless of its cause). For this reason, we analyzed the relationship between ventilation-induced arterial and photoplethysmographic variations avoiding any patient selection. We believe that this study design results in greater external validity than if they were obtained in selected patients. Second, in our protocol the pulse oximeter sensor was placed on fingers in each patient. As judged by visual inspection of the waveform, toes were considered only if the signal from fingers was not "optimal." We considered a waveform as a "high quality" if the visual aspect of the wave was well defined, with marked oscillation between a systolic and a diastolic phase. In his previous paper Dr. Cannesson evaluated the quality of the plethysmographic signal using an index displayed on the pulse oximeter (2). However references regarding this parameter are lacking and, to the best of our knowledge, there are no studies showing any advantages of this method. Moreover this index is not implemented in all devices, precluding its use by the majority of clinicians. Third, I disagree that in our study "it is highly probable that respiratory variations were significantly different between these two sites" (toe and finger): we did not analyze the amplitude of the signal, but rather the percent changes between inspiratory and expiratory phases. There are neither theoretical reasons nor clinical studies to support a difference of plethysmographic pulse variation calculated by finger or toe. In particular, Shelley et al. (3) did not evaluate plethysmographic dynamic indices but the frequency spectrum. Moreover, arterial dynamic indices are calculated similarly by radial and by femoral artery (4,5), despite the fact that the arterial waveform is different in the two vessels. Finally, it is true that each monitor and each pulse oximeter has its own technology, but this should not be a major problem in ventilation-induced photoplethysmographic variation. In fact three different clinical studies confirm the relationship between arterial and photoplethysmographic variation induced by ventilation in different populations of patients (2,6,7); these studies were conducted by different authors, in different countries, with different pulse oximeters.
Finally, if evidence from these analyses does lead to clinical decisions, there is evidence that there is agreement and relationship between arterial and plethysmographic pulse variations (2,6,7).
Footnotes
Dr. Natalini declined to respond to Tavernier et al.
REFERENCES
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