To the Editor:
Recently, Natalini et al. analyzed variations in arterial blood pressure and photoplethysmography during mechanical ventilation (1). We previously reported similar results from patients both in the intensive care unit and in the operating room (2,3). These results suggest that the photoplethysmographic waveform may potentially be used to assess responsiveness to fluid administration in mechanically ventilated patients. However, as stated by Natalini et al., the photoplethysmogram is a highly processed signal and the results from this study should be interpreted with a clear understanding of its nature. In particular, we note that following: 1) The authors studied patients both in the intensive care unit and in the operating room. We assume that several patients in the intensive care unit were given vasopressors during the study. It is now well known that photoplethysmographic amplitude depends on vascular tone and that vasopressors may alter the plethysmographic signal quality. Even if vascular tone remains constant throughout a single respiratory cycle it would have been interesting to differentiate between patients with and without vasopressors. Some devices display a perfusion index that may be used to quantify peripheral vasoconstriction. 2) The authors describe “high quality” waveforms but do not define just what these are. 3) It has recently been demonstrated that the respiratory variations in the plethysmographic signal are strongly influenced by the site of measurement (ear, forehead, finger) (4). In this study, Natalini et al. recorded signals at the finger or at the toe and it is highly probable that respiratory variations were significantly different between these two sites. Consequently, this may have induced a significant bias in their interpretation of the data. 4) Finally, as stated by Dr. Feldman in an editorial related to this article, each monitor and each pulse oximeter has its own technology (5). What has been demonstrated with one still has to be demonstrated with the others. Pulse oximeter waveform analysis to predict fluid responsiveness during mechanical ventilation is very exciting. However, since its interpretation may result in clinical decisions, it is important to define standards and limits before clinical acceptance.
Maxime Cannesson, MD
Olivier Desebbe, MD
Jean-Jacques Lehot, MD, PhD
Department of Anesthesia and Intensive Care
Hospices Civils de Lyon
Louis Pradel Hospital
1. Natalini G, Rosano A, Franschetti ME, et al. Variations in arterial blood pressure and photoplethysmography during mechanical ventilation. Anesth Analg 2006;103:1182–88.
2. Cannesson M, Desebbe O, Hachemi M, et al. Respiratory variations in pulse oxymeter waveform amplitude are influenced by venous return in mechanically ventilated patients under general anaesthesia. Eur J Anaesthesiol 2006;1–7[Epub ahead of print].
3. Cannesson M, Besnard C, Durand PG, et al. Relation between respiratory variations in pulse oximetry plethysmographic waveform amplitude and arterial pulse pressure in ventilated patients. Crit Care 2005;9:R562–R8.
4. Shelley KH, Jablonka DH, Awad AA, et al. What is the best site for measuring the effect of ventilation on the pulse oximeter waveform? Anesth Analg 2006;103:372–7. Table of contents.
5. Feldman JM. Can clinical monitors be used as scientific instruments? Anesth Analg 2006;103:1071–2.