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LETTER TO THE EDITOR

Variations in Photoplethysmographic Waveform During Mechanical Ventilation

Federation of Anesthesia and Intensive Care Medicine; University Hospital of Lille; Lille Cedex, France; btavernier{at}chru-lille.fr

To the Editor:

A recent article by Natalini et al. (1) suggested that ventilation-induced photoplethysmographic pulse variations (PPV_pleth) were similar to those measured in arterial pulse pressure (PPV_art). Agreement between both indices was in fact not very close (limits of agreement: –12% and 12%), but PPV_pleth correctly identified patients likely to respond to fluid administration (as estimated from PPV_art and Down_art measurements). In an accompanying editorial, Feldman suggested that reproducibility of their results may vary with oximeter models (2). Using a monitor from a different manufacturer, we recently found that PPV_pleth predicted stroke volume (SV) response to volume loading with a threshold value similar to that in Natalini’s report (9.5% and 9%, respectively) (3). Agreement between PPV_pleth and PPV_art was also consistent between studies (–14% and 15% in our patients, who were more often hypotensive). However, PPV_pleth predicted fluid responsiveness, and correlated with the fluid-induced change in SV, less accurately than PPV_art or noninvasive PPV obtained using a Finapres device (3). PPV_pleth is thus not equivalent to PPV_art and should still be considered with caution.

Natalini et al. also reported that photoplethysmographic systolic pulse variation (SPV_pleth), but not its Down_pleth component, correlated with its arterial corresponding variable. This suggests that, in contrast with its arterial counterpart (4), Down_pleth may not predict fluid responsiveness. In an unpublished study (5), we measured these parameters in 19 patients in conditions apparently similar to those of Natalini’s study, and found similar correlation between SPV_art and SPV_pleth (r = 0.78; P = 0.001), and between Down_art and Down_pleth (r = 0.77; P = 0.001). In Natalini’s study, almost all patients received PEEP and many had negative Up and Up_pleth. This suggests that inaccurate measurement of Down might have occurred. To be valid in the presence of PEEP, the "plateau" in systolic arterial pressure must be obtained at the level of PEEP. This necessitates an end-expiratory occlusion (6), a maneuver not possible with most respirators in the operating room. Reference systolic pressure measurement at airway pressure less than PEEP (typically when disconnecting the endotracheal tube) results in negative Up and falsely increased Down (depending essentially on the level of PEEP, lung compliance, and preload conditions). We thus believe that, similarly to arterial tracing analysis (4), correct quantification of the expiratory component of SPV_pleth or, probably better, of PPV_pleth (7), could be an informative complement to PPV_pleth and deserves further evaluation.

REFERENCES

1. Natalini G, Rosano A, Franceschetti ME, et al. Variations in arterial blood pressure and photoplethysmography during mechanical ventilation. Anesth Analg 2006; 103:1182–8.[Abstract/Free Full Text]
2. Feldman JM. Can clinical monitors be used as scientific instruments? Anesth Analg 2006;103:1071–2.[Free Full Text]
3. Solus-Biguenet H, Fleyfel M, Tavernier B, et al. Non-invasive prediction of fluid responsiveness during major hepatic surgery. Br J Anaesth 2006;97:808–16.[Abstract/Free Full Text]
4. Michard F. Changes in arterial pressure during mechanical ventilation. Anesthesiology 2005;103:419–28.[Web of Science][Medline]
5. Destandau MS, Szypura AS, Mallat J, et al. Intérêt des variations respiratoires de la courbe d’oxymétrie de pouls pour le monitorage du remplissage vasculaire (abstract). Ann Fr Anesth Réanim 2002; 21:R028.
6. Blanch L, Bernabé F, Lucangelo U. Measurement of air trapping, intrinsic positive end-expiratory pressure, and dynamic hyperinflation in mechanically ventilated patients. Respir Care 2005;50: 110–24.[Medline]
7. Morelot-Panzini C, Lefort Y, Derenne JP, Similowski T. Simplified method to measure respiratory-related changes in arterial pulse pressure in patients receiving mechanical ventilation. Chest 2003;124: 665–70.[Web of Science][Medline]

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