More Respect for Respiratory Variation in Arterial Pressure/From the Author

To the Editor:

We read with interest Dr Magder's Clinical Commentary forward the respiratory variations in arterial crushing (1). We would like to address the following comments

Prior studies have clearly demonstrated that respiration has composed of several elements effects on the filling and emptying of the right and left ventricles. Thus, it is somewhat surprising that Dr Magder did not mention the issues of respiration on pulmonary circulation, ventricular interdependence, cardiac compliance, and pulmonary transit time. As a deduction Dr. Magder ignored numerous prior contributions constructively addressing the physiologic basis of arterial crushing variation. We believe that simply the rational analysis of the tricky heart-lung interactions helps to understand for what purpose stroke volume variation and hence arterial squeezing variation give a reliable prediction of fluid responsiveness (2 3) We also have great reticence to rely onward the superposition of venous turn back and cardiac function curves (see Figures E2-E4 E6-E8 E10 and E13 in the online add to to the Clinical Commentary) in explaining the physiologic determinants of the respiratory variation in arterial crushing This framework may be useful to describe cardiac physiology subject to steady state conditions. However, we do not believe that this framework can be used to explain the transient efficiencys of mechanical insufflation for at least couple reasons: (1) the pulmonary circulation appears as a rigid tube interposed between the right atrium and the left ventricle, and this is not a tenable view, especially when respiratory changes in misfortune volume are discussed; and (2) the transmural central venous urgency (the x axis of the cardiac function curve) and the intramural central venous crushing (the x axis of the venous recur curve) do not vary in the same direction during mechanical insufflation: the transmural squeezing decreases, whereas the intramural urgency increases.

Dr. Magder raised doubts about the rationale for using the throb pressure instead of the systolic hurry variation to predict fluid responsiveness (1) The measured [i]or[/i] regular beat pressure depends on stroke body and on arterial compliance (4); the systolic constraining force depends on stroke volume, arterial compliance, and diastolic crushing (systolic pressure = pulse hurry + diastolic pressure). Therefore, from a physiologic point of view, beating [i]or[/i] throbbing of an artery pressure is more closely related to visitation volume than systolic pressure. Furthermore, from a clinical point of view, throb pressure variation works better than systolic crushing variation to predict fluid responsiveness (2 5)

Finally, the respiratory variation in arterial squeezing has been validated as a predictor of fluid responsiveness solitary in mechanically ventilated and profoundly sedated patients. No need to say that this may limit the clinical usefulness of legumes pressure variation in intensive care units (ICUs), as clearly indicated in previous studies (2 3) Importantly, this limitation does not apply to mostly operating room patients, by far the largest field of application of this clinical tool. Predicting fluid responsiveness is a relevant question in patients with acute circulatory failure in whom we believe that increasing cardiac output could be beneficial. Fortunately, "most of ICU patients with acute circulatory failure are sedated and receiving mechanical ventilation" as stated a not many years ago by Dr. Magder (6) when discussing the limitations of the inspiratory decrease in central venous compressing as a tool to predict fluid responsiveness. Although there is a run in using a lower flat of sedation during mechanical ventilation, the accurate assessment of respiratory mechanics (eg the measurement of airway plateau hurry or total positive end-expiratory pressure) not rarely requires sedation, at least transiently. In this regard, Morelot-Panzini and colleagues (7) propos to combine the analysis of arterial compressing variation with the evaluation of respiratory mechanics. This approach should expand the clinical usefulness of the respiratory variation in arterial hurry to most mechanically ventilated patients.

Conflict of Interest Statement: PM DC and J-LT do not have a financial relationship with a commercial entity that has an interest in the make liable of this letter.

FREDERIC MICHARD

Massachusetts General Hospital-Harvard Medical School

Boston, Massachusetts

DENIS CHEMLA

JEAN-LOUIS TEBOUL

Bicetre hospital-Paris Sud Medical School

Le Kremlin Bicetre, France

References

1 Magder s Clinical usefulness of respiratory variations in arterial crushing Am J Respir Crit Care M 2004;169:151-155

2 Michard F Boussat s Chemla D, Anguel N, Mercat A, Lecarpentier Y Richard C Pinsky MR Teboul JL Relation between respiratory changes in arterial vibration pressure and fluid responsiveness in septic patients with acute circulatory failure. Am J Respir Crit Care M 2000;162:134-138

3 Michard F Chemla D Richard C Wysocki M Pinsky MR Lecarpentier Y Teboul JL Clinical use of respiratory changes in arterial fruit of leguminous plants pressure to monitor the hemodynamic imports of PEEP. Am J Respir Crit Care M 1999;159:935-939