Clinical value of pulse pressure variations in ARDS. Still an unresolved issue?

Excerpt

Volume responsiveness, which refers to the ability of the heart to increase its stroke volume in response to volume load, denotes the presence of a preload reserve. In patients with circulatory shock, it is important to assess volume responsiveness in the decision-making process, which has benefited from the recent development of bedside heart-lung interaction indices. By inducing cyclic changes in intrathoracic pressure and in transpulmonary pressure, i.e., the distending pressure of the lung, mechanical ventilation results in cyclic changes in the preload and afterload of both ventricles. The resulting cyclic changes in stroke volume are assumed to be substantial in the case of cardiac preload reserve and hence of volume responsiveness. Accordingly, pulse pressure variation over the respiratory cycle (ΔPP) reflecting respiratory changes in stroke volume has been shown to be more reliable than traditional markers of preload when assessing volume responsiveness in mechanically ventilated patients with acute lung injury or ARDS [1, 2, 3], or after cardiac surgery [4]. However, like other dynamic indices using heart-lung interaction, ΔPP cannot be used in patients with spontaneous breathing activity or with arrhythmias. In this issue, De Backer et al. tested the hypothesis that low tidal volume ventilation limits the ability of ΔPP to predict volume responsiveness [5]. They showed that ΔPP was a reliable predictor of volume responsiveness in mechanically ventilated patients only when tidal volume was at least 8 ml/kg [5]. They hypothesized that fluctuations in intrathoracic pressure and transpulmonary pressure related to tidal ventilation should be limited when tidal volume is low, such that ΔPP is low even in the case of marked cardiac preload dependency. However, low tidal volumes are generally not applied to subjects with normal lungs, but rather to patients with acute respiratory distress syndrome (ARDS) who exhibit a marked decrease in lung compliance, sometimes associated with reduced chest wall compliance. Consequently, respiratory changes in transpulmonary pressure should still be greater than normal and cyclic changes in intrathoracic pressure still high enough for ΔPP to keep its ability to predict volume responsiveness. From the data of De Backer et al., it was possible to evaluate tidal ventilation changes in transpulmonary pressure and in intrathoracic pressure, although intrathoracic pressure was not measured, but only estimated [5]. In contrast with what was expected from their pathophysiological hypothesis, there were only slight differences between the low and high tidal volume groups in terms of tidal breath-related changes in intrathoracic pressure (3.9 versus 5.9 mmHg) and in transpulmonary pressure (8.6 versus 9.6 mmHg) [5]. …