Preload responsiveness or right ventricular dysfunction?

摘要

Since publication of the study by Michard et al (1), pulse pressure variation (PPV) has been widely used in mechanically ventilated patients to predict their response to fluids. The magic number of 13% was proposed as a cutoff value to classify patients as responders and non-responders to fluids (1). However, after initial enthusiasm, which was understandable because a simple variable had been proposed to allow physicians to accurately monitor fluid needs, studies have reemphasized that this variable has limitations related to cardiac function, respiratory mechanics, and ventilator settings (2). In this issue of Critical Care Medicine, Mahjoub and colleagues (3) report that false positives of PPV (i.e., patients with a PPV >12% without an increase in left ventricular stroke volume >15% after volume expansion) are in part caused by right ventricular (RV) dysfunction. This result was physiologically expected and previously suggested (4). PPV reflects not the preload dependency of the heart but rather the preload dependency of the left ventricle. Such dependency can be intensified by hypovolemia. In this case, PPV is especially mediated by in-spiratory change in pleural pressure (5) and is associated with a large increase in left ventricular stroke volume after volume expansion. Such dependency can also be intensified by severe RV dysfunction. In this case, PPV is especially mediated by inspiratory change in transpul-monary pressure (5), and then volume expansion is unable to increase left ventricular stroke volume and to correct PPV. This situation is mainly encountered in patients with low pulmonary compliance, such as patients ventilated for acute respiratory distress syndrome (ARDS) in whom transpulmonary pressure is abnormally high and RV dysfunction not unusual (6).