Using heart-lung interactions to assess fluid responsiveness during mechanical ventilation

According to the Frank-Starting relationship, a patient is a 'responder' to volume expansion only if both ventricles are preload dependent. Mechanical ventilation induces cyclic changes in left ventricular (LV) stroke volume, which are mainly related to the expiratory decrease in LV preload due to t he inspiratory decrease in right ventricular (RV) filling and ejection. In the present review, we detail the mechanisms by which mechanical ventilatio n should result in greater cyclic changes in LV stroke volume when both ven tricles are 'preload dependent'. We also address recent clinical data demon strating that respiratory changes in arterial pulse (or systolic) pressure and in Doppler aortic velocity las surrogates of respiratory changes in LV stroke volume) can be used to detect biventricular preload dependence, and hence fluid responsiveness in critically ill patients.