Abstract
According to the Frank-Starling 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 the inspiratory decrease in right ventricular (RV) filling and ejection. In the present review, we detail the mechanisms by which mechanical ventilation should result in greater cyclic changes in LV stroke volume when both ventricles are 'preload dependent'. We also address recent clinical data demonstrating that respiratory changes in arterial pulse (or systolic) pressure and in Doppler aortic velocity (as surrogates of respiratory changes in LV stroke volume) can be used to detect biventricular preload dependence, and hence fluid responsiveness in critically ill patients.
Highlights
Volume expansion is a frequently used therapy in critically ill patients with acute circulatory failure
The expected haemodynamic benefit of volume expansion is an increase in left ventricular (LV) stroke volume, and in cardiac output
Intermittent positive-pressure ventilation induces cyclic changes in LV stroke volume, which are mainly related to the expiratory decrease in LV preload due to the inspiratory decrease in right ventricular (RV) filling and ejection (Fig. 3)
Summary
Volume expansion is a frequently used therapy in critically ill patients with acute circulatory failure. ∆down is the difference between the reference systolic pressure and the minimal value of systolic pressure over a single respiratory cycle It reflects the expiratory decrease in LV preload and stroke volume related to the inspiratory decrease in RV stroke volume (see above). The assessment of cardiac preload dependence is useful in predicting volume expansion efficacy, and in predicting the haemodynamic effects of any therapy that induces changes in cardiac preload conditions In this regard, ∆PP has been shown to be useful in monitoring the haemodynamic effects of PEEP in mechanically ventilated patients with acute lung injury. Relationship between the respiratory changes in pulse pressure before volume expansion (Baseline ∆PP) and the volume expansion-induced changes in cardiac index (y-axis) in 40 septic patients with acute circulatory failure. It is likely that analysis of the respiratory changes in LV stroke volume could be useful to monitor the haemodynamic effects of ultrafiltration during dialysis or of any change in ventilatory parameters
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