Mechanical ventilation induces cyclic changes in left ventricular stroke volume. These variations are mainly related to the expiratory decrease in left ventricular preload following the inspiratory decrease in right ventricular filling and ejection. Therefore, the magnitude of the respiratory changes in left ventricular stroke volume reflect the sensitivity of the heart to the cyclic changes in preload induced by mechanical insufflation. At the bedside, the respiratory changes in left ventricular stroke volume can be assessed by the analysis of the arterial pressure (arterial catheter) or aortic blood velocity (echocardiography) wave forms. The respiratory changes in arterial pressure and in aortic blood velocity have been shown to be accurate predictors of fluid responsiveness and of the hemodynamic effects of positive end-expiratory pressure.