Acute respiratory distress syndrome (ARDS) which is a major cause of morbidity and mortality in the intensive care unit, is characterized by decreased lung volume, decreased lung compliance, increased pulmonary vascular resistance and pulmonary hypertension, which ultimately lead to right ventricular dysfunction. Moreover, through biventricular interdependence and inter-ventricular septum shift, the left ventricular filling is limited, which eventually results in a decrease in stroke volume. Due to the high risk of ventilation-induced lung injury, protective lung ventilation is recommended, mainly through a reduction in tidal volume. However, due to the profound hypoxemia inherent to this mode of ventilation, lung recruitment strategies are recommended. When the application of a recruitive positive end-expiratory pressure fails to correct hypoxemia, prone positioning (PP) is recommended. A randomized controlled trial (PROSEVA) reported that early application of prolonged PP sessions in patients with severe ARDS significantly decreased 28- and 90-day mortality. In addition to its respiratory effects (improvement in arterial oxygenation and lung recruitment), PP exerts different cardiovascular effects. By increasing oxygenation and recruiting lung regions, PP can reduce the right ventricular afterload. By increasing the intraabdominal pressure (IAP), PP can increase the venous return and the cardiac preload. This effect might depend on the level of IAP, because a high IAP might collapse the inferior vena cava, especially in the case of hypovolemia. If cardiac preload increases, the resultant effect on cardiac output might depend on the degree of preload responsiveness. Finally, by increasing the IAP, PP can increase the left ventricular afterload. In a recent clinical study, our group showed that PP increases right and left ventricular preload and decreased right ventricular afterload. However, PP increased cardiac output only in patients with preload responsiveness. In this study preload responsiveness was detected by an increase in cardiac output>> 10% during a passive leg raising (PLR) test. Since PP sessions are recommended to last between 16 to 18 hours per day and since patients with ARDS often have an associated sepsis, hemodynamically instability may occur during PP sessions. The therapeutic decision is very tricky since the administration of fluid (the main therapeutic option in case of hemodynamic instability) is risky due to alteration of lung capillary membrane permeability. Thus, it is important to predict the benefits of fluid administration by using indices of preload responsiveness. The recommended indices of preload responsiveness are pulse pressure variation (PPV), stroke volume variation (SVV) and PLR. However, PPV and SVV cannot be used reliably during low tidal volume ventilation. Moreover, conventional PLR cannot be performed during PP. It is thus important to develop other preload responsiveness tests doable during PP. Our group proposed to perform an end-expiratory occlusion (EEO) test to predict fluid responsiveness in the supine position and more recently to perform a tidal volume challenge (TVC) by transiently increasing tidal volume from 6 to 8 mL/kg and observing the changes in PPV.