By inducing cyclic changes in pleural and transpulmonary pressure, mechanical ventilation results in cyclic changes in the preload and afterload, and therefore, the cyclic variation in systolic and pulse pressure. Recently, a systemic review concluded that dynamic preload indicator [pulse pressure variation (PPV), stroke volume variation (SVV)] are highly accurate in predicting volume responsiveness in critically ill patients. However, this technique is limited to patients who receive controlled ventilation with adequate tidal volume (> 8 ml/Kg) and sedation or paralysis is needed to abolish the spontaneous ventilation.