Mechanical ventilation induces cyclic changes in vena cava blood flow, pulmonary artery blood flow, and aortic blood flow. At the bedside, respiratory changes in aortic blood flow are reflected by "swings" in blood pressure whose magnitude is highly dependent on volume status. During the past few years, many studies have demonstrated that arterial pressure variation is neither an indicator of blood volume nor a marker of cardiac preload but a predictor of fluid responsiveness. That is, these studies have demonstrated the value of this physical sign in answering one of the most common clinical questions, Can we use fluid to improve hemodynamics?, while static indicators of cardiac preload (cardiac filling pressures but also cardiac dimensions) are frequently unable to correctly answer this crucial question. The reliable analysis of respiratory changes in arterial pressure is possible in most patients undergoing surgery and in critically ill patients who are sedated and mechanically ventilated with conventional tidal volumes.