Pulsus paradoxus is an exaggeration of the normal inspiratory decline of systemic arterial pressure. Originally described in association with cardiac tamponade, it mechanistically represents complex cardiac and pulmonary interactions. There is growing evidence that parameters frequently used in intensive care units obtained via Swan-Ganz catheters are not reliable indicators of left ventricular end-diastolic volume, particularly in the mechanically ventilated patient. Our hypothesis is that pulsus paradoxus in the mechanically ventilated patient is a sensitive indicator of decreased circulating blood volume.