Abstract: The intra-aortic balloon pump (IABP) is the most widely used cardiac assist device, whose main benefits are augmentation of coronary flow and reduction of left-ventricular afterload. The aim of this study is to investigate the pressure and flow-volume distribution associated with balloon inflation. We hypothesize that in order to displace fluid on both sides of the balloon, a pressure locus must be present along the balloon during inflation. In vitro experiments were performed in two positions, horizontal and angled, using four balloon sizes: 25, 34, 40, and 50 cc. Along and on both sides of each balloon, we measured pressure, flow rate, and calculated flow velocity, volume displacement, wave intensity and energy. A pressure locus was found at the center of each balloon and the average flow volume displaced toward the tip at the horizontal position was about 57% of the balloon volume. In the angled position, the location of the pressure locus was less obvious and average volume displacement toward the top end of the balloon was reduced to 45%. These results confirm the existence of a pressure locus at the center of each of the balloons we tested. Because a clear reduction in flow volume was observed at the angled position, these results may have clinical implications as most patients using IABP in the intensive care units are nursed in semirecumbent position.