We aimed to evaluate the direct effects of intraaortic balloon pumping (IABP) on the phasic coronary flow velocity waveform into myocardium and coronary arteriolar diameter change under absence and presence of severe coronary artery stenosis. In anesthetized open chest dogs, flow velocity of septal artery was measured using 20-MHz Doppler flow velocimeter and diameters of epicardial coronary arterioles were observed using intravital videomicroscope during 2:1 model of IABP. The flow and diameters were compared between the coupled beats of IABP-on and IABP-off. Following control experiments without stenosis, a severe stenosis in the left main coronary artery was introduced by a snare occluder so as to reduce the post-stenotic pressure to about 50 mmHg. Without stenosis, IABP significantly enhanced the systolic retrograde flow, however, which was overcome by much more increased diastolic inflow. Moreover, IABP did enhance early diatolic inflow which is assumed to be beneficial for subendocardial perfusion. The diameters of coronary arterioles increased substantially during diastole by IABP. Under the presence of severe coronary stenosis, IABP failed to affect any flow variables or coronary arteriolar diameter. Thus, these results suggested that IABP has beneficial effects on both the systolic unloading and the diastolic coronary flow augmentation in the heart without stenosis, while the major effect of IABP on the heart with severe stenosis is derived from reduction of oxygen demand by the systolic unloading.

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