Coronary microvascular responses to reductions in perfusion pressure. Evidence for persistent arteriolar vasomotor tone during coronary hypoperfusion.

The goals of this study were to test the following hypotheses: 1) Coronary autoregulatory adjustments to decreases in perfusion pressure occur primarily in coronary arterioles (less than 150 microns in diameter). 2) Small coronary arteries (greater than 150 microns in diameter) can be recruited to participate in the autoregulatory adjustments as perfusion pressure is progressively lowered. 3) Small arterioles are the location of vasodilator reserve in the coronary microcirculation during hypoperfusion. Studies were performed in anesthetized open-chest dogs in which coronary perfusion pressures were reduced to 80, 60, 40, and 30 mm Hg. During reductions in coronary perfusion pressure, measurements were made of systemic hemodynamics, myocardial blood flow (radioactive microspheres), and coronary microvascular diameters. Arterial pressure and heart rate were largely unchanged during the experimental maneuvers. Measurements of microvascular diameters in the beating heart were performed during epi-illumination...