Microvascular Pressure Profiles in the Left and Right Coronary Circulations

Abstract

Most of our knowledge concerning regulation of coronary vascular resistance has been based on measurements obtained from epicardial coronary arteries, and/or measurements of myocardial perfusion. A primary limitation of measurements obtained in this manner is that they cannot distinguish control mechanisms at different levels in the coronary microcirculation. Recently, technological advancements have enabled direct measurements of coronary pressures throughout the microcirculation in the beating heart. These advances have enabled documentation of the microvascular pressure profile in both the left and right coronary circulations during a variety of physiological and pharmacological interventions. The pressure profile can provide an index of vascular resistance, because the steepness of the pressure gradient reflects directly the vascular resistance of a particular vascular segment. The pressure profiles of the coronary micro-circulation exhibit several important characteristics. First, coronary arteries greater than 140–180 μm in diameter constitute approximately 20% of total vascular resistance. Second, vascular resistance in these relatively large coronary arterial vessels is due to active vasomotor tone, because it can be lessened substantially with the pharmacological vasodilator, papaverine. Third, small coronary arterioles less than 150 μm in diameter constitute the majority of coronary resistance. Fourth, resistance in these small arteriolar vessels is due primarily to active vasomotor tone, because the vasodilator dipyridamole will decrease resistance in this segment 12-fold. Fifth, the distribution of microvascular resistance in the left and right coronary circulations is similar. This paper discusses the techniques that have been developed to enable measurements of microvascular pressures in the beating heart, and implications of these measurements towards understanding regulatory mechanisms involved in the control of coronary vascular resistance.