Small vessel phenomena in the coronary microcirculation: Phasic intramyocardial perfusion and coronary microvascular dynamics

Abstract

To place the characteristics of the coronary microcirculation in perspective to another muscular organ system, we have compared various parameters from exchange vessels in the heart and red skeletal muscle (Table 1). The major differences between cardiac and skeletal muscle microcirculations relate to the larger density of capillaries in the heart. This increased density is responsible primarily for a greater capillary filtration coefficient-permeability-surface area product to various solutes, surface area, and decreased intercapillary distance. These features most likely represent an adaptation of the microcirculation of the heart to the very high, continual metabolic demands. Interestingly, capillary permeabilities and reflection coefficients of different solutes are in the same range (although the heart tends to have higher capillary permeabilities). Thus, the adaptation of the coronary circulation to facilitate exchange of nutrients and solutes is mediated via an increase in the numbers of exchange vessels, rather than modifications of the membrane characteristics of these exchange vessels. Within the last decade, there has been much information assimilated on the regulation of the coronary microcirculation. Most of the knowledge has been the result of many indirect approaches to studying the coronary microcirculation (indicator-dilution techniques, nuclidelabeled microspheres, plasma-lymph concentration of solutes). There are relatively few direct observations on regulation of the coronary microcirculation. This is primarily due to difficulties in techniques. Exploration of the phasic nature of intramyocardial perfusion is handicapped by the location of these intramuscular vessels. Visualization of the coronary microcirculation is hampered by movements of the heart, and such measurements are restricted to the superficial layers of the myocardium. It is worth emphasizing that direct observations of red cell velocities in epicardial capillaries, measurements of microvascular caliber, and the pressure profiles in the coronary microcirculation are restricted to the superficial, epicardial layer. It is not unreasonable to speculate that microvascular events and regulation occurring in the subepicardium may be quite different than that in the subendocardium. There are several salient points in this review that are worth emphasizing. First, measurements of phasic blood flow in an epicardial coronary artery do not accurately predict the phasic nature of intramyocardial perfusion. Observed differences between the phasic nature of epicardial blood flow and that in intramyocardial blood vessels is due to epicardial capacitance, which can be modulated by distending pressures and vasomotor tone. Second, extravascular compressive forces markedly influence the phasic nature of intramyocardial blood flow. Augmenting such forces augments systolic-negative blood flow. Third, spatial and temporal heterogeneity of myocardial perfusion occurs in the normal myocardium (with the vasomotor tone intact). Heterogeneity of perfusion is due to twinkling of precapillary sphincters that regulate flow to individual capillary fields. Fourth, the capillary bed of the myocardium is composed of endothelium, which has similar permeability characteristics to most other organ systems in the body (notable exceptions are the brain, with very low permeability, and the liver, with very high permeability). There is, however, a tremendous potential for the filtration of water and solutes from the capillary space to the vascular space due to the very large surface area of the capillary bed, which is much larger than most other organ systems. Fifth, in contrast to many other organ systems, the coronary circulation is characterized by a fairly substantial proportion of total coronary resistance residing in relatively large microvessels (40% to 45% of total coronary resistance is proximal to 100 μm arterioles). Furthermore, this resistance is due primarily to active vasomotor tone rather than structural features of the coronary vasculature system. Direct studies of the coronary microcirculation are yet in the embryonic stages. There are still substantial technical problems that have not been solved. For instance, there are still no measurements of capillary pressures in the heating heart or of microvascular dynamics in subendocardial vessels. Undoubtedly, there should be efforts directed toward development of techniques to enable such measurements. Such endeavors will require the cooperation of physiologists, engineers, computer scientists, and cardiologists to solve these seemingly insurmountable problems.