Abstract
Coronary blood flow is closely regulated to meet the changing metabolic demands of the working myocardium. Resistance of the coronary vasculature is determined by metabolic, myogenic, endothelial, and neural mechanisms. The influence of these control mechanisms varies throughout the coronary circulation, as they have dominant sites of action in vessels of different caliber. Coronary vascular resistance depends upon the coordinated response to these influences. Within a segment of the coronary circulation, resistance may be determined, for example, by competitive interaction between neural vasoconstriction and metabolic vasodilation. Such a system in which control occurs through multiple mechanisms with varying effects allows for precise control of coronary blood flow. This system also provides protection against dysfunction of a single control mechanism. If one fails, other control mechanisms can compensate for that loss of function. Thus, adequate delivery of oxygen and nutrients can be maintained despite potential dysfunction and large fluctuations in metabolic demands of the myocardium. In disease states, these regulatory mechanisms may also fail, and endothelial dysfunction is commonly seen in the setting of cardiac disease. Optimal cardioprotective therapies must target the coronary microcirculation and cardiac myocytes in tandem. Similarly, reversal of cardiac dysfunction requires concomitant amelioration of coronary microvascular dysfunction.
Highlights
Coronary vascular resistance is determined by a variety of mechanisms including coronary vascular anatomy, extravascular cardiac compressive forces, and vascular smooth muscle contraction
The roles of coronary vascular anatomy and cardiac compressive forces in determining coronary blood flow have been the focus of numerous studies and various models describe the contribution of these factors to distribution of blood flow throughout the myocardium [1]; coronary vascular resistance is primarily determined by arteriolar tone
The close match between coronary blood flow and metabolic demand of the myocardium suggests that coronary blood flow is constantly changing and that such precision is due to integrated input from a variety of control mechanisms
Summary
Coronary vascular resistance is determined by a variety of mechanisms including coronary vascular anatomy, extravascular cardiac compressive forces, and vascular smooth muscle contraction. Using stroboscopic illumination synchronized to cardiac motion, Nellis et al [7] were able to visualize epicardial microvessels in the beating right ventricle and make micropuncture measurements of intravascular pressure throughout the coronary tree These investigators observed that in the right ventricle, 70% of coronary vascular resistance was attributed to arteriolar and venular vessels smaller than 140 μm in diameter. Chilian et al [6] adapted this technique to the left ventricle and found that more than 50% of total coronary resistance resides in arterioles less than 150 μm in diameter These techniques have demonstrated that within the microcirculation, responses to vasoactive stimuli are heterogeneous. This schematic emphasizes the fact that the majority of coronary vascular resistance resides in small arteries and arterioles. Neurohumoral and flow-dependent control mechanisms predominate in small arteries, whereas metabolic and myogenic controls are of primary importance in arterioles
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