Coronary diastolic pressure-flow relation and zero flow pressure explained on the basis of intramyocardial compliance

In the controversy about the mechanisms determining the high zero flow pressures and the further interpretation of coronary diastolic pressure flow relations, this paper takes a stand in favor of intramyocardial compliance as the primary cause of the high zero flow pressures. An attempt has been made to estimate the compliance distribution within the coronary circulation and to show the specific effect of intramyocardial compliance on arterial and venous pressure-flow relations. Since no data are available on the distensibility of coronary arterioles and capillaries, these data were taken from studies on mesenteric vessels. Based on these data, it is shown that, depending on the transmural pressure, smooth muscle tone may either increase or decrease arteriolar compliance. A compliance distribution has been proposed based on assumed pressure, volume, and distensibility distributions. For all but the venous division of the circulation, experimental data on volume could be found in the literature. Based on this compartmental analysis, it is predicted that overall intramyocardial compliance may exceed epicardial arterial compliance by a factor 45. The literature presenting functional evidence for intramyocardial compliance effects has been reviewed. Experimental results on venous outflow during long diastoles have been analyzed. Pf = 0 coronary pressure at zero flow, is higher when measured later in diastole. It is shown that this may be explained by charging of intramyocardial compliance in the period before flow ceases. The discrepancy between results on pressure-flow relations in the fully dilated bed and autoregulated bed are related to the differences in pressure, resistance, and compliance distributions.