Interpretation and Physiological Significance of Diastolic Coronary Artery Pressure-Flow Relationships in the Canine Coronary Bed'

Dole et al. (1984) have suggested that the coronary zero flow pressure results from critical closure of microvessels, which they model as pressure-dependent nonlinear resistors. This view differs from the vascular waterfall interpretation of coronary hemodynamics in which microvessels are modeled as collapsible tubes. It is important to determine which model is more descriptive, for an understanding of the back pressure and resistance of the coronary circulation. In the case of a pressure-dependent nonlinear resistor, what is the relationship between crosssectional area and pressure, i.e., its tube law? The solid line in Figure 1 is a plot of the inverse of the square root of the resistance function shown in Figure 10 of Dole et al. (1984) as a function of epicardial artery pressure. This is equivalent to a tube law, since the ordinate is proportional to crosssectional area. However, the tube law at the site of critical closure will differ, because it will be displaced to the left by resistive losses. It may resemble the dashed line; if so, the marked decrease in crosssectional area caused by a slight decrease in pressure is similar to what has been found for collapsible tubes. If a pressure-dependent nonlinear resistor can act