The effect of coronary inflow pressure on coronary vascular resistance in the isolated dog heart.

The shape of the coronary arterial pressure-flow relationship results from the interaction of a number of poorly understood physiological factors. Experiments in which coronary inflow and outflow pressures were coupled so that driving pressure was held constant showed that changes in inflow or outflow pressures altered coronary blood flow: coronary vascular resistance varied inversely with changes inflow pressure below 50 mm Hg and with changes in outflow pressure below 80 mm Hg. The magnitude of the influence of inflow pressure on resistance also depended on the fixed level of outflow pressure, the influence being large when the outflow pressure was low, and small when it was high. Inflow and outflow pressures, then, are two physiological factors which are determinants of the shape of the pressure-flow relationship, and their interaction contributes to the degree of curvature found in a particular relationship. These findings suggest that the use of linear regression in the interpretation of pressure-flow relationships results in poor estimation of resistance and zero-flow pressure. Other experiments measuring regional coronary blood flow using radionuclide-labeled microspheres resulted in the same inverse relationship between inflow pressure and resistance, regardless of mural depth, indicating that inflow pressure may influence resistance by distending vessels, rather than by causing sequential cessation of perfusion in successive transmural layers.