Coronary Pressure-Flow Relationships Controversial Issues and Probable Implications

In studies in which impedance flow through a vascular bed is calculated, driving pressure has traditionally been taken as the difference between inflow and outflow pressures, with the latter often assumed to be equal to right atrial pressure and, therefore, negligible in magnitude. In the coronary bed, ventricular diastolic pressure has sometimes been substituted for right atrial pressure, and has been taken as an index of intramyocardial forces presumed to influence the 'back pressure' opposing coronary inflow. These approximations have continued, despite publications dating back at least 50 years which suggest that inflow in several vascular beds ceases at pressures considerably in excess of right atrial or ventricular diastolic pressure. Values of this socalled 'zero-flow pressure' (Pf-o) as high as 45 mm Hg were reported in the isolated hindlimb of dogs in the 1930's (Whittaker and Winton, 1933; Gomez and Veil, 1936). Influences of vasomotor tone, viscosity, and vessel distention and recruitment were addressed in the 1940's, and nonlinear pressureflow relationships were identified experimentally (Pappeheimer and Maes, 1942; Green et al. 1944). A possible causative role of active tension in vascular smooth muscle was further emphasized in the early 1950's (Burton, 1951; Nichol et al., 1951). Permutt and colleagues' classical studies of pulmonary pressure-flow relationships in the early 1960's called attention to extra vascular compressive forces, as well as vasomotor tone, and proposed a "vascular waterfall' as another explanation for higher-than-expected minimum pressures for forward flow (Permutt et al., 1962; Permutt and Riley, 1963). Mosher et al. (1964) were among the first to describe pressure-flow relations in the coronary circulation. Steady state relationships obtained in five beating hearts by reducing perfusion pressure in step-wise fashion and measuring inflow 30 seconds to 2 minutes later indicated that inflow ceased at an average pressure of 23 mm Hg. Instantaneous relationships obtained by abruptly reducing pressure and measuring flow after only 1-2 seconds (presumably before autoregulatory changes could occur) showed higher values, suggesting in retrospect that Pf_o varied with inflow pressure and that changes in Pf_o were involved in autoregulatory adjustments of flow to changes in pressure. Later, a systolic waterfall was suggested by Downey and Kirk (1975).