The role of autoregulation and tissue diastolic pressures in the transmural distribution of left ventricular blood flow in anesthetized dogs

Abstract

To investigate how autoregulation affects transmural myocardial blood flow, we studied 21 open-chest dogs and varied coronary arterial pressure independently of left ventricular pressure. Total left ventricular myocardial blood flow (F(LV)) and subendocardial:subepicardial blood flow ratio (I:O) were measured with microspheres, 9 μm in diameter, injected during autoregulation or maximal coronary vasodilation. With autoregulation, changing coronary perfusing pressures over a wide range with constant left ventricular work at steady state resulted in constant coronary flow; flows decreased below and increased above this range. When coronary flow was raised or lowered from the autoregulated flow, coronary pressures rose or fell, respectively, to reach values on the pressure-flow curve determined when coronary pressures were varied deliberately. When coronary pressure was changed in autoregulating hearts, F(LV) and I:O ratios were independent of the ratio of diastolic coronary pressure time index (DPTL(c)) to left ventricular systolic pressure time index (SPTI) over the range, 0.45-1.0, but I:O fell with pressure ratios below 0.45 and increased with pressure ratios above one. During vasodilation, F(LV) and I:O ratios varied linearly with DPTI(c):SPTI. Thus, coronary autoregulation is necessary to maintain F(LV) and I:O distribution over a wide range of coronary:ventricular pressure relationships. With maximal vasodilation, flow at low coronary pressures increased from subendocardial to subepicardial layers, thereby suggesting a gradient of diastolic tissue pressures.