Does systolic subepicardial perfusion come from retrograde subendocardial flow?

To examine the influence of cardiac contraction on systolic coronary flow and transmural blood flow distribution, we measured phasic blood flow velocity in distal extramural coronary arteries by Doppler velocimeter and regional myocardial blood flow by radiolabeled microspheres while the heart was beating and during prolonged diastoles in 12 dogs. A servo-controlled coronary perfusion circuit allowed mean coronary pressure to be selected and maintained during beating and diastolic conditions. In epicardial arteries just proximal to their entrance into the myocardium, blood flow was either negligible or reverse in direction during systole. When the heart was beating, subepicardial blood flow was 24.2 +/- 12.3% higher than during asystole (5.05 +/- 0.91 and 4.11 +/- 0.79 ml.min-1.g-1 for beating and prolonged diastoles, respectively; P less than 0.01). In the subendocardium, flow was 49.8 +/- 14.7% lower in the beating condition than during prolonged diastoles (4.23 +/- 1.46 and 8.26 +/- 1.71 ml.min-1.g-1 for beating and asystole, respectively; P less than 0.01). When heart rate was increased stepwise from 60 to 150 beats/min, subendocardial flow fell approximately linearly; flow to the superficial layer was relatively unaffected. In beating hearts, lowering mean left main coronary artery (LMCA) pressure from 80 to 50 mmHg resulted in more systolic reverse flow and a fall in inner-to-outer flow ratio from 0.82 +/- 0.18 to 0.66 +/- 0.34 (P less than 0.05). Because mean LMCA pressure was held constant when the heart was stopped, differences in regional blood flow between beating and diastolic conditions were primarily due to cardiac contraction. Because little or no blood entered the myocardium from the extramural arteries during systole, we conclude that the decrease in subendocardial flow and the increase in subepicardial flow were caused by retrograde pumping of blood from the deep layer to the superficial layer of the left ventricle. Systolic retrograde flow to the subepicardium may help explain this layer's relative protection from ischemia.