Effect of Cardiac Contraction on Regional Myocardial Blood Flow
The effect of myocardial contraction on the regional myocardial flow and its mechanism are still not clear, although these have been the most fundamental questions in the study of coronary blood flow. To evaluate the effect of myocardial contraction on myocardial flow in the different layers and the roles of intramyocardial forces and systolic ventricular pressures in the blood flow distribution, we measured myocardial flow in rabbit hearts during stable systolic contraction with different left ventricular pressures (60 mmHg; n = 5, and 0 mmHg; n = 5) and during stable diastolic arrest (n = 5). We also measured the number and size of the intramyocardial vessels after perfusion fixation (systolic arrest; n = 5, and diastolic arrest; n = 5). In 25 rabbits, hearts were excised and perfused from the aortic root. Systolic arrest was achieved by perfusion of a low Ca2+ Tyrode’s solution containing Ba2+ (2.0 mM). Diastolic arrest was achieved by intraventricular injection of pentobarbital (700–1000 mg) and was maintained by perfusion with St. Thomas’ cardioplegic solution. At perfusion pressure of 100 mmHg, subendocardial flow was lower than subepicardial flow in systolic hearts, regardless of left ventricular pressure, whereas in diastolic hearts, subendocardial flow was higher than subepicardial flow. In systolic hearts with low left ventricular pressure, subendocardial-to-subepicardial flow ratio for a physiologic range of perfusion pressures was lower than in systolic hearts with high left ventricular pressure.