Dilatory capacity of the coronary circulation and its correlation to the arterial vasculature in the canine left ventricle.

The functional capacity of flow limiting myocardial conductance vessels was evaluated in canine hearts. In an isolated heart preparation transmural coronary flow distribution during maximal vasodilation was measured in the unloaded diastolic arrested left ventricle with tracer microspheres. The ratio of subendocardial versus subepicardial (ENDO/EPI) flow in the left ventricular free wall was 1.6. Measurements in 8 different wall layers showed a successive increase in maximal coronary flow from the subepicardium towards the deeper layers. A decreased subendocardial vascular resistance due to a better vascularization is forwarded as a mechanism to compensate for the extravascular compression during cardiac contraction. This statement contradicts the commonly accepted hypothesis that a diminished vascular tone with a reduction of the dilatory reserve in the subendocardium accounts for a homogeneous flow distribution in the normal beating heart. An augmentation of subendocardial supplying vessel capacity could be established from the angiographic determination of the coronary arterial volume of intramural small arteries and arterioles. From a strict parallelity in maximal coronary flow and coronary arterial volume within the wall, it becomes probable that these vascular structures are the flow-limiting factors which determine regional coronary flow reserve in the absence of extravascular compressive forces.