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Myocardial tissue pressure increases from epicardium to endocardium, and in the deeper layers exceeds ventricular blood pressure during one-third of the cardiac cycle (21). The effect of this tissue pressure gradient on local blood flow was studied using the depot clearance technique. Blood flow was found to be at least 25% lower in the deep regions as compared with superficial ones. With total coronary inflow held constant, vagal arrest of the heart removed the tissue pressure gradient, and simultaneously redistributed flow from superficial to deeper layers. We conclude that the gradient in tissue pressure, and hence in the extravascular component of coronary resistance, is at least in part, the cause of the nonhomogeneous blood flow across the wall. By use of the oxygen cathode, a gradient of oxygen tensions was observed which paralleled the blood flow gradient; mean oxygen tension in the subepicardium averaged twice that in the subendocardium. The gradient in oxygen tension appears to be of sufficient magnitude to determine a transmural gradient in aerobic metabolism.
Kirk et al. (Tue,) studied this question.