Subendocardial coronary compression in beating dog hearts is independent of pressure in the ventricular lumen

Ventricular decompression has been shown to have little effect on either the magnitude or transmural distribution of the extravascular resistance in the small hearts of the cat or rabbit. This study tests whether that independence from ventricular pressure also occurs in the large heart of the dog, which should be more representative of the human. We measured regional myocardial flow in each in situ dog heart during three conditions: normally beating, vented beating, and arrest. Hearts were perfused at constant pressure and maximally dilated with 0.4-1.0 mg/min ic adenosine. Total coronary blood flow was measured with an electromagnetic flowmeter, and regional flow was assessed with radiolabeled microspheres. Although arrest resulted in more than a doubling of flow to the inner layer, greatly reducing ventricular pressure by venting had no significant effect on subendocardial flow. Subepicardial flow was minimally affected by either venting or arrest. We conclude that both the magnitude and the transmural distribution of extravascular resistance in the large heart of the dog is unrelated to the pressure in the ventricular lumen.