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Mechanism for Flow Distribution in Normal and Ischemic Myocardium during Increased Ventricular Preload in the Dog

The effects of increased ventricular preload on the distribution of regional blood flow in normal and ischemic myocardium were determined in deep and superficial regions of the left ventricle by using labeled microspheres. The ratio of flow in deep regions to flow in superficial regions in nonischemic tissue was 1.06 ± 0.06 and increased to 1.14 ± 0.06 when left ventricular end-diastolic pressure was raised from 4.8 ± 1 mm Hg to 18 ± 2 mm Hg by intravenous infusion of blood. In ischemic areas the ratio of flow in deep regions to flow in superficial regions fell simultaneously from 0.71 ± 0.12 to 0.45 ± 0.07 (P<0.001). Control of coronary flow during increased preload was studied in a separate preparation in which coronary flow was decreased in steps by reducing coronary perfusion pressure during periods of constant cardiac performance. The increased preload was associated with loss of autoregulation of coronary flow. It is suggested that, in myocardium with unrestricted coronary supply, regional distribution of flow is independent of gradients in tissue pressure during the described changes in preload due to an autoregulatory mechanism. However, when autoregulation of coronary flow is abolished by coronary artery occlusion, the coronary bed is fully dilated, and distribution of flow is directly dependent on gradients in myocardial tissue pressure. Increased ventricular preload consequently augments the underperfusion of the subendocardial regions of an ischemic area.