Gradients in the Regional Rates of Myocardial Rubidium-86 Clearance in Tranquilized Dogs

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Abstract
In order to determine what gradients in local rates of myocardial blood flow are present in the resting hemodynamic state, the regional rates of myocardial clearance of 86 Rb were measured in dogs tranquilized with chlorpromazine. The clearance rate in the left ventricle corresponded to an average coronary flow of 11.7 ml/g per 10 min. Estimated right ventricular flow was 7.1 ml/g per 10 min and left atrial flow 7.9 ml/g per 10 min. High rates of 86 Rb uptake were noted in the atrial appendages and papillary muscles. Uptake in serially sectioned cores from both ventricles was consistently highest near the endocardial surface and decreased toward the epicardium. Selective injections of 86 Rb into the aorta or vena cava demonstrated that isotope penetrated the myocardium from the cavities of both ventricles. In the right ventricle, high subendocardial 86 Rb clearance was not found unless the isotope had been present within its cavity. In the left ventricle, in contrast, more 86 Rb was taken up by the subendocardial zones, even when all of the tracer was distributed by coronary arterial blood. The results support both diffusion of 86 Rb and perfusion of the myocardium by intracavitary blood as mechanisms responsible for the uptake of 86 Rb from the heart cavities.