Regional blood flow in canine myocardium as determined by local washout of a freely diffusible radioactive indicator

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Abstract
The aim of this study has been to examine the utility of the washout of a freely diffusible radioactive indicator as a measure of regional myocardial blood flow in open‐chest anesthetized dogs. The method employed was direct intramyocardial injection of Xenon‐133 followed by measurement of its gamma‐radiation. The experimental washout curves show, after a short acceleration period, monoexponential washout of the indicator over two decades. We found indications of insignificant veno‐arterial shunting by diffusion of the blood flow level measured, insignificant arterial recirculation of the indicator, and minimal radioactive contribution to precordial residue versus time curves arising from right heart or non‐myocardial tissue. We suggest that diffusion equilibrium between the tissue in the counting field and the blood leaving it is maintained during the linear down slope of two decades of the clearance curve, and that local blood flow can be calculated from washout rate constant obtained from this part of the curve. The method employing intramyocardial injection of Xenon‐133 was found to give the same results as atraumatic epicardial labelling indicating negligible effect of the injection trauma and supporting the validity of the local injection method using small volumes (2–10 μl).