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Total and regional coronary blood flow were measured in dogs by left atrial injection of carbonized microspheres labeled with different radioactive isotopes (mean diameter 14 to 61µ). Simultaneously blood was collected at 20 ml/min from a catheter tied into a peripheral artery. The ratios of flow to radioactivity in myocardium and arterial blood should be equal if microspheres are well mixed in the aortic root and are distributed regionally in proportion to flow. This was proved in seven right heart by-pass experiments where coronary venous drainage was measured directly. Also, less than 0.1% of total myocardial radioactivity appeared in coronary venous blood, even with hypoxemia and small microspheres. Total coronary flow in seven conscious dogs averaged 95 to 150 ml/min/100 g heart; and flow to the left ventricle was 111 to 169 ml/100 g. Although not validated independently, there was evidence that values for flow to each ventricle, the atria and the septum were correct. The radioactivity per gram of left ventricular subendocardial muscle was 2.5 times that of subepicardial muscle using microspheres 51 to 61µ in diameter, but the ratios were 1.4 and 1.3 using microspheres of mean diameters 20 to 23µ and 14µ, respectively. It is unlikely that any of these microspheres measure blood flow to small portions of the ventricle.
Domenech et al. (Sat,) studied this question.