Measurement of coronary blood flow by external counting with radioactive rubidium. Critical appraisal and validation of the method.

T HE MYOCARDIAL clearance of potassium (MCK) can be measured by means of precordial counting after single intravenous injection of 42KC1 or 86RbCl.1 It may be considered to measure coronary blood flow per unit mass of myocardium under the assumption that the average extraction of the indicator by the myocardium during the first minutes after injection does not differ significantly from the average total body extraction. Theoretical considerations and experimental evidence suggest that these conditions actually occur in rats, dogs, and humans." 2 When a precordial counter is used to measure myocardial radioactivity, the obtained value for MCK represents the mean flow per unit mass, and it is conventionally expressed per 100 g of myocardium.1 Absolute flow cannot be determined with a precordial counter since individual variability in depth and size of the heart prevents an absolute estimate of myocardial uptake of the radioisotope. If the geometric dependence of the measurement could be overcome, coronary blood flow could be estimated as a fraction of cardiac output or in absolute terms. In fact, under conditions of uniform counting efficiency for the entire heart, the fraction of injectate taken up by the myocardium shortly after injection of 86Rb or 42K would equal the fraction of the cardiac output perfusing the myocardium.1 2 The present series of experiments has been designed with the following purposes: (1)