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ESTIMATION of cardiac output by the indicator-dilution method has been described by Kinsman, Moore, and Hamilton1 utilizing dye; however, radioactive methods for determination of cardiac output owe their inception to Prinzmetal and co-workers,2 who recorded radioactivity as a function of time by means of a counter over the heart after an intravenous injection of radioactive Na24. Nylin and Celander3 utilized red cells tagged with radioactive phosphorus. They determined the dilution by serially sampling arterial blood at short time intervals and using beta-particle assay methods; an external counter was not adaptable to this mnethod. Huff and associates4 used a narrow-angle scintillation counter and radioactive iodinated human albumin and found an excellent correlation of radioactive determinations of cardiac output with those obtained by cardiac catheterization. By this method, the passage of radioactive substance through the heart is detected by placing a gamma-ray detector on the skin between the first and second ribs at the left parasternal line. Count rates as a function of time are obtained that are similar to those obtained by direct arterial blood sampling. The curve written during the initial passage of the radioactive substance through the heart presents two peaks that represent the tracer passage through the right and left ventricles, respectively. The area under the extrapolated curve is divided into the equilibrium counting rate. The dimension of the cardiac output thus obtained is in terms of a multiple or fraction of the ssubject's blood vol-

Radioactive Isotope Determination of Myocardial Blood Flow by Surface Counting and Ratio Formula