An in Vivo Method for the Determination of Cardiac Output

Abstract

The validity of the dye dilution method for the determination of cardiac output has been confirmed by Hamilton and his coworkers (1), as well as Doyle and his group (2). Reports from several centers (3–5) indicate a similar volume of distribution for the dye, T-1824, and radioiodinated human serum albumin during the early moments following an intravenous injection. Pritchard and his associates (6) have described the use of radioiodinated human serum albumin rather than dye in the determination of cardiac output. In their method, blood from an intra-arterial needle is led past the sensitive surface of a scintillation counter, making possible a continuous graphic recording of the arterial concentration of the radioactive material. Huff and his group (7), working in this country, as well as a British group under the direction of Veall (8) have reported the use of an external counter positioned over the chest for the determination of the arterial flow curve. The present study was undertaken to determine the accuracy of the calibration technic required by such external counting methods and to assess the possible clinical applications of the procedure. Methods The equipment for this study consisted of a scintillation counter with a directional lead shield, a scaler, a counting rate computer, and its recorder. The detector is positioned over the second left intercostal space adjacent to the sternum. Five to 15 microcuries of radioiodinated human serum albumin contained in a volume of 0.5 to 1.0 c.c. is injected rapidly into a brachial vein. The changes in the radioactive iodine concentration of the blood which follow are reflected on a curve inscribed by the graphic recorder. Ten minutes later, with the position of the external counter unchanged, a venous blood sample is obtained for purposes of calibration. In 10 patients, the cardiac output value determined by the in vivo counting method was compared with the value obtained simultaneously by analysis of serial arterial blood samples. Calculation of the output value is based on the derivation as described by Veall. In brief the average flow rate is where I is the quantity of tracer injected, C is the instantaneous concentration of the isotope in the mixing chamber, and t is time (Fig. 1). When an external counter is placed over the chamber, the concentration C = XN, where N is the counting rate and X is the constant relating counter efficiency to tracer concentration. Then Equation 1 becomes In a system in which the external counter is placed over two chambers in series, such as the right and left sides of the heart, the curve recorded is a summation of the two separate chamber concentrations. This is shown diagrammatically in Figure 2.