The Factor of Recirculation in Acetylene Method for Determination of Cardiac Output

Abstract

Hamilton, Spradlin, and Saam 1 on the basis of animal experiments have concluded that the results of the acetylene method 2 for determining the cardiac output are vitiated by the return of acetylene to the lungs before the procedure is completed. After using the Marshall-Grollman method for one year, 3 the present writer felt the necessity of studying this question in human subjects. If and when recirculation of acetylene-laden blood occurs in appreciable amounts, it will become manifest by a reduction in the rate of diffusion from the lungs into the blood, a reduction in the diffusion constant K of the system, and a rise in the calculated arterio-venous oxygen difference. In 11 rebreathing experiments on 2 subjects, these data were obtained for successive short intervals during each experiment by drawing several alveolar samples, the breathing and sampling being directed by an accurately timed phonograph record. The results of one typical experiment are graphically presented. If the blood flow through the lungs is fairly constant during the rebreathing experiments, the rate of diffusion of the acetylene from lungs to blood should be proportional to the constantly decreasing alveolar concentration of the gas. Charted as the ordinate against time as the abscissa, the concentration of C₂H₂ should follow an exponential or logarithmic curve, and the logarithms of the concentration should follow a straight line, the slope of which will indicate the rate of diffusion of the gas. In the experiments performed, the break in this line with flattening of the curve indicating a depression in the rate of diffusion of the gas due to recirculation in appreciable amounts, has occurred at 10 seconds or promptly thereafter. This rapid recirculation is undoubtedly due to the increase in blood flow through the lungs occasioned by the deep rapid breathing of the procedure.