Determination of Pi & From the Exponentzd

theoretically that the CO2 time course during rebreathing involves two exponential terms, one of t,hese terms being of the exponential e& negl ig tion. The validity ation,ibl e after the first respira-is shown experimentally. With the aid of this formula it is possible to compute the CO2 tension in the mixed venous blood. Rebreathing is preferred to breath holding as pre-viously described by Dubois and Fenn, since rebreathing yields far more accurate results. The PVcop values computed from the rebreathing values were compared with values obtained by the Haldane method. The mean difference was not statistically significant. With the aid of this ‘exponential’ method for the determination of the T’vco,, the cardiac output was de-termined on the indirect Fick principle. The mean value of nine sul)jects under almost basal conditions was found to be 4.99 I /min. The variations on one and the same subject were less than IO(';. NEW METHOD for the determination of A the CO2 tension of the mixed venous blood is based on the following prin-ciple: during ‘rebreathing ’ the CO2 tension in the bag gradually increases with the number of respirations towards a limiting value, this limit being the CO2 tension of the mixed venous blood. Dubois, Britt and Fenn (I) were the first to suggest the exponential nature of the CO2 time course during rebreathing. This has been fully confirmed by subsequent theo-retical and experimental work done by the author. The theoretical analysis of the CO2 time course during rebreathing (based on the work of Chilton and Stacy (4)) yielded a linear second order difference equation, the Received for publication August 13, 1956. l This paper presents results which have been more fully described in the author’s doctoral thesis A Study of the Carbon Dioxide Time Course During Rebveathing. This work should be looked upon as a continuation o