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It is shown theoretically that the CO2 time course during rebreathing involves two exponential terms, one of these terms being negligible after the first respiration. The validity of the exponential equation 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 previously described by Dubois and Fenn, since rebreathing yields far more accurate results. The PvCOCO2 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 PvCOCO2, the cardiac output was determined on the indirect Fick principle. The mean value of nine subjects under almost basal conditions was found to be 4.99 1/min. The variations on one and the same subject were less than 10%. Submitted on August 13, 1956
J. G. Defares (Mon,) studied this question.