Comparison of cardiac output during exercise by single-breath and CO2-rebreathing methods

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Abstract
Cardiac output (.ovrhdot.Q) was estimated in supine rest and in upright cycling at several work rates up to 200 W in 5 male and 1 female subjects. At least 4 repetitions of both the CO2-rebreathing plateau method (Collier, 1956) and the Kim et al. (1966) single-breath method were performed at each work rate, in a steady state of O2 consumption and heart rate. At supine rest and low work rates, estimates of .ovrhdot.Q were similar by the 2 methods. At higher work rates, the single-breath method significantly (P < 0.05) underestimated the value obtained by CO2 rebreathing. The reason for the difference in estimates of .ovrhdot.Q by the 2 methods was traced to the determination of arterial partial pressure of CO2 (PaCO2) and mixed venous partial pressure of CO2 (P.hivin.vCO2). The estimate of PaCO2 from the single-breath method was .apprx. 88.5% of the estimate from end-tidal PCO2 used with the rebreathing method (P < 0.001). The oxygenated P.hivin.vCO2 calculated from the single-breath .ovrhdot.Q averaged .apprx. 92.5% of the P.hivin.vCO2 from CO2 rebreathing (P < 0.0001). The difference in estimates of .ovrhdot.Q was not eliminated by using a logarithmic form of the CO2 dissociation curve with the single-breath method.