An evaluation of exercise cardiac output using the CO2 rebreathing extrapolation technique

We investigated the reproducibility of the carbon dioxide (CO2) rebreathing extrapolation technique of Defares to determine the mixed venous partial pressure of CO2 (PvCO2) using a direct, rather than the commonly used indirect, extrapolation method. The PvCO2 determinations were made five times a day on four subjects who exercised for 4 h daily on a bicycle ergometer over a period of 13 days. The concentration of CO2 in the rebreathing bag was monitored continuously using a rapid-response infrared capnograph. The mean standard deviation was 1.56 mmHg (0.21 kPa) for a mean PvCO2 of 47.6 mmHg (6.3 kPa), giving a mean coefficient of variation of 3.3%. This level of reproducibility agrees favourably with the reliability of direct measurements of pulmonary arterial partial pressure of CO2. We conclude that the CO2 rebreathing extrapolation technique can give reproducible PvCO2 values when a direct method of extrapolation is used.