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

Abstract

We investigated the reproducibility of the carbon dioxide (CO₂) rebreathing extrapolation technique of Defares to determine the mixed venous partial pressure of CO₂ (P _vCO₂) using a direct, rather than the commonly used indirect, extrapolation method. The P _vCO₂ 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 P _vCO₂ 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 P _vCO₂ values when a direct method of extrapolation is used.