Reliability of the cardiac output measurement with the indirect Fick-principle for CO2 during exercise.

Cardiac output measurements were performed during 50 exercise tests in 16 normal subjects employing the indirect Fick principle for CO2. During sub-maximal steady state exercise the plateau CO2 tension (PplatCO2) was estimated with a rebreathing procedure. The mixed venous CO2 tension (PvCO2) was calculated by subtracting the alveolocapillary CO2 tension difference from the PplatCO2. Compared with data from the literature the most valid calculation of the cardiac output was obtained by using the PvCO2. Cardiac output values, calculated via the PplatCO2 turned out to be too low. The reproducibility was tested by repitition of 18 exercise tests at least after 5 days. The relative standard error of a single observation was 4.1% for the cardiac output, which was found to be as good as that of invasive measurements.