The consistency of cardiac output measurement (CO2 rebreathe) in children during exercise

Exercise cardiac output ($$\dot Q$$) was determined using the CO2 rebreathing equilibrium method. Five repeat tests in 12 boys and two tests over a 4 month interval in 47 boys were performed. Regression equations to predict $$\dot Q$$ from $$\dot V$$O2 were in close agreement with dye dilution studies in boys (Eriksson and Koch 1973). Group mean data were reproducible from trial to trial. The day-to-day variability of $$\dot Q$$, with a coefficient of variation of 7–8%, was found to be higher than when the CO2 method has been applied in adults. This greater variability was related, in part, to a larger biological variation in children as depicted in such relatively simple measures as submaximal exercise heart rate. The larger variability was also related to inaccuracies in the methods of $$P_{aCO_2 } $$ estimation in children. Estimation from end-tidal CO2 concentrations requires further research to establish a correction for the alveolar-arterial gradient during exercise in children. Estimation of the child's dead space in exercise, with subsequent derivation of $$P_{aCO_2 } $$ from the Bohr equation, also could be improved. Nevertheless, $$\dot Q$$ estimates in children exercising above $$\dot V$$O2 1.01·min−1 showed a day-to-day and long term stability acceptable for use in research and clinical studies.