Study objectives: Cardiac output (Q) can be estimated noninvasively during exercise by employing CO2-rebreathing techniques (equilibrium and exponential) to estimate the oxygenated mixed venous PCO2 (PvCO2). It has been found in adults and children that the equilibrium method underestimates Q as a result of overestimation of PvCO2, unless PvCO2 is "downstream corrected." In adults, it has been found that the exponential method does not require downstream correction and yields values similar to those obtained by the equilibrium method with downstream correction. The objectives of this study were as follows: to test whether the exponential method gives similar results to the equilibrium method with downstream correction in children; to verify that downstream correction is required in children; and to test whether a single equation could be used in adults and children to predict Q from oxygen consumption (VO2).
Setting: Exercise laboratory of a university hospital.
Participants: 23 children (16 boys, 7 girls) with a mean age of 11.0±1.9 years (7.1 to 13.9 years), and 12 adults (7 men, 5 women) with a mean age of 33.6±7.2 years (24 to 48 years).
Interventions: While perfonning steady-state exercise on an ergometer, PvCO2 was determined in 14 children using both the equilibrium and exponential methods, and in all other subjects using the equilibrium method alone.
Measurements and results: For the 14 children who underwent testing by both the equilibrium and exponential methods, the unconnected equilibrium PvCO2 was significantly different from both the corrected PvCO2 and the exponential PvCO2. We found a strong relationship between Q (L/min), calculated using the downstream corrected values of PvCO2, and VO2 (L/min) (r2=0.95), and this relationship was similar to that obtained by dye dilution in other studies. When weight was included, it was determined that one equation could be used for children and adults: Q (L/min)=1.42+5.80 · VO2 (L/min)+0.06 · wt (kg), r2=0.97, SEY=0.67.
Conclusions: CO2-rebreathing techniques can be used to determine Q in children; the exponential method gives values that are similar to the equilibrium method with the downstream correction; and one prediction can be used for Q in adults and children.