Study objectives: Cardiac output (Q) can be estimated noninvasively du
ring exercise by employing CO2-rebreathing techniques (equilibrium and
exponential) to estimate the oxygenated mixed venous PCO2 (<P(v)over
bar CO2>). It has been found in adults and children that the equilibri
um method underestimates Q as a result of overestimation of PVCO2, unl
ess <P(v)over bar CO2> is ''downstream corrected.'' In adults, it has
been found that the exponential method does not require downstream cor
rection 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 resul
ts to the equilibrium method with downstream correction in children; t
o verify that downstream correction is required in children; and to te
st whether a single equation could be used in adults aml children to p
redict Q from oxygen consumption (Vo(2)). Design: Descriptive. Setting
: Exercise laboratory of a university hospital. Participants: 23 child
ren (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 performing steady-state
exercise on an ergometer, <P(v)over bar CO2>, was determined in 14 chi
ldren using both the equilibrium and exponential methods, and in all o
ther subjects using the equilibrium method alone.Measurements and resu
lts: For the 14 children who underwent testing by both the equilibrium
and exponential methods, the uncorrected equilibrium <P(v)over bar CO
2> was significantly different from both the corrected <P(v)over bar C
O2> and the exponential <P(v)over bar CO2>. We found a strong relation
ship between Q (L/min), calculated using the downstream corrected valu
es of <P(V)over bar CO2>, and Vo(2) (L/min) (r(2)=0.95), and this rela
tionship 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 . Vo(2) (L/min)+0
.06 . wt (kg), r(2)=0.97, SEY=0.67. Conclusions: CO2-rebreathing techn
iques can be used to determine Q in children; the exponential method g
ives values that are similar to the equilibrium method with the downst
ream correction; and one prediction can be used for Q in adults and ch
ildren.