Oral [^(13)C]bicarbonate measurement of CO_2 stores and dynamics in children and adults

Abstract

During exercise, less additional CO_2 is stored per kilogram body weight in children than in adults, suggesting that children have a smaller capacity to store metabolically produced CO_2. To examine this, tracer doses of [^(13)C]bicarbonate were administered orally to 10 children (8-12 yr) and 12 adults (25-40 yr) at rest. Washout of ^(13)CO_2 in breath was analyzed to estimate recovery of tracer, mean residence time (MRT), and size of CO_2 stores. CO_2 production (VCO_2) was also measured breath by breath using gas exchange techniques. Recovery did not differ significantly between children [73 ± 13% (SD)] and adults (71 ± 9%). MRT was shorter in children (42 ± 7 min) compared with adults (66 ± 15 min, P < 0.001). VCO_2 per kilogram was higher in the children (5.4 ± 0.9 ml.min^(-1).kg^(-1)) compared with adults (3.1 ± 0.5, P less than 0.0001). Tracer estimate of CO_2 production was correlated to VCO_2 (r = 0.86, P < 0.0001) and when corrected for mean recovery accurately predicted the VCO_2 to within 3 ± 14%. There was no difference in the estimate of resting CO_2 stores between children (222 ± 52 ml CO_2/kg) and adults (203 ± 42 ml CO_2/kg). We conclude that orally administered [^(13)C]bicarbonate can be used to assess CO_2 transport dynamics. The data do not support the hypothesis of lower CO_2 stores under resting conditions in children.