Dynamics of exhaled 13-CO_2 following oral administration of ^(13)C-bicarbonate

The ratio of ^(13)CO_2/^(12)CO_2 in the exhaled gas following IV administration of ^(13)C-bicarbonate can be used to estimate VCO_2 at different levels of exercise as well as to gain insight into CO_2 compartmental dynamics. But in children and under field conditions, IV administration is not feasible. To test the efficacy of the oral route, 8 oral and IV studies were performed in 2 subjects at rest and at 2 levels of exercise below the subject's anaerobic threshold. In 2 studies, the oral dose was given 1h and 4h after a meal. while in the remainder,· the subjects were fasted for 12h. Gas exchange was measured simultaneously breath-by-breath and ^(13)CO_2 by isotope ratio mass spectrometry. Mean residence time (MRT) for ^(13)CO_2 was analyzed as the ratio of area under the moment curve to area under the curve, Oral MRT was highly, inversely correlated to VCO_2 (r-.9, p<.05), and positively correlated to IV-MRT (r-.9, p<.05). The oral MRT values were longer by a mean of 8.8 min (p<.05) most likely representing GI transit time for the tracer. During rest, tracer given 1h postprandial resulted in markedly prolonged MRT (90 min compared to 65 min fasted), but by 4h postprandial the MRT was the same as in the fasted state. Oral administration of ^(13)C-bicarbonate may be a useful tool for assessing CO_2-bicarbonate dynamics noninvasively under in vivo conditions of differing metabolic rates