Estimation of CO_2 stores by tracer dilution and VCO_2 kinetics

The dynamics of CO_2 content and storage are critically important in the control of metabolism and ventilation. We compared two methods of estimating the changes in body CO_2 stores due to light and moderate exercise: 1) washout kinetics of exhaled ^(13)CO_2 following IV bolus of labeled bicarbonate, and 2) analysis of pulmonary gas exchange kinetics during the transition from rest to the steady state of exercise. ^(13)CO_2 washout data was fit to a 3 compartment, mammillary model. If it were assumed that the metabolic production of CO_2 occurred in the same compartment for rest and the 2 exercise conditions, then estimates of the increase in total exchangeable CO_2 ranged from 450-1700 mMoles. Analysis of the kinetics of CO_2 elimination at the mouth (VCO_2 breath-by-breath) during the transitions, however, suggested that the total increase in CO_2 stores was much smaller (10-50 mMoles), with 20-50% of the increase in the venous blood stores and 50-80% due to increases in tissue CO_2. The discrepancy in results from these different analyses of CO_2 storage kinetics imply that the compartment associated with the metabolic source of CO_2 may vary under different metabolic conditions. Analysis of VCO_2 kinetics may provide useful information in interpreting the changes in CO_2 flux with exercise estimated from ^(13)C-bicarbonate washout studies.