Rate of elimination of excess CO2 in humans

This study examined the ability of the respiratory system of awake normal subjects to correct an acute disturbance in body CO2 stores produced by rebreathing. Thirteen subjects, after 10 min of O2 breathing, rebreathed CO2 for 4 min in order to increase CO2 stores. The rate of CO2 elimination (VelCO2) after rebreathing was measured breath by breath for the next 10 min. The VelCO2 was highest immediately after the end of rebreathing and then decreased non-linearly toward the pre-rebreathing level as stored CO2 decreased. The time for 90% of this change in VelCO2 to occur (T90) was measured as an index of the rate of correction of body CO2 imbalance. The T90 was independent of the peak PCO2 obtained by rebreathing, and changes in CO2 storage did not produce significant changes in T90. The value of T90 was 120 +/- 48 sec (mean +/- SD) and there was a significant negative correlation between T90 and the slope of the ventilatory response to CO2 among subjects (r = -0.864, P less than 0.001). These results suggest that the capacity to eliminate the acutely stored CO2 in awake normal subjects is dependent on the ventilatory response to CO2. The data were further analyzed by using a mathematical model. Mathematical analysis confirmed the inverse relationship between CO2 sensitivity and T90. Model simulations also suggested that the restoration speed of CO2 balance under hyperoxic conditions is affected by brain blood flow but mainly determined by the sensitivity of central chemoreceptors.