Change in O2 uptake during rebreathing in hyperoxia in man

In pertaining to PO2 dependency of the pulmonary CO diffusing capacity during rebreathing, the O2 uptake (VO2) and cardiac output (Q) were measured at three different PO2 levels between 100 and 500 Torr. Since the VO2 measured by an O2 injection method is strongly influenced in hyperoxia by a gas exchange ratio (R), a simulation method using a R-PCO2 relation during rebreathing was developed. Gas volume in the lung-bag-system needed in the computation was measured from the difference in O2 concentration between before and after injecting a known amount of O2 into the rebreathing circuit. The accuracy of the volume was checked by comparing it with the volume measured successively with a body box. The VO2 was determined by comparing the simulated O2 and CO2 concentrations in rebreathing gas with the measured ones. The VO2 significantly increased by rebreathing in hyperoxia. To analyze the VO2 increase, the Q was computed by dividing the VO2 by the arteriovenous O2 content difference, which in turn was obtained by dividing the slope of the CO2 dissociation curve by that of the R-PCO2 line. The Q was almost linearly related to the VO2. Since there was no difference in VO2 in steady state breathing between normoxia and hyperoxia, the increase in VO2 and Q seemed to occur transiently. This finding is very important in evaluating the PO2 dependency of the pulmonary diffusing capacity for CO.