Assessment of rebreathing O2 consumption in humans with normal and diseased lungs

We investigated sources of error in estimating steady-state O2 consumption (VO2ss) by calculating O2 uptake from an anesthesia bag containing O2, He, and N2 during 10-20 s of rebreathing (VO2rb). In 11 normal resting subjects, VO2rb calculated with end-tidal sampling overestimated VO2ss by 16 +/- 15% (SD) (P less than 0.003). This error was proportional to the increase in pulse rate during rebreathing, so that pulse-corrected VO2rb slightly underestimated VO2ss by 2.1 +/- 12.2% (P = 0.66) in the six subjects who rebreathed 28% O2 in the rebreathing bag but significantly underestimated VO2ss by 7.5 +/- 6.7% (P less than 0.04) in the six subjects who rebreathed 21% O2 in the rebreathing bag. During exercise, VO2rb underestimated VO2ss by 4 +/- 12% (P less than 0.001) and by 7 +/- 6% at O2 consumptions greater than 2,000 ml/min if O2 in the rebreathing bag was kept above 20% throughout rebreathing. We found that VO2rb calculated with end-tidal gas concentrations underestimated VO2ss by 1-43% in patients with moderate-to-severe obstructive lung disease, with even greater errors when mixed expired samples were used. The magnitude of the discrepancy correlated poorly with abnormalities in standard pulmonary function tests. Based on these data, VO2rb closely approximates VO2ss in normal subjects, provided hypoxia during rebreathing is avoided and cardiac acceleration from rebreathing is taken into account during resting measurement.