Pulmonary O2 diffusing capacity at exercise by a modified rebreathing method

The rebreathing technique for the measurement of the pulmonary O₂ diffusing capacity, \(D_{{\text{O}}_{\text{2}} }\) , previously developed for resting conditions [Cerretelli et al., J. appl. Physiol. 37, 526–532 (1974)] has been modified for application to exercise and simplified to one rebreathing maneuver only. The changes consist: in administering in the course of a normoxic exercise a priming breath of an O₂ free mixture just before the onset of rebreathing in order to achieve rapidly the appropriate starting \(P_{{\text{O}}_{\text{2}} }\) values on the linear part of the O₂ dissociation curve as required by the method; in calculating mixed venous blood O₂ tension by extrapolation of the alveolar to mixed venous blood \(P_{{\text{O}}_{\text{2}} }\) equilibration curve, instead of determining it separately. While the mean \(D_{{\text{O}}_{\text{2}} }\) value of 21 measurements on 5 subjects at rest was 30 ml·min⁻¹·Torr⁻¹±3 (S.E.), in 2 subjects exercising on a bicycle ergometer, \(D_{{\text{O}}_{\text{2}} }\) was found to increase from a resting value of about 32 ml·min⁻¹·Torr⁻¹ to 107 ml·min⁻¹·Torr⁻¹ for an eightfold increase of O₂ uptake. The validity and the applicability of the method are critically discussed.