Estimation by a rebreathing method of pulmonary O2 diffusing capacity in man

The pulmonary O 2 diffusing capacity (DO 2) can be estimated in man from the kinetics of PO 2 equilibration between lung gas and mixed venous blood during a rebreathing maneuver, when the following variables are known or can be simultaneously determined: mean rebreathing bag volume (VR), mean lung volume (VA), effective ventilation ([rate of change of V]eff), pulmonary capillary blood flow [rate of change of Q], and slope of the blood O 2 dissociation curve (βO 2). Two rebreathing maneuvers, both performed after breathing 11.5% O 2 in N 2 at steady state, are required. In the first maneuver, an appropriate volume of 8% CO 2 in N 2 is rebreathed for determination of mixed venous blood PO 2 (P(average V)O 2). V)O2. The second maneuver, in which a mixture containing 8% O 2, 7% CO 2, 20% N 2O and 10% He in N 2 is rebreathed, allows the determination of the remaining variables: the rate constant of PO 2 equilibration (KO 2) from the exponential approach of lung PO 2 to P(average V)O 2; Va and [rate of change of V]eff, from He dilution and mixing rate; [rate of change of Q], from the transfer rate of N 2O. The mean DO 2 value found in 19 experiments on 4 healthy young males at rest was 31 ml/min -1/Torr -1. The validity and the applicability of the method are critically discussed.