Estimate of pulmonary diffusing capacity for oxygen during exercise in humans from routine O₂ and CO₂ measurements

A method to estimate pulmonary diffusing capacity for O₂ (D_LO₂) during exercise based on routine O₂ and CO₂ transport variables is presented. It is based on the fitting of a mathematical model to gas exchange data. The model includes V_A/Q heterogeneity (described as two exchanging compartments), diffusion limitation and right-to-left shunt. Mass conservation equations and Bohr integration were solved to calculate partial pressures in each compartment. Diffusion was distributed with perfusion. Two-compartment ventilation and perfusion distributions were estimated at rest during conditions of negligible diffusion limitation. These distributions were used during hypoxic and normoxic exercise to obtain the D_LO₂ from the model computations (D_LO₂2C) compatible with experimental data. Three normals, four sarcoid patients and four patients after lung resection were studied. An independent technique for carbon monoxide was used to provide experimental estimates of D_LO₂ (D_LO₂EXP, rebreathing technique for sarcoid patients and single breath for lung resection). D_LO₂2C was highly correlated with D_LO₂EXP (r² = 0.95, P<0.001) and the slope of the regression line was not statistically different from 1. The mean (D_LO₂EXP - D_LO₂2C) difference was -1.0 ± 7.4 ml min^-1 mmHg^-1. The results suggest that use of a refined analytical procedure allows for assessment of D_LO₂ from routine O₂ and CO₂ measurements comparable with those obtained from independent carbon monoxide techniques. The method may be an alternative for estimates of D_LO₂ during exercise.