Estimating exercise Dl_o2 and diffusion limitation in patients with interstitial fibrosis

Inert gas elimination studies in interstitial fibrosis ascribe all of the resting and most (58-83%) of the exercise (A-a)P_O2 difference to ventilation-perfusion in equality. The previous paper (Hughes, J.M.B., D.N.A. Lockwood, H.A. Jones and R.J. Clark, Respir. Physiol., 1990) suggests from estimates of global Dl_O2/Q̇β ratios a larger role diffusion limitation on exercise. Gas exchange data from that paper was analyzed at rest and on exercise for five patients with interstitial fibrosis. Hypoxemia at rest was attributed toV̇a/Q̇ inequality which was quantified using a log-normal lung model. Dl_O2 was calculated by Bohr integration. The base 10 LogSDQ̇ at rest averaged 0.5 ± 0.1 (SEM) the assumption that V̇a/Q̇ inequality remained unchanged on exercise, Dl_O2 (exercise) was estimated to be 14.3 ± 1.9 ml·min^-1·Torr^-1. At that level of Dl_O2, diffusion limitation accounted for 36% ± 8(SEM)% of the total (A-a)P_O2 difference using the log-normal Q̇a/Q̇ model. But estimates of Dl_O2/Q̇β assuming a homogeneous lung, ascribed 96% of the (A-a)P_O2 gradient on exercise to diffusion limitation. The discrepancy was shown to be related to the shape of the oxygen equilibrium curve and high alveolar P_O2 values. On the other hand, analysis in terms of oxygen contents showed that 68 ± 5% of the (A-a) content difference was accounted for by diffusion limitation. This differs substantially from estimates based on partial pressure alone.