Decreasing diffusion limitation on exercise in lower-graded interstitial lung disease

In this study we investigated the contribution of diffusion limitation to the exercise-induced hypoxaemia in interstitial lung disease (ILD). We applied isotopic analysis to the composition of the stable isotopic oxygen molecules 16O2 and 16O18O in expiratory gas mixtures obtained from six ILD patients and six healthy subjects at rest and during ergometer work (60 W). The changes in the 16O18O/16O2 ratios were interpreted by using the overall fractionation factor of respiration (alpha O) which would be increased towards 1.03 on increasing diffusion limitation. In addition, the O2 partial pressures of alveolar gas and arterial blood (PAO2, PaO2) were determined. In the patients, alpha O was significantly reduced from 1.0066 +/- 0.0004 (mean +/- SD) at rest to 1.0035 +/- 0.0004 during exercise and in the healthy subjects from 1.0072 +/- 0.0008 to 1.0044 +/- 0.0004. Furthermore, the exercise-induced reduction of PaO2 (from 77 to 69 mmHg) was due to a drop of alveolar PO2 found in each patient, whereas in each healthy subject PaO2 was increased on exercise. On the basis of a resistance model we conclude that the patients' data were inconsistent with increasing diffusion limitation but showed an increasing impairment of O2 transport by ventilation.