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

Abstract

 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 (α _O) which would be increased towards 1.03 on increasing diffusion limitation. In addition, the O₂ partial pressures of alveolar gas and arterial blood (P _AO₂, P _aO₂) were determined. In the patients, α _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 P _aO₂ (from 77 to 69 mmHg) was due to a drop of alveolar PO₂ found in each patient, whereas in each healthy subject P _aO₂ 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.