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 isotop ic oxygen molecules O-16(2) and (OO)-O-16-O-18 in expiratory gas mixtu res obtained from six ILD patients and six healthy subjects at rest an d during ergometer work (60 W). The changes in the (OO)-O-16-O-18/O-16 (2) ratios were interpreted by using the overall fractionation factor of respiration (alpha(O)) which would be increased towards 1.03 on inc reasing diffusion limitation. In addition, the O-2 partial pressures o f 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 h ealthy subjects from 1.0072 +/- 0.0008 to 1.0044 +/- 0.0004. Furthermo re, the exercise-induced reduction of PaO2 (from 77 to 69 mmHg) was du e to a drop of alveolar PO2 found in each patient, whereas in each hea lthy subject PaO2 was increased on exercise. On the basis of a resista nce model we conclude that the patients' data were inconsistent with i ncreasing diffusion limitation but showed an increasing impairment of O-2 transport by ventilation.