DLCO/Q AND DIFFUSION LIMITATION AT REST AND ON EXERCISE IN PATIENTS WITH INTERSTITIAL FIBROSIS

Pulmonary diffusing capacity for carbon monoxide (DL(CO)) and pulmonary capillary blood flow (Qp) were measured on exericse in patients with a low DL(CO) with the aim of predicting, from the overall DL/Qp ratio, diffusion limitation for oxygen and relating it to the fall in arterial oxygen saturation actually observed. Five patients with cryptogenic fibrosing alveolitis (DL(CO) ranging from 20-54% predicted normal) exercised for 5 min at a work load equal to 60% of their maximum (45 to 90 watts). At 5 min (and previously at rest) they rebreathed rapidly for 15 sec from a 1.0 L bag containing helium (He), sulphur hexafluoride (SF6) and freon-22, 30% oxygen in argon and < 1 ppm C-11-labelled carbon monoxide. Pulmonary capillary blood flow (Qp) and diffusing capacity (DL(CO)) were measured from flow-weighted breath-by-breath concentrations of freon-22 and 11CO, after correction for gas mixing delays (using He and SF6). Oxygen saturation (Sa(O2) (ear oximetry), M(O2) and M(CO2) and cardiac frequency were measured. PA(O2) (ideal) was derived and mixed venous O2 saturation and content were calculated (Fick); Pa(O2) and PvBAR(O2) were derived from standard dissociation curves. For comparison, DL(CO) and Qp were measured in a similar fashion in five normal subjects exercising at 60 watts. Mean DL(CO) in patients with fibrosis was 9.62 (SD 2.88) ml . min-1, mm Hg-1 on exercise and mean Qp was 10.48 (SD 1.79) L . min-1 giving mean DL(CO)/Q ratios of 0.92 (SD 0.28). At 60 watts mean DL(CO)/Qp in normal subjects was 2.54 (SD 0.3), 2.76-times greater than in patients. Sa(O2)% fell in patients by 3-15% on exercise. Predictions of alveolar-end capillary P(O2) gradients from these overall DL/Q gradients showed that diffusion limitation accounted for 99% of the alveolar-arterial P(O2) gradient on exercise in fibrosing alveolitis. The analysis in the companion paper (Hempleman and Hughes, 1990) suggests that this simple approach overestimates the contribution of diffusion limitation by about 30%.