Follow-up desaturation in 6 minute walk tests and lung function decline in COPD patients

Background: Whether patients who develop desaturation or repeatedly desaturate during 6 minute walk tests (6MWT) over time have poorer lung function prognosis than patients who do not desaturate is unknown. Aim: To examine associations between desaturation in 6MWT over time and lung function decline. Methods: In the Bergen COPD cohort study 2006-10, 307 patients, 40-76yrs, performed 6MWT at baseline and at follow-up 1 yr later. Oxygen saturation (SpO2) was measured before and after tests. Desaturation was defined as 4% reduction between pre- and post-test SpO2, and post-test SpO2<90%. FEV1 and FVC were measured at follow-up and yearly for 3 years. General estimating equation (GEE) analyses were performed adjusting for baseline values of sex, age, body composition, smoking, FEV1 % predicted, FVC % predicted, resting arterial oxygen tension, exacerbations the last yr before inclusion, comorbidity, and walking distance in the 6MWT. Results: 23% of the patients desaturated at baseline 6MWT. One yr later 32% of the patients desaturated, 13% were new desaturators and 19% desaturated in both tests. Desaturation only at follow-up was associated with 2.18% FVC decline per year after covariate adjustment, compared with no desaturation and 0.78% decline per year (desaturation at follow-up coeff -.014 (95%CI -.03,-.0006)). Subjects with desaturation in both tests later decreased FVC with 1.43% per year (not significantly different from no desaturation). Desaturation and FEV1 decline were not associated. Conclusion: Follow-up desaturation in the 6MWT was associated with increased decline in FVC later on. Onset of desaturation may represent a vulnerable time window in disease progression.