Oxygen desaturation in the six minute walk test is a predictor of decline in muscle mass in COPD patients

Background: The six minute walk test (6MWT) is a simple exercise test to measure functional status in COPD patients, and can also provide information regarding exertional oxygen desaturation. Little is known on whether oxygen desaturation can predict decline in muscle mass. Method: 370 COPD patients, aged 40-76 yrs, in the Bergen COPD Cohort Study 2006-09, performed the six minute walk test at baseline in 2006, and were followed for 3 years. Oxygen saturation (SpO2) was measured by pulse oximetry immediately before and after the test. Oxygen desaturation was defined as a 4% reduction between pre and post SpO2, and a post SpO2 below 90%. Fat free mass index (FFMI, kg/m2) was used as a proxy for muscle mass. Change in FFMI was estimated by generalized estimating equation (GEE) models with robust standard errors, including the time by desaturation interaction. The multivariable adjustment included baseline values of sex, age, body composition (cachectic, normal, obese), smoking habits, FEV 1 in % predicted, resting arterial oxygen tension (PaO 2 ), number of exacerbations the last year before inclusion, Charlson comorbidity score, and walking distance measured during the 6MWT. Result: 23% of the patients desaturated during the 6MWT. The average yearly decline in fat free mass during follow up was 0.03 kg/m2 (95% CI -0.08, 0.02). Those who desaturated declined on average 0.151 kg/m2 (95%CI -0.28, -0.03) more during follow-up than those who did not desaturate. Age, baseline cachexia, and lower FEV1 were also associated with faster decline in FFMI over 3 years. Conclusion: Oxygen desaturation in the 6MWT was a significant predictor for decline in fat free mass in a three year follow-up.