Expiratory muscle strength and functional exercise tolerance in adults with cystic fibrosis: a cross-sectional study.

OBJECTIVE: A 6-minute walk work (6MWW), that is the product of distance walked at the 6-minute walk test (6MWT) multiplied by body weight, has been suggested as an appropriate variable for estimating functional capacity. Under the hypothesis that 6MWW strongly correlates with expiratory muscle strength (PE max), as found in Chronic Obstructive Pulmonary Disease (COPD), the aim of the study was to determine the degree of this correlation in adults with cystic fibrosis. METHODS: A cross-sectional study was carried out at the Regional Referral Centre for Cystic Fibrosis, where patients with cystic fibrosis attending their regular follow-up, aged 18 years or older with mild-to-normal lung disease, were asked to participate. Dietitians determined nutritional status before subjects performed spirometry and static measurement of respiratory muscles. The 6MWT was performed as the last study procedure. Spearman correlation test was used to correlate PE max with 6MWW and other study variables. RESULTS: Twenty-five cystic fibrosis subjects aged 18-30 years and with FEV1 values ranging from 42.4% to 123.4% predicted completed the study. PE max strongly correlated with 6MWW (rho .72, p = .0001) whereas it did not correlate with other study variables, including distance at the 6MWT. Distance walked multiplied by body weight (p = .0019) and body mass index (p = .0238) were significantly lower in those with PE max below the lower limits of normal. CONCLUSION: Increased functional exercise tolerance is related to higher static expiratory pressure in patients with mild-to-normal cystic fibrosis lung disease. Difference in exercise performance might reflect a possible difference in body composition that needs to be further investigated.