Six-minute walk test vs cardiopulmonary exercise test in the assessment of exercise tolerance in adults with cystic fibrosis

Background: Reduced exercise tolerance is a clear hallmark of cystic fibrosis (CF) and also an important prognostic factor. To date, it is not well established whether six-minute walk test (6MWT) and cardiopulmonary exercise test (CPET) provide similar sensitivity results in the assessment of exercise tolerance in adults with CF. Objective: The aim of the study was to compare 6MWT and CPET in the evaluation of exercise tolerance in CF adults with mild to moderate lung impairment. Methods: 30 stable CF adults (20 males, 10 females; age 32.9±8.8 years; FEV1 70.2±19.1% predicted, BMI 22.6±2.7 kg/m2) and 15 age-sex-BMI matched healthy controls (HC) were recruited. All participants underwent 6MWT and incremental CPET on cycle-ergometer: maximal distance at 6MWT (6MWD) and peak oxygen uptake (V'O2 peak) at CPET were measured, together with other physiologic variables. Lower limits of normal values (LLN) were calculated from the 5th percentile of HC results’ distributions. Results: 6MWD was similar between CF patients and HC (656.3±65.5 vs 719.6±100.9 m, p=ns). By contrast, V'O2 peak was significantly lower in CF patients, both as normalized for body weight (28.7±5.0 vs 38.1±4.2 ml/min/kg, p<0.0001) and as percentage of predicted value (76.3±13.4 vs 98.2±13.1%, p<0.0001). When compared to LLN, 6MWD was reduced in only 36.7% of CF patients, whilst a reduction in V'O2 peak was found in 66.7% of them. 14 of 19 (73.7%) CF patients with normal 6MWD showed reduced exercise tolerance if assessed by V'O2 peak. Conclusions: Despite of the advantage of being an easy test to perform, 6MWT is not as accurate and sensitive as CPET in detecting exercise intolerance in CF adults.