Muscle size and cardiorespiratory response to exercise in cystic fibrosis

The mechanism responsible for diminished exercise performance in cystic fibrosis (CF) is not clear. We hypothesized that reduced muscle size, rather than an intrinsic muscle defect, was the primary factor in such diminished exercise performance. Twenty-two subjects with CF (14 females and eight males, aged 6.5 to 17.7 yr, with FEV1 of 46% to 111% predicted) participated in a study of this hypothesis, and were compared with healthy children tested in the same laboratory. Muscle size was estimated from midthigh muscle cross-sectional area (CSA) obtained by magnetic resonance imaging, and fitness was determined by progressive cycle ergometer exercise testing with breath-by-breath measurements of gas exchange. Peak oxygen consumption ((V) over dot O-2) was reduced in CF subjects (956 +/- 81 [mean +/- SEM] ml/min, as compared with 1,473 +/- 54 ml/min in controls; p < 0.00001). Surprisingly, CF subjects had a lower peak (V) over dot O-2 per CSA (mean for CF subjects 79 +/- 3% predicted, p < 0.0001) than did controls, whereas muscle CSA in CF subjects was not significantly smaller than in controls. The scaling parameters of peak (V) over dot O-2 and muscle CSA did not differ significantly between healthy controls (0.80 +/- 0.16) and CF subjects (1.03 +/- 0.12). Indexes of aerobic function that are less effort-dependent than peak (V) over dot O-2 were also lower in the CF subjects (e.g., the slope of (V) over dot O-2 versus work rate [WR] (Delta>(*) over bar * (V) over dot O-2/Delta WR) was 68 +/- 2% predicted; p < 0.005). The study data did not support the initial hypothesis, and suggest a muscle-related abnormality in oxygen metabolism in patients with CF.