Am. J. Respir. Crit. Care Med., Volume 162, Number 5, November 2000, 1823-1827

Muscle Size and Cardiorespiratory Response to Exercise in Cystic Fibrosis

CHUANPIT MOSER, PORNCHAI TIRAKITSOONTORN, ELIEZER NUSSBAUM, ROBERT NEWCOMB, and DAN M. COOPER

Department of Pediatrics, University of California Irvine Medical Center, Irvine; and Division of Pediatric Pulmonology, Miller Children's at Long Beach Memorial Medical Center, Long Beach, California

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 ( O₂) 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 O₂ per CSA (mean for CF subjects 70 ± 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 O₂ 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 O₂ were also lower in the CF subjects (e.g., the slope of O₂ versus work rate [WR] ( O₂/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.

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