Analysis of factors limiting maximal exercise performance in cystic fibrosis

Abstract

1. Maximal exercise capacity in cystic fibrosis is influenced by both pulmonary and nutritional factors: lung disease by limiting maximal achievable ventilation, and malnutrition through a loss of muscle mass. The associated reduction in everyday activities may result in peripheral muscle deconditioning. 2. We studied 14 stable patients with cystic fibrosis (six males, eight females) and 14 healthy control subjects (seven males, seven females) in order to assess the influence of these factors on exercise performance. Subjects underwent anthropometry to estimate muscle mass, spirometry to assess ventilatory capacity, a 30 s sprint on an isokinetic cycle ergometer to assess maximal leg muscle performance, and progressive cycle ergometry to assess overall exercise capacity. 3. Compared with control subjects, the patients with cystic fibrosis were of similar age and height but weighed proportionately less [% ideal weight (mean ± SD): 94.3 ± 9.64 versus 109.5 ± 11.82] and showed evidence of airflow limitation [forced expiratory volume in 1.0 s (FEV1.0) 72.5 ± 24.78 versus 112.6 ± 14.25% of predicted]. 4. The patients with cystic fibrosis did less absolute (5.1 ± 1.89 versus 7.3 ± 1.97 kJ) but similar relative maximal (11.5 ± 3.41 versus 13.1 ± 3.55 kJ/kg lean body mass) sprint work. During progressive exercise, the group with cystic fibrosis achieved lower absolute [maximal O2 consumption (V̇O2 max.) 1.8 ± 0.527 versus 3.0 ± 0.655 litres/min] and relative (V̇O2 max./kg lean body mass: 40.5 ± 9.23 versus 53.0 ± 11.62 ml min-1 kg-1) work levels. 5. Step-wise linear regression was carried out with V̇O2 max. as the dependent variable and lean body mass, FEV1.0 and 30 s sprint work as independent variables. V̇O2 max. was best accounted for by a two-factor equation of FEV1.0 and 30 s sprint work (r2 = 0.882), with FEV1.0 being relatively more important. 6. While noting the importance of lung disease as a limiting factor, these results suggest that peripheral muscle function is a more sensitive determinant of maximal performance than lean body mass. The 30 s sprint work results imply that the effects of mild malnutrition in cystic fibrosis are quantitative, affecting the size of the muscle mass but not the performance of the remaining muscle.