Determinants of Exercise Capacity Assessed With the Modified Shuttle Test in Individuals With Cystic Fibrosis

Patients with cystic fibrosis develop decreased exercise capacity. However, the main factors responsible for this decline are still unclear. Thus, the objective of this study was to evaluate the factors influencing exercise capacity assessed with the modified shuttle test (MST) in individuals with cystic fibrosis.

A cross-sectional study was carried out in subjects with a diagnosis of cystic fibrosis who were 6-26 y old and were regularly monitored at 2 cystic fibrosis reference centers in Brazil. Individuals who were unable to perform the tests or who exhibited hemodynamic instability and exacerbation of respiratory symptoms were excluded. Anthropometric, clinical, and genotype data were collected. In addition, lung function and exercise capacity were evaluated with the MST.

73 subjects (mean age 12.2 ± 4.9 y and FEV₁ 76.8 ± 23.3%) were included. The mean distance achieved in the MST was 765 ± 258 m (71.6% of predicted). The distance achieved on the MST correlated significantly with age (r = 0.49, P < .001), body mass index (r = 0.41, P < .001), resting heart rate (r = -0.51, P < .001), and FEV₁ (r = 0.24, P = .042). Subjects with FEV₁ > 67% of predicted (P = .02) and those with resting heart rate < 100 beats/min (P = .01) had a greater exercise capacity. Resting heart rate, age, and FEV₁ (%) were found as significant variables to explain the distance achieved on the MST (R² = 0.48, standard error = 191.0 m).

The main determinants of exercise capacity assessed with the MST in individuals with cystic fibrosis were resting heart rate, age, and lung function.