Time constant of inspiratory muscle relaxation in cystic fibrosis.

Background: Patients with cystic fibrosis (CF) are at increased risk of inspiratory muscle fatigue and respiratory failure. The time constant (τ) of the inspiratory muscle relaxation is a simple bedside test of muscle fatigue. We have compared patients with CF and healthy controls regarding τ and hypothesized that it is negatively associated with severity of lower airway obstruction.

Methods: For this cross-sectional study, τ after maximal inspiration and spirometric indices (forced expiratory volume in 1 s (FEV1) and forced vital capacity (FVC)) were measured.

Results: Fifty-three CF patients (median age 14 y (interquartile range: 11-19.5)) and 53 age- and sex-matched healthy control subjects (14 y (11-19.5)) were recruited. Application of a general linear model revealed that health status (CF vs. non-CF) had a significant effect on τ (P < 0.001), but age group and the interaction of age group with health status did not have significant effects on τ (P = 0.10 and P = 0.71, respectively). Participants with CF had significantly higher τ (253 (188-406)) than control subjects (117 (81-185)) (P < 0.001) and τ was negatively related to FEV1 (r = -0.205; P = 0.031) and FVC (r = -0.294; P = 0.002).

Conclusion: Patients with CF have higher τ than healthy controls but the correlation of τ with expiratory flow function is modest.