Time constant of respiratory muscle relaxation in patients with cystic fibrosis

Background: Patients with cystic fibrosis (CF) are at increased risk of respiratory muscle fatigue and respiratory failure, secondary to lower airway obstruction and malnutrition. Testing for respiratory muscle fatigue involves complex methodology and the physical presence of the patient in a research facility. The time constant of respiratory muscle relaxation (τ) is a simple bedside test which can be alternatively used to assess CF patients for respiratory muscle fatigue. Measures: For this cross-sectional study, we measured body mass index z-score (BMI), upper arm muscle area (UAMA), Forced Expiratory Volume in 1 second z-score (FEV 1 ), Forced Vital Capacity z-score (FVC), maximal inspiratory pressure (Pi max ), τ after maximal inspiration (τ-Pi max ), maximal expiratory pressure (Pe max ) and τ after maximal expiration (τ-Pe max ). Results: Fifty-three CF patients [median age 14 years (interquartile range: 11-19.5)] and 53 age- and sex-matched healthy controls [14 years (11-19.5)]were recruited.Pi max and Pe max did not differ significantly between patients and controls.Median (IQR) τ-Pi max in CF subjects [253(188-406)] was significantly increased compared to median (IQR) τ-Pi max in controls [117(81-185); p max in CF subjects [232 (105-344)] was significantly increased compared to median (IQR) τ-Pe max in controls [101 (78-143); p max was significantly related to FEV 1 (r=-0.205, p=0.031) and FVC (r=-0.294, p=0.002) but not to BMI or UAMA. Conclusions: Our findings suggest that CF patients are predisposed to respiratory muscle fatigue as described by slow respiratory muscle relaxation, which is significantly associated with severity of spirometric abnormalities.