The Effect Of Bronchodilators On Forced Vital Capacity In Patients With Idiopathic Pulmonary Fibrosis

Rationale: Longitudinal change in forced vital capacity (FVC) is a key measure of disease progression in patients with idiopathic pulmonary fibrosis (IPF) and has been the primary endpoint for many clinical trials of novel therapies in this disease. Concomitant obstructive disease is common in IPF and can also affect FVC, thereby reducing the precision of FVC as a measure of IPF severity. The use of bronchodilators (BD) in patients with IPF may result in a more precise measurement of FVC by treating reversible airways obstruction. The objective of this study was to determine the effect of inhaled BD on FVC measurement in patients with IPF. Methods: Patients were retrospectively identified from an ongoing longitudinal cohort of IPF patients at UCSF. Patients were included if they had at least one spirometry test with pre and post-BD values recorded. Patient information was obtained from the medical chart. The intra-test difference between pre-BD and post-BD FVC was obtained using a paired t-test. Mixed effects model regression was performed to identify predictors of response to BD. Inter-test precision of FVC was assessed in the subgroup of patients who had sequential spirometry tests (over 6 months) by comparing the standard deviations of the 6-month change in pre-BD FVC and post-BD FVC. Results: There were 261 patients who met inclusion criteria for this study, contributing 476 unique spirometry tests. The mean age was 69.8 years and 77% were male. Self-reported diagnosis of asthma and COPD were present in 17% and 28%, respectively and 73% were current or former smokers. The mean FVC pre-BD was 2.78L and the mean post-BD FVC was 2.81L (intra-test difference = 0.02L, p = 0.044). None of the baseline patient characteristics were significant predictors of FVC change with BD. The mean inter-test change in pre-BD FVC over 6 months was -0.13L with a SD of 0.29L. The mean inter-test change in post-BD FVC over 6 months was -0.14L with a SD of 0.26L. The inter-test precision was significantly better using post-BD FVC (10% difference in SD, p = 0.046). Conclusion: In patients with IPF, the intra-test difference in FVC following BD administration is minimal. However, post-BD FVC measured over time has a smaller standard deviation and therefore better precision than pre-BD FVC. This can have implications for powering of randomized controlled trials and suggests that clinical studies in IPF could use sample sizes when using post-BD FVC as the primary endpoint.