Background: Isolated small airways obstruction is common in general populations. We aimed to investigate whether it is associated with impaired cardiorespiratory fitness.

Methods: We analysed data from 222 patients referred for cardiopulmonary exercise test (CPET) and spirometry at Cambridge University Hospitals NHS foundation trust. We investigated the association between isolated small airways obstruction, defined as the mean forced expiratory flow rate between 25% and 75% of the forced vital capacity (FEF_25?75) less than the lower limit of normal (<LLN) in the presence of a normal forced expiratory volume in 1s to FVC ratio (FEV₁/FVC ?LLN), and peak oxygen uptake (peak V_O2), end-exercise oxygen pulse (V_O2/HR), and breathing reserve (L/min) using multivariable regression analyses. We re-ran the analyses defining isolated small airways obstruction using the forced expiratory volume in 3 s to FVC ratio (FEV₃/FVC) <LLN.

Results: The prevalence of isolated small airways obstruction was 9% and 7% for FEF_25-75 and FEV3/FVC respectively. Isolated small airways obstruction was associated with a reduced end-exercise breathing reserve (FEF_25-75: ?-18.24 L/min, 95%CI -28.65, -7.64; FEV₃/FVC: ?= -15.13 L/min, 95%CI -27.50, -2.77. It was also associated with greater odds of ventilation limitation, defined as a breathing reserve <15L or 15% at peak exercise without cardiovascular limitation (FEF_25-75: OR=32.65, 95%CI 5.82, 183.32; FEV₃/FVC: OR= 6.81, 95%CI 1.60, 28.9). Isolated small airways obstruction was not associated with peak V_O2 or oxygen pulse.

Conclusion: Individuals with isolated small airways obstruction are more likely to experience ventilation limitation on exercise.