Small airways response to naturalistic cat allergen exposure in subjects with asthma

Background

It is currently unclear whether the small airways (diameter <2 μm) contribute significantly to late asthmatic reactions to inhaled allergen.

Objectives

We sought to determine whether naturalistic exposure to cat allergen induced late responses in the small airways as measured by pulmonary function testing and high-resolution computed tomography (HRCT) of the chest performed at end-expiration.

Methods

In a group of 10 subjects with cat-induced asthma, physiologic studies (spirometry and lung volumes, including closing volume) and HRCT were performed before and 6 and 23 hours after a cat room challenge that caused a 20% or greater acute fall in FEV₁.

Results

There was no significant decline in FEV₁ at 6 or 23 hours after cat exposure. Forced expiratory flow at 25% to 75% of forced vital capacity was significantly decreased at 6 hours after the challenge and returned to normal by 23 hours. HRCT image analysis as well as closing volume demonstrated increased air trapping from baseline at both 6 and 23 hours after the challenge. In addition, image analysis demonstrated a significant increase in small airways hyperresponsiveness to methacholine at 23 hours after the challenge. No significant mean changes were noted in lung volumes at either 6 or 23 hours or in PC₂₀ FEV₁ at 23 hours postchallenge.

Conclusion

These findings demonstrate that naturalistic exposure to cat allergen results in significant small airways obstruction and hyperresponsiveness persisting for at least 23 hours, at which time these changes cannot be detected by conventional physiologic measures.

Clinical implications

Physiologically silent distal lung inflammation persists after an antigenic challenge.

Key words: Asthma, bronchial hyperreactivity, Fel d 1 protein, image analysis, computer-assisted, tomography, x-ray computed

Abbreviations used: BDP-CFC, Beclomethasone–dipropionate chlorofluorocarbon, BDP-HFA, Beclomethasone dipropionate hydrofluoroalkane, CRC, Cat room challenge, CV, Closing volume, CV/VC%, Closing volume as a percent of the vital capacity, FEF_25-75, Forced expiratory flow at 25% to 75% of forced vital capacity, FRC, Functional residual capacity, FVC, Forced vital capacity, HRCT, High-resolution computed tomography, HU, Hounsfield unit, LAC, Lung attenuation curve, MCT, Methacholine challenge test, RV, Residual volume, TLC, Total lung capacity