Effects of inhaled steroids on methacholine-induced bronchoconstriction and gas trapping in mild asthma.

According to a recent hypothesis, airway smooth muscle regulates
airway calibre mostly at high lung volume, whereas the mucosa and adventitia
dimensions dominate at low lung volumes. It was thought that if inhaled steroids
decrease the thickness of airway wall in asthma, then forced vital capacity (FVC),
which reflects the functional changes at low lung volume, should decrease less during
induced bronchoconstriction than flow at high volume.
The study was conducted in 31 mild asthmatics under control conditions and during
a methacholine challenge before and after 4-weeks treatment with inhaled fluticasone
dipropionate (1.5 mg daily, 16 patients) or placebo (15 patients).
After fluticasone dipropionate treatment, control forced expiratory volume in one
second (FEV1), and maximal flow at 50% of control FVC during forced expiration
after a maximal (V'max,50) and a partial inspiration (V'p,50) significantly increased.
During methacholine challenge, FVC decreased less than did FEV1 or V'max,50, and so
did inspiratory vital capacity compared to V'p,50. Both the provocative dose of methacholine
causing a 20% fall in FEV1 and the bronchodilator effect of deep inhalation
significantly increased. The latter was assessed by means of the regression coefficient
of all V'max,50 plotted against V'p,50. No significant changes in these parameters
occurred after placebo.
These data show that inhaled steroids remarkably blunt the occurrence of gas
trapping during induced bronchoconstriction in mild bronchial asthma, possibly due
to their effect on airway wall remodelling.