Asthma is a disease characterized by inflammation which affects both proximal and distal airways. This French study evaluated the prevalence of small airway obstruction (SAO) in 441 stable asthmatics with both normal FEV1 and normal FEV1/FVC and treated with inhaled corticosteroids (ICSs) and long acting β2-agonists (LABAs).
The prevalence of SAO was estimated by spirometry and plethysmography.
Small airway obstruction (SAO) was defined by the presence of one or more of the following criteria:
- functional residual capacity (FRC) higher than 120% predicted
- residual volume (RV) higher than predicted + 1.64 residual standard deviation (RSD)
- RV/total lung capacity (TLC) higher than predicted + 1.64 RSD
- forced expiratory flow (FEF)25–75% lower than predicted − 1.64 RSD
- FEF50% lower than predicted − 1.64 RSD
- slow vital capacity (SVC) − FVC higher than 10%
At least one criteria of SAO was found in 52% of patients, mainly lung hyperinflation (39%). In asthmatics with normal FEV1 and FEV1/FVC, treated with ICS/LABA, small airway obstruction (SAO) is found in more than half of the patients indicating that the routinely used lung function tests can underestimate dysfunctions occurring in the small airways.
Here is what wheezing sounds like (click to play the embedded video):
The findings of this study are important because the step-up/step-down approach to asthma management is based on symptoms and spirometry:
Asthma Treatment Options in 6 Steps (click to enlarge the image).
Small airway impairment in moderate to severe asthmatics without significant proximal airway obstruction. Thierry Perez, Pascal Chanez, Daniel Dusser, Philippe Devillier. Respiratory Medicine, Volume 107, Issue 11 , Pages 1667-1674, November 2013.
Image source: Spirometry, from Wikipedia, the free encyclopedia, GNU Free Documentation License.