The definition of asthma has evolved from that of an episodic disease characterized by reversible airways constriction to a chronic inflammatory disease of the airways, with at least partially reversible airway constriction.
The contribution of the distal airways to the asthma phenotype carries implications for the delivery of inhaled medications to the appropriate areas of the lung and for the monitoring of the response to asthma treatment.
Common spirometric parameters offer limited information with regard to the peripheral airways, and it is therefore necessary to move beyond FEV1.
Flow-Volume loop showing successful FVC maneuver. Positive values represent expiration, negative values represent inspiration. The trace moves clockwise for expiration followed by inspiration. Image source: Spirometry, from Wikipedia, the free encyclopedia, GNU Free Documentation License.
Several functional parameters and inflammatory markers can be employed to evaluate distal lung function. Extrafine formulations deliver inhaled drugs throughout the bronchial tree (both large and small airways).
The role of small airways in monitoring the response to asthma treatment: what is beyond FEV1? N. Scichilone et al. Allergy, 2009.