Effects of cool, dry air stimulation on peripheral lung mechanics in asthma

We have previously demonstrated that peripheral airway resistance (Rp) rise s more in asthmatics than in nonasthmatic control subjects after segmental challenge with cool, dry air. To better understand this rise in Rp, we used a stop-flow method to measure the decay of segment pressure with time that yielded information on airway resistance (Raw), final plateau pressure (Pp ), and peripheral lung compliance (Cp). After stop-flow maneuvers in all se ven asthmatics and all seven normal subjects, pressure decayed smoothly wit hout an initial sudden drop. This finding suggests that Raw was negligible and that the predominant site of flow resistance was the collateral pathway s of the obstructed segment. Asthmatics had a significantly higher Pp and l ower Cp at baseline than did normal subjects, but neither Pp nor Cp changed after challenge. Pp and Rp were significantly correlated. When interpreted in terms of a single-compartment nonlinear model, we concluded that Rp is predominantly determined by the resistance of the collateral airways rather than the more proximal airways. We also concluded that, compared with norm al subjects, asthmatics have (1) more collateral airway narrowing and closu re and lower segmental compliance, and (2) after challenge, increased colla teral airway narrowing or closure without a change in compliance of the dis tal lung parenchyma. These results reflect the fundamental differences in p eripheral lung mechanics between asthmatic and nonasthmatic subjects and in their response to directly instilled cool, dry air.