Airway and lung tissue mechanics in asthma - Effects of albuterol

We examined the partitioning of total lung resistance (RL) into airway resi stance (Raw) and tissue resistance (Rti) in patients with mild to moderate asthma (baseline FEV1, 54 to 91% of predicted) before and after albuterol i nhalation. An optimal ventilator waveform was used to measure RL and lung e lastance (EL) in 21 asthmatics from approximately 0.1 to 8 Hz during tidal excursions. Analysis of the RL and EL provided separate estimates of airway and lung tissue properties. Eleven subjects, classified as Type A asthmati cs, displayed slightly elevated RL but normal EL. Their data were well desc ribed with a model consisting of homogeneous airways leading to viscoelasti c tissues before and after albuterol. The other 10 subjects, classified as Type B asthmatics, demonstrated highly elevated RL and an EL. that became h ighly elevated at frequencies above 2 Hz. These subjects required the inclu sion of an airway wall compliance in the model prealbuterol but not postalb uterol. This suggests that the Type B subjects were experiencing pronounced constriction in the periphery of the lung, resulting in shunting of flow i nto the airway walls. Spirometric data were consistent with higher constric tion in Type B subjects. Both groups demonstrated significant (p < 0.05) de creases in Raw and tissue damping after albuterol, but tissue elastance dec reased only in the Type B group. The percent contributions of Raw and Rti t o RL were similar in both groups and did not change after albuterol. We con clude that in asthma, Raw comprises the majority (> 70%) of RL at breathing frequencies. The relative contributions of Raw and Rti to RL appear to be independent of the degree of smooth muscle constriction.