Airway and Lung Tissue Mechanics in Asthma

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Abstract
We examined the partitioning of total lung resistance (R L ) into airway resistance (Raw) and tissue re- sistance (Rti) in patients with mild to moderate asthma (baseline FEV 1 , 54 to 91% of predicted) be- fore and after albuterol inhalation. An optimal ventilator waveform was used to measure R L and lung elastance (E L ) in 21 asthmatics from approximately 0.1 to 8 Hz during tidal excursions. Analysis of the R L and E L provided separate estimates of airway and lung tissue properties. Eleven subjects, classified as Type A asthmatics, displayed slightly elevated R L but normal E L. Their data were well described with a model consisting of homogeneous airways leading to viscoelastic tissues before and after al- buterol. The other 10 subjects, classified as Type B asthmatics, demonstrated highly elevated R L and an E L that became highly elevated at frequencies above 2 Hz. These subjects required the inclusion of an airway wall compliance in the model prealbuterol but not postalbuterol. This suggests that the Type B subjects were experiencing pronounced constriction in the periphery of the lung, resulting in shunting of flow into the airway walls. Spirometric data were consistent with higher constriction in Type B subjects. Both groups demonstrated significant (p , 0.05) decreases in Raw and tissue damp- ing after albuterol, but tissue elastance decreased only in the Type B group. The percent contribu- tions of Raw and Rti to R L were similar in both groups and did not change after albuterol. We con- clude that in asthma, Raw comprises the majority ( . 70%) of R L at breathing frequencies. The relative contributions of Raw and Rti to R L appear to be independent of the degree of smooth muscle constriction. Kaczka DW, Ingenito EP, Israel E, Lutchen KR. Airway and lung tissue mechanics in asthma: effects of albuterol. AM J RESPIR CRIT CARE MED 1999;159:169-178.