The volume fraction of the extracellular matrix (ECM) within the layer of airway smooth muscle (ASM) is increased in subjects with fixed airflow obstruction. We postulated that changes in ECM within the ASM layer will impact force transmission during induced contraction and / or in response to externally applied stresses like a deep inspiration (DI). Subjects were patients undergoing lung resection surgery who were categorized as unobstructed (n=12) or obstructed ‘fixed’ (n=6), based on pre-operative spirometry. The response to a DI, assessed by the ratio of isovolumic flows from maximal and partial inspirations (M/P ratio), was also measured pre-operatively. M/P ratio was reduced in the obstructed group (p=0.02).Post-operatively, bronchial segments were obtained from resected tissue and luminal narrowing to acetylcholine and bronchodilation to simulated DI were assessed . Airway wall dimensions and the volume fraction of ECM within the ASM were quantified. Maximal airway narrowing to acetylcholine (p=0.01) and the volume fraction of ECM within the ASM layer (p=0.02) were increased in the obstructed group, without a change in ASM thickness. While bronchodilation to simulated DI was not different between obstructed and unobstructed groups, it was correlated with increased M/P ratio (bronchodilation / less bronchoconstriction) (p=0.03). The volume fraction of ECM was inversely related to FEV%predicted (p=0.04) and M/P ratio (p=0.01). Results show that in subjects with fixed airflow obstruction the mechanical behaviour of the airway wall is altered, and there is a contemporaneous shift in the structural composition of the ASM layer.

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