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Maintenance of airway caliber in isolated airways by deep inspiration and tidal strains

¹Department of Biomedical Engineering, Boston University, Boston, Massachusetts; and ²Discipline of Physiology, School of Biomedical, Biomolecular, and Chemical Sciences, University of Western Australia, Crawley, Western Australia, Australia

Submitted 15 November 2007 ; accepted in final form 6 June 2008

Deep inspirations (DIs) are large periodic breathing maneuvers that regulate airway caliber and prevent airway obstruction in vivo. This study characterized the intrinsic response of the intact airway to DI, isolated from parenchymal attachments and other in vivo interactions. Porcine isolated bronchial segments were constricted with carbachol and subjected to transmural pressures of 5–10 cmH₂O at 0.25 Hz (tidal breathing) interspersed with single DIs of amplitude 5–20 cmH₂O, 5–30 cmH₂O, or 5–40 cmH₂O (6-s duration) or DI of amplitude 5–30 cmH₂O (30-s duration). Tidal breathing was ceased after DI in a subset of airways and in control airways in which no DI was performed. Luminal cross-sectional area was measured using a fiber-optic endoscope. Bronchodilation by DI was amplitude dependent; 5–20 cmH₂O DIs produced less dilation than 5–30 cmH₂O and 5–40 cmH₂O DIs (P = 0.003 and 0.012, respectively). Effects of DI duration were not significant (P = 0.182). Renarrowing after DI followed a monoexponential decay function to pre-DI airway caliber with time constants between 27.4 ± 4.3 and 36.3 ± 6.9 s. However, when tidal breathing was ceased after DI, further bronchoconstriction occurred within 30s. This response was identical in both the presence and absence of DI (P = 0.919). We conclude that the normal bronchodilatory response to DI occurs as a result of the direct mechanical effects of DI on activated ASM in the airway wall. Further bronchoconstriction occurs by altering the airway wall stress following DI, demonstrating the importance of continual transient strains in maintaining airway caliber.

bronchodilation; airway smooth muscle; bronchial segments