Individual Canine Airways Responsiveness to Aerosol Histamine and Methacholine in Vivo

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Abstract
Inhalational challenges to histamine and methacholine (Mch) are commonly used to measure airway reactivity. In this study, we investigated the serial distribution of individual airway responsiveness in vivo following stimulation with histamine or methacholine by directly measuring airway area with high resolution computed tomography (HRCT). Anesthetized dogs were challenged on separate days to increasing aerosol concentrations of histamine or Mch. Airway areas of 13 or 14 individual airways in each dog ranging in size from 1.1 to 16.9 mm in diameter at baseline were measured. Dose response curves for each agonist showed a similar responsiveness to each agonist over all airway sizes measured. There was a slightly greater individual airway responsiveness to Mch throughout the airway tree, and this greater responsiveness was also present in those airways that are responsible for the decreased Cdyn observed with Mch challenge. We conclude that in vivo aerosol challenges with histamine or methacholine do not lead to preferential construction of large or small airways.