Relationship between tracheal mucosal thickness and vascular resistance in dogs

We have measured changes in tracheal mucosal thickness and tracheal vascular resistance in the dog. A probe was used to detect changes in height with time of the tracheal epithelium relative to an underlying cartilage. Tracheal vascular resistance was determined by perfusing a cranial tracheal artery at constant flow and measuring inflow pressure. Various drugs injected close-arterially were tested in 20 greyhounds anesthetized with pentobarbital sodium. Bradykinin, histamine, and methacholine significantly (P less than 0.01) decreased vascular resistance (-39.3 +/- 3.7, -47.3 +/- 4.2, and -22.5 +/- 5.2%, respectively) and increased the thickness of the mucosa (119.0 +/- 25.0, 61.9 +/- 25.0, and 46.3 +/- 6.4 micron). Substance P, vasoactive intestinal peptide, prostaglandin F2 alpha, and prostaglandin E, had large vasodilator actions (-31.4 +/- 5.0, -34.3 +/- 2.2, -21.9 +/- 2.8, and -31.5 +/- 2.4%) but only small effects on mucosal thickness (12.3 +/- 3.9, 13.0 +/- 3.4, 16.7 +/- 6.5, and 8.7 +/- 2.9 micron, respectively). Phenylephrine hydrochloride increased vascular resistance (19.8 +/- 1.7%) and decreased mucosal thickness (-23.9 +/- 3.1 micron). Thus airway vascular resistance and mucosal thickness always change in opposite directions, but drugs have different relative actions on the two variables. Even with large vasodilatations, the absolute changes in mucosal thickness were small and were unlikely to have an appreciable effect on tracheal airway resistance.