Vasomotion influences airflow in peripheral airways

The purpose of this study was to test the hypothesis that blood flow, by its effect on blood volume, influences airflow resistance in peripheral airways. In conscious ewes, forced sinusoidal flow oscillations (5 Hz) were applied through a balloon-tipped, dual-channel fiberoptic bronchoscope placed in a segmental bronchus, and peripheral airflow resistance (R(p)) was determined from flow and bronchial pressure. Drugs with predominant vascular or airway smooth muscle effects were administered locally through the bronchoscope. Nitroglycerin (NTG) produced a dose-dependent increase in mean R(p) (+288% at 1,000 μg), which was blocked by methylene blue (p<0.05) and not reversed by atropine. Carbachol (CARB) also increased mean R(p) in a dose-dependent manner (+605% at 400 μg); this effect was not blocked by methylene blue, but it was reversed by atropine (p<0.05). The increase in mean R(p) after a single dose of NTG (250 μg) was sustained for at least 20 min and transiently reversed by vasopressin (0.2 units, p<0.05) but not by isoproterenol (100 μg). Conversely, the sustained increase in R(p) after a single dose of CARB (50 μg) was transiently reversed by isoproterenol (p<0.05) but not by vasopressin. We conclude that NTG increased R(p) by vasodilation and CARB by bronchoconstriction. This supports the hypothesis that vasodilation limits airflow in the lung periphery, presumably because of vascular congestion.