Effect of muscle constriction on flow-limiting collapse of isolated canine trachea

Before and after trachealis muscle contraction, induced by application of acetylcholine, the relationship of changing pressures to flow through excised canine tracheas was examined in a mechanical model simulating isovolume pressure-flow relationships in vivo. At the same time, the change in cross-sectional lumen area at a point in the downstream third of the trachea was measured using slit-lamp transillumination and cinematographic measurements. Trachealis muscle contraction reduced the uncompressed lumen area but inhibited dynamic collapse as driving pressures increased. As a result, following contraction, the initial slope of the tracheal pressure-flow (TPF) curve changed, indicating increase in resistance at low flows, but higher flows were achieved at higher driving pressures. Results suggest that smooth muscle tone of large airways may affect maximum expiratory flow (Vmax) and explain apparent discrepancies between in vivo measurements of resistance and Vmax. The magnitude of the Bernoulli effect contributing to airway narrowing was demonstrated by measurements of lateral airway pressures well in excess of driving pressure.