Mechanical properties of the passive pharynx in Vietnamese pot-bellied pigs. II. Dynamics

We described the dynamic mechanical properties of the passive pharynx in Vietnamese pot-bellied pigs and the effects of caudal tracheal displacement. During general anesthesia and neuromuscular blockade, airflow through the upper airway (V)over dot and pharyngeal cross-sectional area were measured during ramp decreases in pressure downstream from the pharynx (Pdown). Measurements were made with 0, 1, and 2 cm of caudal tracheal displacement. Airflow limitation and/or negative pressure dependence (NPD) were observed in all animals. Tracheal displacement (2 cm) increased maximal (V)over dot ((V)over dot(max)) by 205.1+/-105.1% (P<0.05) relative to the value with no displacement and increased the magnitude of NPD, expressed as percent decrease in (V)over dot from (V)over dot(max), from 22.9&PLUSMN;27.4 to 56.6&PLUSMN;37.5% (P<0.05). Initial decreases in Pdown narrowed all levels of the pharynx, but, once (V)over dot(max) was reached, further decreases in Pdown narrowed the hypopharynx but not the nasopharynx and oropharynx. We conclude that the hypopharynx is the flow-limiting site in the pig pharynx. Tracheal displacement not only improved airflow dynamics as (V)over dot(max) increased but also resulted in pronounced NPD.