Effect of hypercapnia on upper airway resistance and collapsibility in anesthetized dogs.

The upper airway (UAW) is intrinsically unstable and susceptible to collapse when the negative inspiratory intraluminal pressure exceeds the stabilizing forces which prevent obstruction. In the present study we evaluated mechanisms by which UAW patency is maintained in the presence of increased inspiratory flows when respiration is stimulated. In seven anesthetized dogs breathing spontaneously through a low tracheostomy, the UAW was isolated by a second tracheostomy directed rostrally. UAW pressure-flow relationship and stability against collapse were evaluated during steady flow in the inspiratory direction while the animals were breathing 100% O2 or a hypercapnic gas mixture. The pressure-flow curves of the isolated UAW demonstrated the characteristic pattern of collapsible tubes. Steady state hypercapnia resulted in lower UAW resistance during both inspiration and expiration. UAW resistance decreased linearly as PCO2 and ventilation increased over the course of CO2 rebreathing. In addition, during hypercapnia the critical negative intraluminal pressure required to induce UAW collapse and obstruction increased from -4.3 +/- 0.9 to -8.5 +/- 1.5 SE cm H2O (p less than 0.01), indicating increased stability of the UAW. Since hypercapnia is known to stimulate UAW muscles, our findings suggest that increased UAW muscle activity improves UAW patency both by decreasing their resistance to airflow, and by increasing UAW walls rigidity and stability against collapse.