PCO₂ affects tracheal tone during apnea in anesthetized dogs

We hypothesized that CO₂, like hypoxia and withdrawal of pulmonary slowly adapting receptor input, would cause tracheal constriction during neural apnea (absence of phrenic activity). In seven anesthetized paralyzed dogs ventilated to neural apnea, we increased arterial PCO₂ (Pa(CO₂) in steps by adding CO₂ to the inspirate while keeping ventilation constant. Increases in Pa(CO₂) caused tracheal constriction during neural apnea in all dogs; 69 ± 26 (SD)% of the change in tracheal diameter occurred during neural apnea. Average sensitivity of tracheal diameter to CO₂ was 0.44 mm/Torr Pa(CO₂). Our data suggest that central chemoreceptor inputs to brain stem neurons controlling smooth muscle of the extrathoracic airway bypass central mechanisms generating inspiration.