Effect of upper airway cooling and CO₂ on diaphragm and geniohyoid muscle activity in the rat

Upper airway (UA) reflexes play an important role in regulating breathing and UA patency, but the effects of UA CO₂ and cooling on ventilation and UA muscle activity are controversial. Diaphragm and geniohyoid electromyographic activities were recorded in anaesthetized rats, breathing spontaneously through a low-cervical tracheostomy. Warmed, humidified air containing 0 or 10% CO₂ and cooled, room humidity air were applied at constant flow to the UA through a high-cervical tracheostomy. Spontaneous tracheal airflow, UA airflow and temperature, blood pressure, and rectal temperature were recorded. In all animals, the geniohyoid muscle had phasic inspiratory activity, which slightly preceded diaphragmatic activity. CO₂ had no effect on mean peak integrated diaphragmatic activity and variable effects on geniohyoid activity. The coefficients of variation of these activities were unaffected by CO₂. Similar results were obtained following bilateral mid-cervical vagotomy. Cool air decreased respiratory frequency (78 ± 8%) (mean ± SD % of control), peak inspiratory flow (78 ± 5%) and diaphragmatic activity (77 ± 4%), and increased geniohyoid activity (149 ± 11%). Cutting the superior laryngeal nerves abolished these effects. In conclusion, whilst moderate upper airway cooling inhibits breathing and excites geniohyoid muscle activity, upper airway carbon dioxide has minimal effect.