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Effect of upper airway cooling and CO2 on diaphragm and geniohyoid muscle acti- vity in the rat. K.D. O'Halloran, A.K. Curran, A. Bradford. ERS Journals Ltd 1996. ABSTRACT: 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 anaes- thetized rats, breathing spontaneously through a low-cervical tracheostomy. Warmed, humidified air containing 0 or 10% CO2 and cooled, room humidity air were ap- plied at constant flow to the UA through a high- cervical tracheostomy. Spontane- ous 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 inte- grated diaphragmatic activity and variable effects on geniohyoid activity. The co- efficients of variation of these activities were unaffected by CO2. Similar results were obtained following bilateral mid-cervical vagotomy. Cool air decreased res- piratory 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.