Respiratory effects of cold air breathing in anesthetized cats

Respiratory effects of cold air breathing were studied in anesthetized cats. Two different protocols were used: the air temperature was either lowered in an isolated segment, constituted by the larynx and oropharynx or in lower airways, so that the cats inspired the cold air directly. Temperatures ranged between 37 and 8 degrees C (first protocol) or between 37 and 15 degrees C (second protocol). When the temperature fell below 15 degrees C in the upper segment, marked increase in lung resistance occurred, without any significant changes in ventilatory variables nor in diaphragmatic electrical activity. The section of superior laryngeal nerves abolished this bronchomotor effect. In present experimental circumstances, thermal changes measured in lower airways when cats breathed cold air were mainly located in the cervical trachea. An increase in lung resistance and weak but significant changes in the diaphragmatic electromyogram began when the inspired air temperature fell below 25 degrees C. A selective local block of conduction in small vagal fibres by procaine or section of vagus nerves suppressed all these effects. In all cases the cold-induced changes in lung mechanics began very early (less than 10 sec) but continued for few minutes after the physiological temperature range had been restored in airways. The present data strongly suggest that the bronchomotor response to cold air breathing is a reflex, mediated by afferent fibres in the superior laryngeal nerves and in the vagus nerves.