Respiratory reflexes in response to nasal administration of halothane to anesthetized, spontaneously breathing dogs

Objective-To characterize and determine the sensory innervation of respirat ory reflexes elicited by nasal administration of halothane to dogs. Animals-10 healthy Beagles. Procedure-Dogs underwent permanent tracheostomy and, 2 to 3 weeks later, we re anesthetized with thiopental and a-chloralose administered IV. The nasal passages were functionally isolated so that halothane could be administere d to the nasal passages while dogs were breathing 100% O-2 via the tracheos tomy. Respiratory reflexes in response to administration of halothane at co ncentrations of 1.25, 1.75, and 2.5 times the minimum alveolar concentratio n (MAC), and 5% (administered in 100% O-2 at a flow rate of 5 L/min) were r ecorded. Reflexes in response to administration of 5% halothane were also r ecorded following transection of the infraorbital nerve, transection of the caudal nasal nerve, and nasal administration of lidocaine. Results-Nasal administration of halothane induced an inhibition of breathin g characterized by a dose-dependent increase in expiratory lime and a resul tant decrease in expired volume per unit time. Effects were noticeable imme diately after the onset of halothane administration and lasted until its ce ssation. Reflex responses to halothane administration were attenuated by tr ansection of the caudal nasal nerve and by nasal administration of lidocain e, but transection of the infraorbital nerve had no effect. Conclusions and Clinical Relevance-Nasal administration of halothane at con centrations generally used for mask induction of anesthesia induces reflex inhibition of breathing. Afferent fibers in the caudal nasal nerve appear t o play an important role in the reflex inhibition of breathing induced by h alothane administration.