Halothane Depresses the Response of Carotid Body Chemoreceptors to Hypoxia and Hypercapnia in the Cat

Abstract

Halothane inhibits the ventilatory responses to hypoxia and hypercapnia. To determine whether this inhibition was mediated by peripheral chemoreceptors, the effect of halothane on the response of carotid body chemoreceptors to these stimuli was measured. Cats were decerebrated under brief halothane anesthesia, paralyzed and ventilated. Chemoreceptor activity was recorded from single- or few-fiber preparations of carotid sinus nerve, and the inspiratory drive was recorded from the whole phrenic nerve. Steady-state responses were measured at 3 levels of CO2 tension (19-92 mm Hg) during hyperoxia and at 4 levels of O2 tension (35-450 mm Hg) at a fixed PaCO2 [arterial partial pressure of CO2]. Both responses were measured before, during and after 0.5-1.0% halothane was inspired. The halothane inhalation was maintained for at least 30 min before the responses were obtained. Halothane reduced the slope of chemoreceptor response to hypercapnia to .apprx. 48% of the control slope. The response to hypoxia was reduced to .apprx. 58% of the control response. The increase in firing after i.v. nicotine (100 .mu.g), summed for 20 s, was reduced to 25% of the prehalothane control value; that after NaCN (25 .mu.g) was reduced to 17% of the control value. The effect of halothane was prompt (1/2 complete in 1-2 min) and reversible. This finding explains some of the inhibition of the ventilatory responses to hypoxia and hypercapnia caused by halothane.