Effects of halothane on the phrenic nerve responses to carbon dioxide mediated by carotid body chemoreceptors in vagotomized dogs.

BACKGROUND

Previous studies in dogs showed that the phrenic nerve response to an acute hypoxic stimulus was dose dependently depressed by 0.5-2.0 minimum alveolar concentration (MAC) of halothane but not abolished. Because a carbon dioxide stimulus is transduced by a different mechanism in the carotid body chemoreceptors (CBCRs) than is a hypoxic stimulus, inhalational anesthetics may preferentially depress one of these transduction processes, the central neuronal processing, or both, of the integrated responses to these two types of inputs.

METHODS

Carotid body chemoreceptor stimulation was produced by short (1-1.5 s), bilateral, 100% carbon dioxide in saline infusions into the carotid arteries during neural inspiration in unpremedicated, halothane-anesthetized, paralyzed, vagotomized dogs during constant mechanical ventilation. The phrenic neurogram quantified the neural inspiratory response. Four protocols were performed in the study: (1) the dose-dependent effects of halothane anesthesia (0.5-2.0 MAC) during hyperoxic hypercapnia on phrenic nerve activity, (2) the effects of three background levels of the partial pressure of carbon dioxide (PaCO2) on the magnitude of the carbon dioxide infusion responses at 1 MAC halothane, (3) the effects of anesthetic type on the magnitude of the carbon dioxide infusion response, and (4) the effects of CBCR denervation.

RESULTS

Peak phrenic nerve activity (PPA) increased significantly during the carbon dioxide-stimulated phrenic burst in protocols 1-3; after denervation there was no response (protocol 4). Halothane produced a dose-dependent reduction in the PPA of control and carbon dioxide infusion-stimulated phrenic bursts and in the net carbon dioxide response. The net PPA responses for the different PaCO2 background levels were not different but were somewhat larger for sodium thiopental anesthesia than for 1.0 MAC halothane.

CONCLUSIONS

The phrenic nerve response to an acute, severe carbon dioxide stimulus was dose dependently depressed by surgical doses of halothane. The observed responses to carbon dioxide infusion were mediated by the CBCRs because they were eliminated by CBCR denervation. These results suggest that the CBCR transduction and central transmission of the carbon dioxide signal in terms of inspiratory excitatory drive are not abolished at surgical levels of halothane anesthesia.