Dose-dependent effects of halothane on the phrenic nerve responses to acute hypoxia in vagotomized dogs

Background: Previous studies in dogs and humans suggest that the carotid body chemoreceptor response to hypoxia is selectively impaired by halothane. The present studies in an open-loop canine preparation were performed to better delineate the effects of anesthetic concentrations of halothane an the carotid body chemoreceptor-mediated phrenic nerve response to an acute hypoxic stimulus. Methods: Three protocols were performed to study the effects of halothane anesthesia on the phrenic nerve response to 1 min of isocapnic hypoxia (partial pressure of oxygen [Pa-O2] at peak hypoxia, 35-38 mmHg) in unpremedicated, anesthetized, paralyzed, vagotomized dogs during constant mechanical ventilation. In protocol 1, the dose-dependent effects of halothane from 0.5-2.0 minimum alveolar concentration (MAC) on the hypoxic response during moderate hypercapnia (partial pressure of carbon dioxide [Pa-CO2], 60-65 mmHg) were studied in 10 animals. In protocol 2, the hypoxic responses at 1 MAC halothane near normocapnia (Pa-CO2, 40-45 mmHg) and during moderate hypercapnia were compared in an additional four animals. In protocol 3, the hypoxic response of 4 of 10 dogs from protocol 1 was also studied under sodium thiopental (STP) anesthesia after they completed protocol 1. Results: Protocol 1: Peak phrenic nerve activity (PPA) increased significantly during the hypoxic runs compared with the isocapnic hyperoxic controls at all halothane doses. The phrenic nerve response to the hypoxic stimulus was present even at the 3 MAC dose. Protocol 2: The net hypoxic responses for the two carbon dioxide background levels at 1 MAC were not significantly different. Protocol 3: The net hypoxic response of PPA. for the STP anesthetic was not significantly different from the 1 MAC halothane dose, Bilateral carotid sinus denervation abolished the PPA response to hypoxia. Conclusions: The phrenic nerve response to an acute, moderately severe isocapnic hypoxic stimulus is dose-dependently depressed but not abolished by surgical doses of halothane, This analysis does not suggest a selective depression of the carotid body chemoreceptor response by halothane. The observed hypoxic phrenic response was mediated hy the carotid body chemoreceptors in vagotomized dogs because bilateral carotid sinus denervation abolished all increases in PPA.