Effects of halothane and isoflurane anesthesia on sympathetic adrenal nerve responses to carbon dioxide challenge in rats.

We studied the influence of two volatile anesthetics, halothane and isoflurane, on the circulatory and sympathetic nerve responses to carbon dioxide (9% CO2) in rats. Systolic blood pressure was depressed throughout the CO2 challenge and after an initial reduction, a gradual increase was observed in heart rate. Sympathetic adrenal nerve action potentials (SANA) significantly increased in contrast to negative responses in the circulatory functions. SANA responses against time were trapezoid in shape. There were no significant differences in SANA responses between 1% (1 MAC) and 1.5% (1.5 MAC) halothane groups, nor between 1.4% (1 MAC) and 2% (1.5 MAC) isoflurane groups. Halothane and isoflurane, therefore, did not produce dose-dependent effects on sympathetic response to hypercapnia within these concentrations. The maximum changes in SANA from the baseline values were 110% and 40% for the halothane and isoflurane groups, respectively. The sympathetic reflex response to hyperacapnia was retained at 1.5 MAC for both anesthetics, though isoflurane depressed these responses more markedly than halothane. Our results suggest that halothane is a more preferable anesthetic than isoflurane when viewed from the standpoint of preservation of sympathetic nerve response in such undesirable situations as severe hypercapnia occurring during anesthesia.