Effects of halothane, thiamylal, and ketamine on central sympathetic and vagal tone

The effects of increasing doses of halothane, thiamylal, and ketamine on central sympathetic tone (ST) and vagal tone (VT) in cats were studied. Compound action potentials were recorded simultaneously from the cervical sympathetic trunk and the vagus nerve. After full-wave rectification, they were integrated for continuous monitoring of the tonic levels of activity. ST and VT changed characteristically with different anesthetics. Halothane depressed ST and VT equally to approximately 70%, 60%, and 30% of the control level (70% N2O in O2) at end-tidal halothane concentrations of 0.5%, 1.0%, and 2.0%, respectively. When thiamylal was given intravenously at incremental doses (3, 6, 9, and 12 mg/kg), ST was markedly reduced to 10% of the control level. The reduction in VT was relatively small and the autonomic balance shifted in the vagodominant direction. Intravenous ketamine (2, 4, 6, and 8 mg/kg) changed neither ST nor VT significantly. Halothane and thiamylal markedly reduced central sympathetic and vagal outflows that play a role in peripheral homeostatic regulation. We suggest that these two anesthetics effect attenuated or altered autonomic regulation. Ketamine produced little change in central autonomic outflow.