[Depressant effects of inhalational anesthetic halothane on autonomic nervous activities]

To evaluate the effects of halothane on central nervous function, preganglionic sympathetic and vagal tones (ST, VT), ganglionic transmission and somato-sympathetic reflex potential (SSRP) were studied under different concentrations of halothane in 17 cats and 6 dogs. Compound action potentials of the cervical sympathetic trunk and the vagus nerve were recorded simultaneously and analyzed quantitatively. Relative ganglion-blocking potencies of halothane, enflurane and methoxyflurane were examined by recording evoked potentials from the postganglionic nerve of the stellate ganglion. SSRP was recorded from the cervical sympathetic trunk followed by stimulation of the ipsilateral radial nerve. Mean arterial pressure was kept constant at 100 mmHg to eliminate barostatic response. Halothane affected sympathetic and parasympathetic outflows and moderately blocked ganglionic transmission compared with other inhalational anesthetics and markedly diminished SSRP. ST and VT levels were almost equally depressed and attenuated to about 70, 60, 50 and 30% of the control level (N2O 75% in oxygen) with additional halothane concentrations of 0.5, 1.0, 1.5 and 2.0 MAC, respectively. The magnitude of SSRP was decreased to 68, 34, 24 and 13% with the similar concentrations of halothane, respectively. Halothane moderately depressed ganglionic transmission to about 70% and enflurane markedly decreased it to about 20% at anesthetic depth of 2 MAC, respectively. Methoxyflurane, however, did not depress ganglionic transmission at anesthetic depth of 4 MAC. It was demonstrated that halothane markedly depressed the sympathetic and vagal outflows from the nervous system without changing the balance between the central sympathetic and vagal tones, and decreased ganglionic transmission and somato-sympathetic reflex.