Characteristics of muscle nerve sympathetic activity during general anaesthesia in humans.

General anaesthesia influences the cardiovascular system at different levels. To increase our knowledge of how sympathetic outflow is affected by anaesthetic interventions, we used direct microelectrode recordings of muscle sympathetic activity (MSA) in the peroneal nerve in 18 patients scheduled for ENT-surgery. During induction of anaesthesia (propofol 2.1 mg/kg or methohexitone 1.4 mg/kg), all patients showed reductions in MSA (from 41 +/- 4 to 20 +/- 4 bursts/min). With endotracheal intubation there was a sudden increase of activity and in several patients the normal pulse-synchrony of MSA was lost temporarily. Maintenance of anaesthesia with nitrous oxide (n = 12) increased MSA and methohexitone (n = 2), propofol (n = 3) and isoflurane (n = 8) decreased MSA, while the effect of halothane (n = 3) varied. Baroreflex mechanisms were still operative but seemed to be depressed in relation to anaesthetic agent and depth. Laryngeal and surgical stimuli caused increases in MSA and blood pressure, lasting several minutes after the stimulation. It is concluded that the strength of MSA is profoundly influenced by the choice of anaesthetic agent. A suppression of activity is more common than an increase. Qualitatively, several sympathetic reflexes operate in a similar way during light anaesthesia as in awake subjects, but are depressed or absent during deep anaesthesia.