Effects of dexmedetomidine on isoflurane requirements in healthy volunteers.: 2:: Auditory and somatosensory evoked responses

The anaesthetic-sparing activity of dexmedetomidine during isoflurane anaesthesia was examined, using the end-point of lack of response to tetanic nerve stimulation. Nine subjects were given two doses of dexmedetomidine (target plasma concentrations of 0.3 ng ml(-1) and 0.6 ng ml(-1), respectively) and saline an separate occasions. We measured auditory (AER) and somatosensory (SER) evoked responses at end-tidal isoflurane concentrations of 0.2-1.4%. Pa and P25-N35 amplitudes increased as isoflurane concentration was reduced (P<0.001). Dexmedetomidine had no significant effect on this relationship. In contrast, P15-N20 (SER) amplitude increased (P<0.001) as isoflurane concentration was reduced. The dose of dexmedetomidine had a significant interaction with this trend (P<0.002). Decreasing the concentration of isoflurane at the high dose of dexmedetomidine had less impact on P15-N20 amplitude than decreasing isoflurane at the low dose or with saline. The mechanism by which dexmedetomidine spares isoflurane is discussed in the light of these evoked response changes.