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

The anaesthetic-sparing activity of dexmedetomidine during isoflurane anaes thesia was examined, using the end-point of lack of response to tetanic ner ve stimulation. Nine subjects were given two doses of dexmedetomidine (targ et 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 somatosens ory (SER) evoked responses at end-tidal isoflurane concentrations of 0.2-1. 4%. Pa and P25-N35 amplitudes increased as isoflurane concentration was red uced (P<0.001). Dexmedetomidine had no significant effect on this relations hip. In contrast, P15-N20 (SER) amplitude increased (P<0.001) as isoflurane concentration was reduced. The dose of dexmedetomidine had a significant i nteraction with this trend (P<0.002). Decreasing the concentration of isofl urane at the high dose of dexmedetomidine had less impact on P15-N20 amplit ude than decreasing isoflurane at the low dose or with saline. The mechanis m by which dexmedetomidine spares isoflurane is discussed in the light of t hese evoked response changes.