Differential effects of propofol and nitrous oxide on posterior tibial nerve somatosensory cortical evoked potentials during alfentanil anaesthesia

We have studied differential effects of propofol and nitrous oxide on posterior tibial nerve somatosensory evoked potentials (PTN-SEP) during a continuous infusion of alfentanil. In study 1, 14 patients received initially 66% nitrous oxide in oxygen, which was replaced 90 min after incision by propofol 6 mg kg^-1 h^-1. This substitution resulted in a significant increase in mean P1N1 amplitude from 1.01 (SD 1.14) μV to 2.61 (2.17) μV (P < 0.01), while amplitudes of later peaks were unaffected. Latencies of peaks N1, P2 and N2 increased after the substitution. In study 2, 30 patients undergoing spinal surgery received either alfentanil-nitrous oxide anaesthesia (group l, n = 15) or alfentanil-propofol anaesthesia (group ll, n = 15). P1N1 amplitude was significantly greater in group ll (3.24 (1.08) μV) than in group l (1.64 (0.97) μV) (P < 0.01). Latencies of peaks P2 and N2 were of significantly greater duration in group II than in group I. Because early cortical PTN-SEP peaks were preserved better during alfentanil-propofol anaesthesia we conclude that this combination may be a suitable alternative to alfentanil-nitrous oxide anaesthesia, when spinal cord function monitoring with PTN-SEP is indicated.