The influence of nitrous oxide to supplement fentanyl low-dose propofol anesthesia on transcranial myogenic motor-evoked potentials during thoracic aortic surgery

Objective: Intraoperative monitoring of myogenic motor evoked potentials to transcranial electrical stimulation (tc MEPs) is a new method to assess the integrity of the motor pathways. The authors studied the effects of 50% nitrous oxide (N2O) and a low-dose propofol infusion on tc MEPs paired electrical stimulation during fentanyl anesthesia with partial neuromuscular blockade. Design: Cross-over study. Setting: St Antonius Hospital, Nieuwegein, The Netherlands. Participants: Ten patients scheduled to undergo surgery on the thoracoabdominal aorta were studied; 6 women aged 54 to 69 years and 4 men aged 68 to 77 years. Interventions After achieving a stable anesthetic state and before surgery tc MEPs were recorded during four 15-minute periods: (I) air/oxygen (O-2; F1O2 = 50%); propofol target blood concentration, 0.5 mu g/mL; (II) N2O/O-2 (F1O2 = 50%); propofol target blood concentration, 0.5 mu g/mL;(III) N2O/O-2 (F1O2 = 50%; propofol target blood concentration, 1.0 mu g/mL; and (IV) air/O-2 (F1O2 = 50%); propofol target blood concentration, 1.0 mu g/mL. Measurements and Main Results: Tc MEPs were recorded from the right extensor digitorum communis muscle and the right tibialis anterior muscle. The right thenar muscle was used for recording the level of relaxation; the T1 response was maintained at 40% to 70% of the control compound muscle action potential. There was no significant difference in onset latency among the four phases. The addition of N2O and doubling the target propofol infusion to 1.0 mu g/mL resulted in a 40% to 50% reduction of tc MEP amplitude recorded in the extensor digitorum communis muscle and tibialis anterior muscle (p < 0.01). During each phase, tc MEPs could be elicited and interpreted, except in one patient, in whom no tc MEPs could be elicited in the leg because of technical problems. Conclusion: The data indicate that tc MEP monitoring is feasible during low-dose propofol, fentanyl/50% N2O in O-2 anesthesia and partial neuromuscular blockade. Copyright (C) 1999 by W.B. Saunders Company.