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 (O2; F(I)O2 = 50%); propofol target blood concentration, 0.5 microg/mL; (II) N2O/O2 (F(I)O2 = 50%); propofol target blood concentration, 0.5 microg/mL; (III) N2O/O2 (F(I)O2 = 50%; propofol target blood concentration, 1.0 microg/mL; and (IV) air/O2 (F(I)O2 = 50%); propofol target blood concentration, 1.0 microg/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 microg/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 02 anesthesia and partial neuromuscular blockade.