The effect of neuroleptanalgesia (droperiodol-fentanyl) on motor potentials evoked by transcranial magnetic stimulation in the monkey.

Motor evoked potentials (MEPs) after transcranial magnetic stimulation (TMS) have been examined in 12 monkeys under neuroleptanalgesia (NLA). Compound muscle action potentials were recorded from abductor policis brevis (APB) and gastroncnemius (GN) muscles contralateral to the stimulation site. After obtaining baseline tracings during emergence from methohexitone, 10 mg/kg i.m., NLA was induced using droperiodol, 0.3 mg/kg i.v. followed by fentanyl, 0.006 mg/kg i.v. Sequential MEP recordings were obtained 10 min after i.v. droperiodol, 2, 8, and 16 min after i.v. fentanyl, and during recovery. Replicable TMS MEPs were consistently recorded under NLA. However, droperidol and fentanyl caused significant stimulation threshold elevation, amplitude depression, and latency delay compared to control values (p <0.01). Ten minutes after droperiodol administration, the APB-GN threshold, amplitude, and latency values (mean +/- SD) were 0.81 +/- 0.2-0.84 +/- 0.1 T (baseline 0.57 +/- 0.1-0.59 +/- 0.1 T), 3.4 +/- 2.1-4.0 +/- 2.5 mV (baseline 8.0 +/- 3.7-9.0 +/- 3.7 mV), and 15.8 +/- 1.3-21.1 +/- 1.2 ms (baseline 14.9 +/- 1.2-20.1 +/- 1.3 ms), respectively. Addition of fentanyl resulted in further response deterioration. Two minutes after fentanyl injection, the APB-GN threshold, amplitude, and latency values were 0.88 +/- 0.18-0.95 %% 0.15 T, 2.1 +/- 1.7-2.0 +/- 2.1 mV, and 16.0 +/- 1.4-21.9 +/- 1.3 ms, respectively. Subsequent MEPs revealed gradual response improvement but, in contrast to baseline, remained markedly altered (p <0.05). During the recovery period (53 +/- 6 min), the APB-GN threshold, amplitude, and latency measurements were 0.66 +/- 0.1-0.77 +/- 0.2 T, 4.4 +/- 3.1-4.2 +/- 2.9 mV, 15.5 %% 1.4-20.9 +/- 1.7 ms, respectively. We conclude that, in a primate model, NLA maintains measurable TMS MEPs. Nevertheless, droperiodol and fentanyl produce significant and prolonged response alterations. Knowledge of these changes, while administering NLA drugs intraoperatively, is essential to interpretation of MEP data.