Etomidate dose-response on somatosensory and transcranial magnetic induced spinal motor evoked potentials in primates

There is growing interest and need to monitor reliably both motor (MEP) and somatosensory (SEP) evoked potentials under anesthesia. On a pre-established primate model, the present study examined the effect of incremental etomidate (ET) dosages on spinal neural MEPs to transcranial magnetic stimulation (TMS) and posterior tibial rate (PTN) SEPs. Through a small thoracic T11-T12 laminotomy, an insulated double bipolar electrode was inserted epidurally in seven cynomolgus monkeys. Spinal TMS-MEPs, PTN-SEPs, and frontal EEG were tested against graded increase of ET doses. Etomidate 0.5 mg kg(-1) i.v. was initially given and followed by 30 min continuous infusion of 0.01 mg kg(-1) min(-1), 0.018, 0.032, 0.056, 0.1, and 0.18 mg kg(-1) min(-1) in that order. Measurable spinal MEPs and SEPs were recorded under deep ET anesthesia (total 12.38 mg kg(-1) cumulative dose over 180 min). The EEG showed marked slow wave and graded burst suppression at cumulative dose of greater than or equal to 3.14 mg kg(-1). The direct (D) and subsequent initial indirect (I) waves (I-1, I-2, I-3) were reproducible at doses < 0.18 mg kg(-1) min(-1) infusion. The latter I-waves (I-4 and I-5) showed graded loss at infusion dosage 0.056 mg kg(-1) min(-1). Etomidate remains an anesthetic of attractive features in neuroanesthesia. In the primate model, neural MEPs-SEPs were reproducible despite the exceedingly high dose of ET and markedly depressed EEG. Moreover, MEP-SEP can be monitored during ET burst-suppressive neuroprotective state. The study may set a model in humans for intra-operative multi-modality neurophysiologic recording under ET-based anesthesia.