Etomidate dose-response on somatosensory and transcranial magnetic inducedspinal 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-establish ed primate model, the present study examined the effect of incremental etom idate (ET) dosages on spinal neural MEPs to transcranial magnetic stimulati on (TMS) and posterior tibial rate (PTN) SEPs. Through a small thoracic T11 -T12 laminotomy, an insulated double bipolar electrode was inserted epidura lly 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 ane sthesia (total 12.38 mg kg(-1) cumulative dose over 180 min). The EEG showe d marked slow wave and graded burst suppression at cumulative dose of great er than or equal to 3.14 mg kg(-1). The direct (D) and subsequent initial i ndirect (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 o f attractive features in neuroanesthesia. In the primate model, neural MEPs -SEPs were reproducible despite the exceedingly high dose of ET and markedl y depressed EEG. Moreover, MEP-SEP can be monitored during ET burst-suppres sive neuroprotective state. The study may set a model in humans for intra-o perative multi-modality neurophysiologic recording under ET-based anesthesi a.