Effect of time and dose on scalp-recorded somatosensory evoked potential wave augmentation by etomidate

Bolus etomidate transiently increases the amplitude of scalprecorded somatosensory evoked potentials (SSEPs). The reproducibility of this augmentation and its dose-response relationship are unknown. In unpremedicated patients, we studied the effect on the SSEP of repetitive administration of single doses of etomidate (0.1 mg/kg i.v. bolus) in six patients and increasing doses of etomidate in six additional patients. Anesthesia was induced with fentanyl (15-20 microg/kg i.v.) plus thiopental (1-2 mg/kg i.v.) and maintannined with 0.4-0.8% isoflurane in oxygen, and the surgical incision was infiltrated with bupivicaine (0.5% without epinephrine). Etomidate administration was delayed for 30 min following anesthesia induction. In group 1, 0.1 mg/kg etomidate was administered intravenously as a bolus three times at 30-min intervals. In group 2, 0.05, 0.1, or 0.2 mg/kg was administered at 30-min intervals in random order in each patient. SSEPs were measured immediately before and once each minute for 5 min after etomidate administration following nondominant median nerve stimulation. In group 1, administration of 0.1 mg/kg etomidate (three trials) increased latency of an early negative wave (N20; latency approximately 20 ms) and a positive wave following N20 (P23; latency approximately 23 ms) by 1.0-1.4 and 1.3-2.6 ms, respectively (p < 0.05). P15N20 amplitude was increased by approximately 50% (range 36-76%; p < 0.05) and N20P23 amplitude was increased to 174% of control (range 173-178%; p < 0.05) and the amplitude increase was similar during the three etomidate administrations for both P15N20 and N20P23. Latency remained elevated by approximately 1.5 ms and amplitude remained elevated (P15N20 = 138%; N20P23 = 150%) 5 min following injection. Mean arterial blood pressure was unchanged by 0.1 mg/kg etomidate. In group 2, 0.05 mg/kg etomidate altered neither amplitude nor latency. However, 0.1 and 0.2 mg/kg increased N20P23 amplitude to 161 +/- 33 and 230 +/- 10% of control (p < 0.05), respectively. N20 and P23 latency were increased by 0.1 mg/kg etomidate by approximately 1.0 ms, while 0.2 mg/kg increased N20 latency by 1.0 ms and P23 latency by 1.5 ms. Bolus administration of etomidate (0.1 mg/kg) reproducibly increased SSEP amplitude and a larger dose (0.2 mg/kg) further increased amplitude augmentation. Thus, intermittent injection of etomidate can be used to augment small SSEP waves with reproducible increases in wave amplitude.