Study Objective: To measure the incidence of awareness during induction of anesthesia with etomidate and fentanyl, and to model its frequency as a function of dose of etomidate.

Design: Prospective cohort study.

Setting: Anesthesia department of a university hospital.

Patients: 30 ASA physical status I, II, and III patients undergoing elective general surgery.

Interventions: Patients were assigned to one of three groups of etomidate (0.2 mg/kg, 0.3 mg/kg, 0.4 mg/kg) and received fentanyl (2 μg/kg) and 2 × ED₉₅ of cisatracurium (0.1 mg/kg). Neuromuscular block was monitored with a peripheral nerve stimulator. Intubation was performed after maximum T₁-depression. To identify awareness, the isolated forearm technique (IFT) was used. The IFT was performed by prompting the patient every 20 seconds. Only a verified response was considered a positive IFT response. Anesthesia was maintained with isoflurane in oxygen/air and fentanyl.

Measurements and Main Results: Maximum neuromuscular block occurred after 352 ± 96 seconds and intubation was performed 424 ± 86 seconds after loss of consciousness (LOC). Awareness was dose dependent: 80% of patients receiving 0.2 mg/kg etomidate, 70% of patients receiving 0.3 mg/kg etomidate, and 20% of patients receiving 0.4 mg/kg etomidate had a positive IFT response. Awareness occurred in one patient 3 minutes after LOC, in 65% during laryngoscopy, and in 30% within the following 120 seconds. One patient had explicit recall without finding awareness unpleasant. Hemodynamic parameters did not differ between patients with a positive or a negative IFT response.

Conclusions: The incidence of awareness during bolus induction can be modeled as dose dependent. However, when combining a short-acting induction drug and a delayed-onset neuromuscular blocker, the continuous infusion of the hypnotic drug may prevent awareness during induction.