Prevention of etomidate induced myoclonus Introduction Etomidate is a popular hypnotic agent, and it has a stable hemodynamic profile with minimal histamine release [1]. However, etomidate often induces spontaneous movements, especially myoclonic activity, and can enhance focal epileptogenic activity in patients with epilepsy [2]. Myoclonus has been described as the involuntary contracture of some muscle fibers, which involves a whole muscle, or different muscles of one group, leading to short observable movements of the corresponding body parts. Myoclonus is observed in 50-80% of non-premedicated patients after etomidate administration [3], and it can be a problem in patients who had open-global injury or full-stomach conditions. In patients with an open-global injury, myoclonus after etomidate administration increases the risk of vitreous prolapse because of high intraocular pressure [4]. The incidence of myoclonus can be reduced by various agents including benzodiazepines, magnesium sulfate, rocuronium and opioids [1,4-7]. Remifentanil is a selective μ-receptor agonist with similar pharmacodynamic properties to other opioid receptor agonists such as fentanyl [8,9], and it is metabolized rapidly with a very short acting duration independent of infusion time. However, it can cause adverse effect like bradycardia and/or chest wall rigidity even at the low dose of 1.0 μg/kg [8,10-12]. The aim of this study was to investigate the safe effect site concentration of remifentanil for preventing myoclonus induced by etomidate injection without generating side effects. Materials and Methods The study protocol was approved by the Institutional Review Board of the Clinical Research Ethics Committee. Adult patients with American Society of Anesthesiologists (ASA) physical status I-III and who were scheduled for elective surgery under general anesthesia were selected for this study and the three groups were demographically similar (Table 1). Patients with neuropsychological disease or abnormality of the adrenal gland were excluded. Thirty minutes before Committee. Adult patients Table 1. Demographic Data Age (yr) Gender (M/F) Weight (kg) Group N (n = 33) 41.9 ± 16.7 17/16 65.4 ± 12.4 Group R (n = 33) 44.2 ± 15.0 16/17 62.8 ± 10.9 Group Q (n = 33) 42.2 ± 18.3 16/17 65.3 ± 13.2 Group N: normal saline, Group R: pretreatment with 2 ng/ml of remifentanil, Group Q: pretreatment with 4 ng/ml of remifentanil. 128 Vol. 61, No. 2, August 2011 with American Society of Anesthesiologists (ASA) physical status I-III and who were scheduled for elective surgery under general anesthesia were selected for this study and the three groups were demographically similar (Table 1). Patients with neuropsychological disease or abnormality of the adrenal gland were excluded. Thirty minutes before arrival at the operating room, midazolam 0.05 mg/kg and glycopyrrolate 0.2 mg were injected intramuscularly as premedication. Venous access with an IV canula was placed in the patient’s antecubital fossa. On arrival at the operating room, patient monitors such as oxygen saturation (SpO2) by pulse oximeter probe, electrocardiography (ECG), and a blood pressure cuff were applied. The standard bispectral index (BIS) was monitored (BIS Quatro,Aspect Medical System Inc.) by placing the electrode on the forehead of each patient and the BIS score was monitored. The patients, who had provided written, informed consent, were allocated in a randomized fashion by sealed envelope into the 3 pretreatment groups (n = 33 per group) according to the effect site concentration of remifentanil (Ultiva, Glaxo- Wellcome, München, Germany) of 0, 2 or 4 ng/ml (Group N: 0 ng/ml, Group R: 2 ng/ml, Group Q: 4 ng/ml). A target controlled infusion (TCI) system for remifentanil was adopted to produce an accurate desired plasma concentration. Each effect site concentration (2 or 4 ng/ml) was set by an infusion pump (OrchestraⓇ, Fresenius Vial, France) with the Minto model. The concentration of 0 ng/ml of remifentanil was set by a syringe filled with saline. BP and HR were recorded at baseline, 210 seconds after injection of etomidate 0.3 mg/ kg, 30 seconds, 1 min and 3 min after intubation. When the SQI (signal quality index) was maintained over 90, the BIS was measured continuously throughout the procedure and recorded at baseline, the end of etomidate administration, and every 30 seconds after etomidate administration until 2 minutes. After preoxygenation for 1 minute, one of the solutions (remifentanil 50 μg/ml or saline) was infused through the infusion pump. Any side effects, such as cough, sedation, chest wall rigidity, or apnea, were checked by an anesthesiologist who was blinded to the groups. The Ramsay sedation scale was used for evaluating sedation and sedation was indicated by a 4 on the Ramsay sedation scale, which was no response Table 2. Ramsay Sedation Scale Score 1 2 3 4 5 6 Response Anxious or restless or both Cooperative, orientated and tranquil Responding to commands Brisk response to stimulus Sluggish response to stimulus No response to stimulus Sedation was indicated by 4 on the Ramsay sedation scale. www.ekja.org