This randomized study was designed to evaluate the analgesic effectiveness and to determine the optimal dose of remifentanil when administered as intermittent bolus injection during infiltration of local anesthetics for patients undergoing plastic surgery.

Forty five ASA I or II patients undergoing plastic surgery were randomly assigned to one of the three bolus doses of remifentanil injection. Five minutes after receiving midazolam 0.05 mg/kg IV, remifentanil was injected intravenously one minute before the infiltration of local anesthetics: A bolus of remifentanil 0.25 µg/kg was injected in Group R0.25 (n = 15), 0.50 µg/kg in Group R0.5 (n = 15), or 0.75 µg/kg in Group R0.75 (n = 15). After assessment of the patients' Observer's Assessment of Alertness/Sedation (OAA/S) scale, the subjective pain level during local anesthetic infiltration was evaluated. Respiratory rate, SpO₂, mean blood pressure, and heart rate were recorded during the study period. Postoperative nausea/vomiting and patient's satisfaction were checked by telephone call.

OAA/S scale values were similar among the three groups (3-4). The number of the patients who complained of moderate or severe pain level is significantly higher in group R0.25 than in group R0.5 or group R0.75 (P < 0.05). Adverse events such as hypoventilation, desaturation, and bradycardia were observed with significantly high frequency in R0.75 than in other two groups (P < 0.05). All three groups showed no occurrence of hypotension or postoperative nausea/vomiting.

The above results demonstrate that intermittent bolus injection of remifentanil 0.50 µg/kg is recommended to accomplish desirable pain control during local anesthetic infiltration in those who received midazolam 0.05 mg/kg.