BACKGROUND: Pre-operative anxiety is known to cause an increase in plasma catecholamine levels, which, in turn, attributes to the decrease in serum K⁺ concentration. Midazolam is one of the most commonly used premedication agent for the purpose of anxiolysis in the pre-operative period. In this study, by measuring serum K⁺ concentration, we investigated the optimal intramuscular injection time for midazolam which maximally prevents the reduction of serum K⁺ concentration.

METHODS: One hundred twenty three patients undergoing breast surgery were randomly assigned to one of 5 groups. Control group (n = 24) had no premedication. 30-minute group (n = 30), 60-minute group (n = 25), 90-minute group (n = 22) and 120-minute group (n = 22) received IM injection of midazolam (0.05 mg/kg) at 30, 60, 90, and 120 minutes before induction of anesthesia, respectively. Anxiety level, serum K⁺ concentrations, blood pressures and heart rates of all patients were measured at 7:00pm of the day before surgery and immediately before induction of anesthesia.

RESULTS: Serum K⁺ concentrations of all groups showed significant decrease in the preinduction time compared with those values at 7:00 pm of the day before surgery. There were no significant changes of heart rates in all groups. Blood pressures immediately before induction of anesthesia elevated compared with those at 7"00 pm of the day before surgery in all groups. Immediately before induction of anesthesia, the K⁺ levels of 60-minute and 90-minute groups were higher than those of the control group. At this time, the rates of hypokalemia (K⁺<3.5 mEq/L) of 60-minute and 90-minute groups were lower than those of the control group. 30-minute and 60-minute groups had significantly lower anxiety levels than the control group.

CONCLUSIONS: When we consider anxiety and serum potassium levels, the optimal intramusculr injecting time of midazolam was 60 minutes before induction of anesthesia.