Dissolving methohexital in a lipid emulsion reduces pain associated with intravenous injection

Abstract

Pain often accompanies intravenous injection of 1% methohexital. The aim of the present study was to test whether pain on injection could be reduced by dissolving methohexital in a lipid emulsion (study A) and whether this would affect anesthetic potency (study B). In study A, 24 healthy volunteers, 36 ± 1 yr (mean ± SE), were given 1 ml 1% methohexital in saline, 1 ml 1% methohexital in lipid emulsion, and 5 ml 0.1% methohexital in saline in random order. The injections were given in a small vein in the forearm at 5- min intervals. One minute after each injection, the subject was asked to assess the injection pain on a visual analog scale (0-100 mm). The pain score (median [range]) was 44.5 (0-77) after 1% methohexital in saline, 0.5 (0-26) after 1% methohexital in a lipid emulsion, and 1.0 (0-26) after 0.1% methohexital in saline. The pain score for 1% methohexital in saline was significantly greater than those for the other two solutions (P < 0.001 for each comparison). In study B, 42 patients, 41 ± 3 yr, were given 1% methohexital in lipid emulsion (n = 22) or 1% methohexital in saline (n = 20). A bolus of either solution was administered over 10 s, and the patient was considered asleep if there was no gross movement or response to verbal command 40-70 s after injection. Using these criteria, the methohexital dose required for satisfactory induction in 50% of patients (ED50) was 1.2 ± 0.1 mg/kg for 1% methohexital in lipid emulsion and 1.1 ± 0.1 mg/kg for 1% methohexital in saline (not significant). It is concluded that dissolving methohexital in a lipid emulsion almost abolishes pain on injection but does not reduce anesthetic potency.