Sex- and age-related differences in morphine requirements for postoperative pain relief

Sex-related differences in the perception of pain and susceptibility to opioids remain a matter of debate. Intravenous morphine titration used to obtain pain relief in the immediate postoperative period is a unique clinical model for assessing the effect of sex on reported pain. Because of the wide variation in dose requirements for pain management, the authors conducted a prospective study in a large population and also assessed the effect of aging.

Intravenous morphine titration was administered as a bolus of 2 (body weight 60 kg) during the immediate postoperative period. The interval between each bolus was 5 min. The visual analog pain scale (VAS) threshold required to administer morphine was 30, and pain relief was defined as a VAS score of 30 or less. Data are expressed as mean +/- SD.

Data from 4,317 patients were analyzed; 54% of the patients were male, and 46% were female. The mean morphine dose required to obtain pain relief was 11.9 +/- 6.8 mg or 0.173 +/- 0.103 mg/kg. Women had a higher initial VAS score (74 +/- 19 vs. 71 +/- 19; P < 0.001) and required a greater dose of morphine (0.183 +/- 0.111 vs. 0.165 +/- 0.095 mg/kg; P < 0.001). In contrast, no significant difference was noted in elderly (aged > 75 yr) patients (0.163 +/- 0.083 vs. 0.157 +/- 0.085 mg/kg).

Women experienced more severe postoperative pain and required a greater dose (+11%) of morphine than men in the immediate postoperative period. This sex-related difference disappeared in elderly patients.