The influence of transcutaneous electrical stimulation (TES) and psychological factors in determining the intensity of acute postoperative pain was examined in a prospective, double-blind controlled trial completed by 30 patients having elective surgery. Psychometric tests were administered prior to surgery. Postoperative pain was assessed by cumulative morphine requirement (M48) administered intramuscularly, and the mean score of a visual analogue scale of pain (VAS), in the first 48 h following surgery. M48 was significantly correlated with the VAS score (r = 0.62, p < 0.001), and with the psychometric test scores for trait-anxiety (r = 0.70, p < 0.001) and neuroticism (r = 0.67, p < 0.001). Though patients treated with TES required 25% less morphine than those treated with placebo, the difference was not significant using monovariate analysis and applying unpaired two-tailed Student's t-test (p < 0.2). When the contribution of neuroticism to the variance of M48 was adjusted using multiple regression analysis, the effect of TES became significant at the 0.05 level. Covariance analysis showed that TES contributed some 19% to the explained variance of M48 while neuroticism contributed about 80%, and there was no interaction between these two factors. These findings allow a degree of prediction of the individual patient's postoperative pain and narcotic requirement, and point to a strong correlation between postoperative pain perception and personality.

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