Meperidine serum concentrations and analgesic response in postsurgical patients

We examined the relationship between serum concentrations of meperidine hydrochloride and analgesic response in postsurgical patients allowed to use patient-controlled analgesia (PCA) and compared these findings with those of patients receiving conventional intramuscular (IM) dosing. Six patients who had undergone abdominal surgery were randomly assigned to receive postoperative analgesia with either PCA or IM therapy. A sequence of five-point pain and sedation scores and serum meperidine concentrations were obtained in all patients the day after surgery. Minimum effective concentration (MEC) was defined as that concentration of meperidine at which patients felt pain relief as indicated by a decrease in pain rankings. The mean MEC for patients using PCA, 296 +/- 112 ng/mL, was significantly lower than the mean MEC in patients receiving IM dosing (551 +/- 164 ng/mL, p less than 0.05). The mean maximum change in meperidine concentrations in the PCA group, 177 +/- 88 ng/mL, was significantly lower than that of the IM group (484 +/- 125 ng/mL, p less than 0.05). Mean maximum changes in pain and sedation scores for patients in the PCA group were not significantly different from those of the IM group. During this investigation patients using PCA experienced smaller swings in meperidine concentrations than did patients receiving IM injections. MEC analysis suggests that PCA patients may experience pain relief at lower meperidine concentrations than those needed by IM patients.