Absence of the preemptive analgesic effect of dextromethorphan in total knee replacement under epidural anesthesia

Previous studies have shown that dextromethorphan (DM), a N-methyl-D-aspartate (NMDA) receptor antagonist, produces a preemptive analgesic effect on post-operative pain. The aim of this study was to further examine the preemptive analgesic effect of intramuscular (i.m.) DM injection on unilateral total knee replacement (TKR).

Sixty-four ASA I-III patients scheduled for unilateral TKR surgery were randomly allocated into three groups in a prospective double-blind manner. All patients received epidural anesthesia without any premedication. An initial bolus dose of 2% lidocaine (15-20 mL) followed by a maintenance dose of 8-10 mL/h was decided. Fentanyl (1.5 micrograms/kg) and diazepam (2 mg) were given i.v. before epidural catheter insertion. The epidural catheter was placed via the L2-L3 or L3-L4 interspace and advanced for 5 cm cephalad [corrected]. Patients received i.m. injection of 20 mg chlorpheniramine (CPM) before surgery as control (group C, n = 22). For the study groups, patients were given an i.m. injection containing 40 mg DM and 20 mg CPM, before (group B, n = 22) or after surgery (group A, n = 20), respectively. Postoperation, patients received intravenous morphine by means of a patient controlled analgesia (PCA) device for pain relief. The time to the first pull of PCA trigger, morphine consumption, worse pain scores (resting and incidental), and analgesics related side effects were recorded at 1, 2, 4, 8, 24, 48 and 72 h after surgery.

The time from the end of operation to the first PCA trigger were 31.2 +/- 5.2 min in group C, 67.3 +/- 11.1 min in group B (P < 0.05, compared with group C) and 61.8 +/- 7.2 min in group A (P < 0.05, compared with group C) respectively. The relevant pain score at resting, observed at the 8 h postoperatively was respectively 4.2 +/- 0.1 in group C, 3.7 +/- 0.2 in group B (P < 0.05, compared with group C) and 3.4 +/- 0.2 in group A (P < 0.05, compared with group C); and at the 24 h was 3.1 +/- 0.2 in group C, 2.4 +/- 0.2 in group B (P < 0.05, compared with group C) and 2.5 +/- 0.1 in group A (P < 0.05, compared with group C) respectively. There were no significant differences in actual morphine delivery and frequency of PCA triggering at all time among the three groups. Moreover, there was also no significant statistic difference in morphine-associated side effects among the three groups.

In the present study, we failed to observe any preemptive analgesic effect of DM (40 mg, i.m.) on postoperative pain in patients who received TKR under epidural anesthesia, however, DM given either before or after surgery augmented other analgesic (morphine) to offer a better pain relief.