Epidural ketamine for postoperative analgesia.

·T WO factors have increased interest in ketamine for postoperative analgesia. The first was the discovery of the N-Methyl-D-aspartate (NMDA) receptor and its role in central pain processing and spinal cord neural plasticity.1 Ketamine is one of two clinically useful NMDA receptor antagonists available (the other is dextromethorphan). Ketarnine binds non-competitively to the PCI, (phencyclidine) recognition site in the NMDA receptor channeU In addition, interest in the concept of preemptive analgesia makes ketamine a natural candidate for investigation of postoperative pain relief since blockade of the NMDA receptor reduces noxious stimulusinduced allodynia and hyperalgesia. To avoid the well-known psychomimetic effects ofketarnine, investigators have focused on the use of smaller doses than are required for general anaesthesia and alternative administration routes for postoperative analgesia. Spinal administration would seem to be especially interesting due to the proximity of the NMDA receptors and the potential for decrease in dose requirements. Reports on intraspinal administration of ketamine include caudal subarachnoid ketarnine administration, with or without local anaesthetic agents, lumbar epidural ketamine alone or with local anaesthetic agents and/ or opioids and thoracic epidural ketarnine with opioids. There are several case reports, comments and uncontrolled studies related to epidural ketamine analgesia3-5 but only a few controlled clinical trials.6-8 Although two observa· tiona! studies claimed epidural ketamine is effective as a postoperative analgesic3•4 other uncontrolled trials could find little or no postoperative analgesic effect. 5 Similarly, properly controlled trials using either small (4-8 mgf or large (30 mg)6•8 doses were unable to document effective postoperative epidural analgesia. Epidural ketamine may have an adjuvant effect when added to epidural morphine or local anaesthetic agents9 Wong et al.8 administered 10 mg ketarnine and 0.5 mg morphine epidurally to patients scheduled for Editorial