[Lack of pre-emptive analgesic effect of low-dose ketamine in postoperative patients. A prospective, randomised...

NMDA receptors are assumed to play an important role for neuronal plasticity. In vitro and animal experiments confirmed that NMDA antagonistic drugs can prevent hyperexitability of dorsal root neurons after strong pain stimuli. Clinical data, however, are more or less controversial in this respect. It was the aim of the present prospective, randomised, double-blind study to verify if low-dose preoperative ketamine, an NMDA antagonist, provides relevant postoperative analgesia in surgical patients and to re-examine positive results published by other investigators. 80 ASA I-II patients undergoing elective laparoscopic or proctologic surgery received at induction of general anaesthesia a single i.v. bolus dose of either ketamine 0.15 mg/kg or placebo (0.9% NaCl). Postoperative analgesia was provided by i.v. patient-controlled analgesia (PCA) using the opioid piritramide. Cardiovascular parameters, respiration, sedation, cumulative piritramide consumption and pain scores (visual analogue scale 1-10, verbal rating scale 0-4) were monitored at 1, 2, 3, 4, 5, 6, 12 and 24 hours after surgery. Additionally, a retrospective pain score was documented after the 24 hours observation period. There was no statistically significant difference in any study parameter. Cumulative PCA piritramide consumption after 24 hours was 25.0+/-16.2 mg in the ketamine group and 29.5+/-20.4 mg in the placebo group. Ketamine-specific side effects such as hallucinations or bad dreams were not observed. It is concluded that under the study conditions used, low dose ketamine, contrary to previously reported results [30], does not provide a clinically relevant pre-emptive analgesic effect in postoperative patients.