Very-low-dose ketamine (0.05 mg . kg(-1) . h(-1)) potentiated morphine-ropivacaine analgesia and reduced postthoracotomy pain.

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Simultaneous use of small doses of ketamine with morphine enhances the pain relief produced by morphine.

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The co-administration of ketamine and morphine as a mixture is not recommended for postoperative pain relief. As an adjunct in multimodal analgesia, low-dose ketamine infusion and the administration of dextromethorphan may be able to improve postoperative pain status. Memantine exhibits the greatest potency among NMDA receptor antagonists. In future, research should consider the perioperative infusion of ketamine followed by long-term administration of memantine for the prevention of persistent pain.