The Use of Ketamine as Rescue Analgesia in the Recovery Room Following Opioid Administration. A Double-blind Randomised Trial in Postoperative Patients

Some patients require large doses of opioids to control postoperative pain, which can result in a prolonged period of poor pain control, and potentially increased side effects associated with large morphine doses. This may be due to insufficient morphine dose to that individual or acute tolerance ( 1 ). Ketamine is not just an anaesthetic agent but at lower doses is known to provide efficacious analgesia ( 2, 3 ). Ketamine has been shown to have a marked analgesic effect on high intensity nociceptive stimuli ( 4 ) as exhibited in postoperative pain. When given for opioid analgesia resistant cancer pain in bolus doses at two different concentrations it has been shown to be effective and have a morphine-sparing effect, without undue complications ( 5 ).

Ketamine has been suggested to work pre-emptively and also by many other routes other than intravenously ( 6 - 9 ) .

Previous studies have compared morphine with morphine and ketamine administered as PCA or intramuscularly ( 10 - 12 ) in postoperative patients with varying effects. Javery et al. ( 11 ) showed that pain scores were lower in patients who received ketamine but Reeves et al. in a later but similar study showed no significant difference ( 13 ).

The authors have noted that in the postoperative situation with morphine resistant pain, a bolus dose of ketamine not only leads to a marked decline in pain but it also remains efficacious for several hours. This prolonged effect was also noted in opioid resistant cancer pain ( 5 ). This indeed may have relevance to the prevention of onset of chronic post surgical pain ( 14 ) and earlier discharge from the Post Anaesthetic Care Unit.

Morphine and ketamine are not without side effects. Respiratory depression, nausea, vomiting and vivid dreams, being well documented will hence be a secondary endpoint. A quality of recovery score will also be measured ( 15 ) and four hours postoperatively.

This study is designed to compare a morphine regimen in the form of a standard Post Anaesthetic Care Unit pain protocol with a bolus dose of ketamine to be implemented if the pain protocol has been inadequate. Any patient in pain, despite two doses of morphine will be included. Thereafter the patients will be randomised to receive either a further solution of ketamine or continuation of the morphine protocol. This randomised, double-blinded, trial will be based in the Post Anaesthetic Care Unit under close anaesthetic and nursing staff supervision.

Inclusion criteria

• 1. Those patients requiring the routine pain protocol to be implemented as used in the Post Anaesthetic Care Unit who need more than two doses of morphine (and having received intraoperative morphine).

Exclusion criteria

• Exclusion Criteria

• Known allergy to morphine or ketamine.
• Past history of major psychiatric disturbance or currently taking psychiatric medication/s.
• Chronic morphine usage.
• Chronic pain syndrome or chronic painful medical condition.
• Unable to obtain a reliable pain score in recovery due to language barriers or residual anaesthesia.
• Known pregnancy.
• Cases where primary anaesthetist prefers alternate therapy.
• Aged less than 18 years.
• Weight less than 50 kilograms or greater than 100 kilograms.
• Use of ketamine intraoperatively.
• Use of major regional block.