Postoperative pain in a patient on buprenorphine
A 44-year-old woman on long-term buprenorphine/naloxone 16/4 mg daily for opioid use disorder undergoes an emergency laparotomy for a perforated sigmoid colon. She receives a standard general anaesthetic with fentanyl and morphine. In recovery she is agitated, with a pain score of 9/10, respiratory rate 22/min, and has received morphine 10 mg IV in divided doses over 40 minutes with no relief. The surgical registrar asks you to prescribe "something stronger - she is clearly opioid tolerant". She last took her buprenorphine 20 hours ago.
1. Explain why she is in severe pain despite morphine, and how buprenorphine's pharmacology changes your approach.
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Buprenorphine is a partial mu-agonist with very high receptor affinity and slow dissociation. At 16 mg daily it produces near-maximal receptor occupancy. Two things follow:
- Ceiling effect on analgesia. Increasing the dose does not produce proportionally more analgesia, so standard morphine titration behaves unpredictably - some effect, but limited, and the patient may still report severe pain.
- Competitive antagonism of full agonists. Because buprenorphine binds so tightly, morphine and fentanyl must compete for a small number of free receptors. This is why 10 mg of morphine has produced little relief. Higher doses of pure agonist can overcome this in part, but the response is blunted and the therapeutic window is narrow.
This is not simply "opioid tolerance". Calling it tolerance and escalating morphine risks sedation and respiratory depression without analgesia, because the dose-response curve is flattened and then steepens unpredictably.
Also note she is 20 hours since her last dose: buprenorphine's long half-life means receptor occupancy is still substantial, and she may additionally be in withdrawal if she has missed doses, which itself drives agitation and pain behaviour. Agitation with a respiratory rate of 22 is more consistent with pain and withdrawal than with opioid toxicity.
2. Give a concrete analgesic plan for the next 24 hours, including the role of regional techniques and how you would handle her buprenorphine.
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Multimodal, opioid-sparing, and planned rather than reactive.
Regional anaesthesia is the highest-value intervention. A laparotomy for perforated colon is a midline incision; a thoracic epidural or a bilateral transversus abdominis plane / rectus sheath block will reduce opioid requirement substantially and avoid the receptor competition problem altogether. Discuss with the anaesthetist on call and check coagulation and sepsis status before neuraxial techniques. If neuraxial is contraindicated, a continuous local anaesthetic wound catheter is a reasonable alternative.
Scheduled non-opioid agents. Regular paracetamol, regular NSAID only if renal function and bleeding risk allow after major abdominal surgery, plus ketamine as a low-dose infusion (for example 0.1 to 0.2 mg/kg/hour) which is particularly useful in opioid-tolerant patients. Magnesium and lidocaine infusions are options depending on local protocol and cardiac monitoring.
Opioid strategy. If a full agonist is needed, it must be titrated to effect in a monitored setting, with small aliquots and frequent reassessment, accepting that the ceiling is higher than usual. Patient-controlled analgesia with a background infusion is generally avoided; bolus-only PCA with close supervision is safer.
Buprenorphine management. Do not stop it abruptly - that precipitates withdrawal and worsens pain. Options are to continue it at the same dose and accept a blunted response to full agonists while relying on regional and multimodal techniques, or, in a supervised inpatient setting, to split the dose or reduce it and transition, with addiction medicine input. The decision should be made with the patient and the addiction team, not unilaterally.
Involve the acute pain service and addiction medicine early, and document a plan that covers the transition back to her usual regimen before discharge.