Anaes · Anaesthetic adjuncts
Subanaesthetic ketamine as an analgesic adjunct
Also known as Low-dose ketamine · NMDA antagonist adjunct · Opioid-sparing ketamine · Anti-hyperalgesic ketamine · Esketamine analgesia
Subanaesthetic-dose ketamine is the prototypical NMDA-receptor antagonist used not to induce anaesthesia but to prevent the central sensitisation that drives postoperative pain, opioid tolerance and opioid-induced hyperalgesia. At a tenth to a half of the induction dose, given as a 0.1 to 0.5 mg per kg bolus or a 0.1 to 0.5 mg per kg per hour infusion (taught dose band; confirm local protocol), it is an effective opioid-sparing adjunct: a 70-study systematic review found reduced opioid consumption with longer time to first analgesic across all studies. The framework rests on five exam-critical ideas: the analgesia is mechanistically an ANTI-HYPERALGESIC action, not a simple analgesic one, because NMDA blockade preferentially silences the wind-up and central sensitisation that amplify pain after tissue injury; the same mechanism directly counteracts the NMDA-driven pathway by which opioids themselves produce hyperalgesia and tolerance, so ketamine is uniquely valuable in the opioid-tolerant patient; it is effective for neuropathic pain by both the intravenous and the topical route; its S-enantiomer esketamine preserves the analgesic effect with fewer psychotomimetic emergences and a rapid antidepressant action; and even at these low doses the sympathomimetic cardiovascular stimulation and the psychotomimetic effects persist, so patient selection matters. Built on the intravenous-ketamine postoperative-analgesia systematic review (Laskowski 2011), the ketamine chronic-pain risks-benefits review (Niesters 2014), the NMDA-antagonist PK/PD review (Niesters 2012), the topical ketamine neuropathic-pain study (Rav 2026), the perioperative ketamine and esketamine fatigue review (Al Subhi 2026), the opioid-sparing anaesthesia and PONV trial (Sung 2026), the neuro-glial marker work in sinus surgery (Rizopoulou 2026), the dexamethasone and rebound-pain study (Hong 2026), and the palliative opioid prescribing review (Chen 2026).
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Red flags
- Even at subanaesthetic doses ketamine retains its PSYCHOTOMIMETIC potential — vivid dreams, hallucinations and emergence phenomena occur in a minority of patients on a low-dose infusion, and are the commonest reason the infusion is stopped. Benzodiazepine premedication reduces but does not abolish the risk; the opioid-tolerant and the psychiatrically unwell patient are more susceptible.
- The sympathomimetic cardiovascular stimulation persists at adjunct doses — heart rate, blood pressure and cardiac output rise. This is desirable in the shocked patient but a hazard in uncontrolled hypertension, ischaemic heart disease, aortic dissection or a recent myocardial infarction, where the surge in afterload and myocardial work can precipitate ischaemia.
- Ketamine does NOT cause respiratory depression or histamine release, and it preserves airway reflexes better than other sedatives — but a rapid or large subanaesthetic bolus can still produce transient apnoea and airway loss. Monitoring and rescue airway equipment must be present; it is not a licence for unmonitored ward infusion.
- Hypersalivation and nystagmus occur even at low dose; combined with preserved airway reflexes the secretions can precipitate laryngspasm, particularly in children.
- In the catecholamine-depleted heart (prolonged severe shock, chronic severe heart failure) the indirect sympathomimetic effect fails and the direct negative inotropic effect is unmasked — ketamine can then cause hypotension rather than the expected pressor response, even at adjunct doses.
- Tolerance and dependence can develop with repeated or prolonged subanaesthetic administration (the chronic-pain and palliative populations); ketamine is a drug of potential misuse and prescribing must follow institutional and regulatory controls, especially for take-home or palliative use.
Meet the patient
A chronic-pain patient on long-term opioids presents for major abdominal surgery. Escalating morphine has stopped working — the pain is wider and the dose keeps climbing. The acute-pain service starts a low-dose ketamine infusion, and overnight the morphine requirement falls and the pain contracts again. That is the anti-hyperalgesic effect in action.[7][8]
The one concept that organises the whole topic is that ketamine is an anti-hyperalgesic, not a simple analgesic. It does not dull an unchanging pain — it stops pain from amplifying. Every clinical use flows from that single mechanism.[9]
References9ShowHide
- [1]Rav E, et al. Topical Amitriptyline, Ketamine, and Lidocaine Cream for Neuropathic Pain Control in Pediatric Oncology Patients J Pain Symptom Manage, 2026.PMID 42362167
- [2]Al Subhi M, et al. Effect of Perioperative Ketamine and Esketamine on Postoperative Fatigue: A Systematic Review and Meta-Analysis of Randomized Controlled Trials Medicina (Kaunas), 2026.PMID 42356168
- [3]Rizopoulou S, et al. Variations in S-100Β and Neuron-Specific Enolase Levels During Functional Endoscopic Sinus Surgery Under Moderately Controlled Hypotension Using Four Distinct Anesthetic Protocols: A Randomized Controlled Study Medicina (Kaunas), 2026.PMID 42356019
- [4]Sung TY, et al. Effect of Opioid-Sparing Anesthesia on Postoperative Nausea and Vomiting After Breast Surgery: A Single-Center Randomized Controlled Trial J Clin Med, 2026.PMID 42355627
- [5]Hong B, et al. Consistent analgesic effect of intravenous dexamethasone on rebound pain after brachial plexus block: a causal machine learning approach Korean J Pain, 2026.PMID 42357812
- [6]Chen J, et al. Top Ten Tips Palliative Care Clinicians Should Know About Navigating Outpatient Opioid Prescribing J Palliat Med, 2026.PMID 42363735
- [7]Laskowski K, et al. A systematic review of intravenous ketamine for postoperative analgesia Can J Anaesth, 2011.PMID 21773855
- [8]Niesters M, et al. Ketamine for chronic pain: risks and benefits Br J Clin Pharmacol, 2014.PMID 23432384
- [9]Niesters M, et al. Pharmacokinetic and pharmacodynamic considerations for NMDA receptor antagonists in the treatment of chronic neuropathic pain Expert Opin Drug Metab Toxicol, 2012.PMID 22871026