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NMDA Receptor Antagonist — Opioid-Sparing Analgesic Pregnancy: Use with caution — limited data; increases uterine tone at high doses; used in obstetric anaesthesia at low doses

Ketamine (Sub-anaesthetic — Orthopaedic Pain)

Brand names: Ketalar

Adult dose

Dose: 0.1–0.5 mg/kg IV bolus; 0.1–0.2 mg/kg/h IV infusion (sub-anaesthetic analgesic dose)
Route: Intravenous
Frequency: As required (bolus) or continuous infusion
Max: 0.5 mg/kg per bolus for analgesic use; anaesthetic doses are higher
Sub-anaesthetic doses for analgesia are distinct from anaesthetic induction doses (1–2 mg/kg IV). Administer in monitored setting. Often combined with low-dose midazolam (1–2 mg) to reduce dissociative side effects. Use preservative-free preparation for intrathecal/epidural use.

Paediatric dose

Dose: 0.5–1 mg/kg
Route: IV or IM
Frequency: As required
Max: Specialist guidance
Paediatric procedural analgesia/sedation — IM ketamine (4–6 mg/kg) for procedures in children who cannot tolerate IV; emergence reactions less common in children <15 years

Dose adjustments

Renal

No dose adjustment required at sub-anaesthetic doses; use with caution in severe renal impairment

Hepatic

Use with caution in hepatic impairment — norketamine accumulation; reduce dose

Paediatric weight-based calculator

Paediatric procedural analgesia/sedation — IM ketamine (4–6 mg/kg) for procedures in children who cannot tolerate IV; emergence reactions less common in children <15 years

Clinical pearls

  • NMDA receptor antagonism prevents and reverses central sensitisation — unique analgesic mechanism explains opioid-sparing effect and utility in opioid-tolerant patients
  • Opioid-sparing: sub-anaesthetic ketamine (0.3 mg/kg bolus peri-operatively) reduces 24-hour opioid consumption by 30–40% in arthroplasty and major orthopaedic surgery
  • Emergency Medicine use (BOAST guideline): ketamine IM 0.25–0.5 mg/kg for procedural analgesia in fractures — particularly useful in pre-hospital and ED settings where IV access is difficult
  • Dissociative side effects: minimised at sub-analgesic doses (<0.3 mg/kg); benzodiazepine co-administration (midazolam 0.05 mg/kg IV) further reduces emergence reactions
  • MHRA: Ketamine is a Class B / Schedule 4 Part 1 controlled drug in UK (2014) due to bladder toxicity from recreational misuse — prescriptions require CD documentation

Contraindications

  • Uncontrolled hypertension (≥180/100 mmHg)
  • Ischaemic heart disease (increases HR and BP)
  • History of psychosis or schizophrenia
  • Severe hepatic impairment

Side effects

  • Dissociative effects — dreaming, hallucinations, emergence delirium (most common concern)
  • Tachycardia and hypertension — sympathomimetic effect
  • Laryngospasm (at higher doses — airway reflexes preserved but not infallible)
  • Hypersalivation — co-prescribe glycopyrronium or atropine
  • Bladder dysfunction — chronic use causes ketamine cystitis (recreational use context)

Interactions

  • CNS depressants — additive sedation; reduces ketamine dose required
  • Benzodiazepines — reduce emergence reactions; often co-administered
  • Theophylline — increased seizure risk; avoid combination
  • Thyroid hormones — enhanced hyperthermia and hypertension

Monitoring

  • Blood pressure and heart rate during infusion
  • Oxygen saturation and respiratory rate
  • Sedation level
  • Emergence reactions — reassurance and calm environment

Reference: BNFc; BNF 90; NICE NG124 (Hip Fracture); BOAST Open Fracture Guidelines; MHRA Ketamine Scheduling 2014; SPC Ketalar. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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