Ketamine infusions for acute and chronic pain

Subanesthetic ketamine is an opioid-sparing adjunct for painful surgery and opioid-tolerant patients, with moderate evidence in CRPS; keep within consensus dose ceilings and avoid it with poorly controlled cardiovascular disease or psychosis.

1. Painful surgery is the core acute indication. Consensus guidance says subanesthetic infusions should be considered for painful surgery (grade B, moderate certainty) and may be considered for opioid-tolerant or opioid-dependent surgical patients (grade B, low certainty) ¹.
2. Respect the dose ceilings. Acute pain boluses should not exceed 0.35 mg/kg, and infusions generally not 1 mg/kg per hour where intensive monitoring is unavailable ¹.
3. Evidence is strongest in CRPS among chronic pain conditions. Moderate evidence supports infusions such as 22 mg/h for 4 days or 0.35 mg/kg/h for 4 hours daily over 10 days, with benefit up to 12 weeks (grade B) ².
4. Spinal cord injury pain: weak, short-term evidence. Infusions give short-term improvement only (grade C, low certainty); one 80 mg 5-hour regimen helped at 2 weeks but not afterward ².
5. Screen for contraindications. Avoid ketamine with poorly controlled cardiovascular disease or certain poorly controlled psychoses (grade B), and severe hepatic impairment; with raised intracranial or intraocular pressure, avoid it or use low doses with extreme caution ².
6. Sickle cell crisis has its own regimen. ASH suggests subanesthetic ketamine for hospitalized opioid-refractory pain, starting at 0.1–0.3 mg/kg/h, maximum 1 mg/kg/h, in centers with expertise ³.

Repeated or multiday infusions carry cumulative risks: hepatotoxicity and cystitis appear more likely with higher doses and repeated exposure, so plan monitoring before scheduling serial treatments ².

• In opioid-tolerant back surgery patients, intraoperative ketamine reduced opioid consumption for up to 6 weeks ¹.
• Ketamine may be considered for opioid-tolerant patients with sickle cell pain and as an opioid-limiting adjunct in sleep apnea (grade C) ¹.
• Liver enzyme elevations reported with ketamine return to normal after discontinuation ².
• A 30% or 2-point fall in pain corresponds to patients rating themselves "much improved" ².
• The multisociety postoperative pain guideline lists IV ketamine as an option within multimodal postoperative analgesia ⁴.
• Beyond NMDA antagonism, ketamine acts at mu-opioid, muscarinic, monoaminergic and GABA receptors ¹.
• In ventilated surgical ICU patients, infusions of 0.06–0.30 mg/kg/h reduced opioid consumption ¹.
• Dosages below the anesthetic range can still put patients at risk of aspiration and other adverse effects ¹.

• Infusing ketamine in a patient with unstable angina or uncontrolled hypertension ².
• Expecting durable benefit in spinal cord injury pain ².
• Using anesthetic induction doses for analgesia ¹.
• Assuming one infusion protocol fits all; trial regimens for chronic pain varied widely in dose and duration ².