Ketamine for Analgesia in the ED

Case

A 47-year-old construction worker without past medical history presents to your emergency department after falling off of scaffolding; he complains of left lower leg pain. On presentation he is well appearing but distressed in pain. Exam is notable for an open fracture of the tibia and fibula. The foot is warm and well perfused. You initiate appropriate radiology and call orthopedics, order pre-operative labs and 8 mg IV morphine. Several minutes later the nurse approaches you and tells you that the patient refused the morphine. On further questioning, the patient reports that he is a recovered heroin addict, clean for 10 years, and cannot touch opiates. Is there anything else you can give him to relieve his pain?

Ketamine for Analgesia

Ketamine was developed in the 1960s in a successful effort to synthesize a dissociative anesthetic that didn’t make people as crazy as phencyclidine, PCP, which was developed in the 20s. Ketamine exerts its CNS effects via antagonism at the NMDA receptor, which, in high doses (> 1 mg/kg), causes dissociation, a cataplectic state where patients perceive no sights, sounds, or pain and cannot interact. Ketamine is commonly used as a procedural sedation agent and RSI induction agent at this dose range. In much smaller doses, however, ketamine provides excellent analgesia with no or minimal effects on perception or emotion.

When should I use ketamine for analgesia?

1. When opiates are not tolerated. Some patients seem to be more sensitive to the side effects of opiates and suffer more nausea and pruritus than benefit from analgesia.
2. When opiates are ineffective. Patients who use daily opiate analgesia may have intractable pain when they suffer an acutely painful condition, pain that does not respond to IV opiates.
3. When opiates are not desired by the patient, as may be the case in recovered opiate addicts.
4. When opiates are not desired by the physician, as may be the case in patients who use daily opiate analgesics and where concern exists for recreational motives, or contributing to opioid hyperalgesia syndrome.
5. When the patient has marginal hemodynamics in addition to an acutely painful condition, and even minor compromise to blood pressure or respiration could be dangerous.

How do I use ketamine for analgesia?

Ketamine can be used as a bolus, generally 10 or 20 mg. If you use 10 mg, you will usually have little effect on perception but sometimes not an adequate analgesic effect. If you give 20mg, you will almost always take care of your patient’s pain but that may push the patient beyond analgesic into the recreational dose range, which means they will be talking to you, but stoned. It’s not a big deal if that happens, and most of the time the patient will think it’s a good thing. If not, it will be metabolized soon. Which is actually a weakness of bolus ketamine for analgesia – it wears off in 15 minutes. Pushing ketamine accentuates its psychiatric effects. So unless the painful stimulus is going to disappear, you are better off writing for a drip. 20 mg over 10 minutes as a loading dose, then 20 mg/hour, titrated to effect.

Further Reading