Ketamine analgesic mechanism
Advanced, Pain, Pharmacology
Drug Class : Phencyclidines
How it differs from most other induction agents : In addition to being able to induce anesthesia, it also has significant analgesic properties
Proposed analgesic benefits : Ketamine has been shown to inhibit nociceptive central hypersensitization. It also has been shown to reduce the incidence of acute tolerance after opioid administration.
Ketamine is also well known for its reputation of the negative side effects of hallucinations, hypertension & tachycardia. Titrating propofol with BIS/EMG to a level of 75 or less provides a numerically reproducible basis to administer 50 mg ketamine without producing any of the historically negative side effects
Principal Site of Action : Antagonism of NMDA receptors. Ketamine, 50 mg (independent of adult body weight*) intravenously, 3-5 minutes prior to surgical stimulation (local anesthetic injection or scalpel incision) effectively blocks 98-99% of NMDA receptors in the midbrain.
- The midbrain is a very small part of the entire brain mass and the NMDA receptors do not comprise the entire midbrain. After observing both 100 pound adult females and 250 pound adult males remain motionless in response to local anesthetic injection, it was concluded that among anesthetic agents, the effective dose of ketamine is unrelated to body weight.
This approach to non-opioid preemptive analgesia, informally known as the ‘nifty fifty,’ blocks the most potent stimulus to the brain; i.e. the violation of the boundary between the world of self and the world of danger.