Ketamine: Reinventing Chronic Pain Management

AUGUST 07, 2014
Jeannette Y. Wick, RPh, MBA, FASCP

For patients who respond poorly or incompletely to opioids, ketamine may be the answer.
In the middle of the past century, phencyclidine hydrochloride—called PCP or angel dust on the street—was developed to be a safe, effective anesthetic that did not cause cardiovascular and respiratory depression. However, its propensity to cause convulsions at high doses and long-lasting psychoactive side effects during emergence from anesthesia destroyed its potential.

Ketamine—a PCP derivative—was synthesized in 1963 and was tested on 20 prison volunteers in 1965. One-tenth as potent as PCP, ketamine was intended to induce anesthesia like PCP, but with greater specificity and fewer side effects.1 The FDA approved it in 1970, and its widespread use in the Vietnam conflict theater catapulted its popularity.2 Today, ketamine is used less and less in the operating suite.3 Although ketamine’s psychomimetic side effects are milder than those of PCP, they can be problematic (Table 12-10).

Recreational abuse has dogged ketamine since its approval. Abusers have injected, inhaled, and smoked ketamine, revealing characteristics of the drug that would otherwise remain unknown. Researchers hypothesize that abusers may develop tolerance because ketamine induces liver enzymes.11 Abusers rarely experience withdrawal, instead reporting a sensation called the K-hole—a constellation of visual hallucinations, dissociation, and out-of-body, and sometimes, near-death experiences. Heavy, prolonged ketamine use can cause cognitive and psychological impairment.4,12-15

Up to one-third of chronic ketamine abusers develop dose-dependent urinary tract symptoms within weeks to years: lower urinary tract irritation (vesicopathy), hydroureter, and hemorrhagic or ulcerative cystitis.13,16,17 The symptom etiology remains unclear, but may be direct toxic damage, immune system activation, or the effect of unknown bacteria.16,18 Long-term complications include hepatotoxicity (jaundice, itching, or elevated liver enzyme levels, especially in alcoholic patients) and/or cholangiopathy.19,20 Some long-term abusers develop corneal edema.21 These complications reverse after cessation of ketamine use.17,20,21 Clinically, the most common side effects of ketamine are inebriation, mental alteration, headache, hypertension, and altered liver enzymes.22

Newer, cleaner drugs or biologics are replacing ketamine in the operative suite. Yet ketamine is finding a new place in clinical therapy. Ketamine, an N-methyl-D-aspartate (NMDA)–receptor antagonist, is becoming an option for perioperative pain management among patients with opioid tolerance, acute hyperalgesia, and chronic neuropathic pain.1

NMDA Receptors
NMDA receptors are 1 of 3 glutamategated ion receptors. Gated by a magnesium ion, they normally open only briefly to allow calcium ions and other cations to enter the cell. Calcium activates second- messenger systems, causing neuronal hyperactivity.1,22-24 NMDA receptors may be involved in neuronal survival and maturation, synaptic plasticity, and memory. Abnormal NMDA function may cause neurologic disorders including Alzheimer’s disease, amyotrophic lateral sclerosis, depression, epilepsy, multiple sclerosis, Parkinson’s disease, and schizophrenia.25 Unrelenting NMDA receptor excitation allows continuous calcium influx into the cell and creates hyperexcitability. This presents clinically as opioid tolerance, hyperalgesia, and allodynia.22,26,27

Ketamine is the most potent clinically available, uncompetitive, open-channel NMDA-receptor blocker (it only works if the receptor is activated and the channel is open). Ketamine depresses the thalamus and limbic systems, preventing central nervous system centers from receiving or processing sensory input. This creates anesthesia, analgesia, and amnesia, and sometimes unpleasant psychomimetic effects or emergence phenomena.23,28,29

Sympathetic cardiovascular stimulation caused by ketamine is unique among intravenous anesthetics: it inhibits neuronal catecholamine reuptake, thereby increasing heart rate, cardiac output, and systemic and pulmonary blood pressure.30,31 Theoretically, ketamine use should be avoided in patients with prolonged QT syndrome.32 Ketamine inhibits neuronal serotonin reuptake, causing an emesis that is reversed by 5-HT–receptor blockers.33,34