Watch for QTc interval prolongation in patients taking antipsychotics

March 12, 2018|Family Practice News

REPORTING FROM NPA 2018

LAS VEGAS – Many potential but rare side effects, primarily arrhythmias, can occur from taking antipsychotics, according to Carrie L. Ernst, MD.

Risk factors for QT prolongation and torsades de pointes (TdP) include medications such as antiarrhythmics, many antibiotics such as macrolides and quinolones, antifungals, antiemetics, antimalarials, methadone, and antipsychotics and other psychotropics. Other risk factors include cardiac disease, electrolyte abnormalities, being female, age 65 and older, being on another medication that inhibits metabolism of the drug, genetic predisposition to prolonged QT, and impaired liver function.

According to the American Heart Association, prolonged QTc in males is more than 450 milliseconds, while prolonged QTc in females is more than 460 milliseconds. “The more prolonged the QTc interval is, the greater the risk for arrhythmias,” said Dr. Ernst, a consultation liaison psychiatrist at the Icahn School of Medicine at Mount Sinai, New York. Dr. Ernst spoke at an annual psychopharmacology update held by the Nevada Psychiatric Association. “Most experts agree that a QTc of greater than 500 millisecond represents a risk factor for TdP. When I see a patient with a QTc over 500 milliseconds, I’m going to think very carefully about whether to use an antipsychotic or not,” she said.

All antipsychotics can cause an increase in the QT interval and can cause TdP, although some likely confer a greater risk than others. “It’s most likely not an idiosyncratic effect but rather a dose-related effect,” she said. “So the higher the dose, the more the risk.” There are some data suggesting that phenothiazines, particularly thioridazine, seem to present the greatest risk for prolonging the QTc interval. The low potency typical antipsychotics in general are thought to carry a higher risk than the high-potency agents, but IV haloperidol has been linked to prolonged QTc and TdP. “It’s difficult to know whether that’s the impact of the drug or a reflection of the fact that patients on IV haloperidol are usually medically complicated patients,” Dr. Ernst noted. “Unfortunately, there is limited direct comparison data between agents. Atypicals are only implicated in TdP in rare case reports, and there is less available data for the newest atypicals.”