Certain cardiac and noncardiac drugs induce QT prolongation (prolonged recovery after contraction of the heart muscle) and increase the risk of torsade de pointes, a type of ventricular tachycardia (abnormally fast heartbeat) that may progress to ventricular fibrillation (rapid, irregular heartbeat that can quickly lead to death).
Unfortunately, drugs with the potential for QT prolongation, particularly some antibiotics and a wide variety of other drugs, or drugs that inhibit their removal from the body are prescribed frequently in outpatient settings and often together. Combined prescribing of these drugs may increase the risk for cardiac arrest or arrhythmias, suggests a study that was supported in part by the Agency for Healthcare Research and Quality (HS10385).
Researchers at the Centers for Education and Research on Therapeutics (CERTs) at the University of Arizona Health Sciences Center and Duke University Medical Center used the administrative claims database of a national pharmaceutical benefit manager to measure the frequency of overlapping prescriptions for adults for 50 drugs with QT-prolonging potential and 26 drugs that inhibit their metabolic clearance in the outpatient setting in 1999. Nearly 1.1 million individuals filled 4.4 million prescriptions for QT-prolonging drugs. Of these, 9.4 percent filled overlapping prescriptions for two or more QT-prolonging drugs or drugs that inhibit their clearance.
Twenty-two percent of those who filled overlapping prescriptions were elderly, and 74 percent were women; both of these groups are more susceptible than younger people to drug-induced arrhythmias. Antidepressants were involved in nearly 50 percent of cases. For the five most commonly prescribed QT-prolonging drugs in this study, serious cardiac problems (atrial fibrillation, cardiac arrest, and ventricular arrhythmia) were rare (less than 1 in 1,000 patients). The additive effects of taking two QT-prolonging drugs might result in 2 in 1,000 patients suffering a serious cardiac problem. Synergistic effects of both drugs would affect considerably more patients.
See "Prescription of QT-prolonging drugs in a cohort of about 5 million outpatients," by Lesley H. Curtis, Ph.D., Truls Ostbye, M.D., Ph.D., Veronica Sendersky, Pharm.D., and others, in the February 1, 2003, American Journal of Medicine 114, pp. 135-141.