Drugs of special concern are those that frequently inhibit the metabolism of other agents, including erythromycin, clarithromycin, ketoconazole, itraconazole, amiodarone, and quinidine, and many antidepressants and antiretroviral agents. Of the deaths associated with drug-induced QTc prolongation related to the prokinetic agent cisapride, many were due to drug interactions with an imidazole or macrolide antibiotic. In these cases, increased serum concentrations of cisapride occurred due to inhibition of the cytochrome P450 CYP3A4 isoenzyme.9
If treatment with a drug that has the potential for causing QTc prolongation is begun, tell your patient to report any “potential cardiac” symptoms, such as palpitations, syncope, or near-syncope with or without palpitations, to a member of the healthcare team. Always be on the lookout for any concomitant conditions or treatments that can cause hypokalemia (e.g., diuretic use, gastroenteritis, diarrhea, excessive vomiting), or other agents that inhibit drug metabolism.
Obtaining a complete medication history, including the use of herbal products and over-the-counter medications, can help identify and prevent QTc prolongation from a drug interaction. A routine, 12-lead electrocardiogram (EKG) should be utilized during treatment to detect asymptomatic QTc prolongation or abnormal postectopic QTc intervals. Additionally, any patient predisposed to QTc prolongation should have an EKG performed before commencing treatment as well as after treatment is complete. If a drug prolongs the QTc interval beyond normal limits, the benefit of continuing the drug should be weighed against the risk of serious adverse cardiac events.10 TH
Michele B Kaufman, PharmD, BSc, RPh, is a freelance medical writer based in New York City.
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