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QT-prolonging Drugs in LQTS — EECC MCQ

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ModerateCardiac PharmacologyQT-prolonging Drugs in LQTSEECC

A 40-year-old man with long QT syndrome (QTc 520 ms, LQT2 genotype) is prescribed a macrolide antibiotic by his GP for a chest infection. Why is this problematic?

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Correct answer: C — Many macrolides (erythromycin, clarithromycin, azithromycin) prolong the QT interval by blocking the hERG potassium channel — in patients with congenital LQTS this can precipitate torsades de pointes and sudden death; an alternative antibiotic should be used

Drug-induced QT prolongation is a major safety concern, particularly in patients with underlying LQTS. Many drugs block the hERG (KCNH2/Kv11.1) potassium channel, which conducts the IKr current — the same current deficient in LQT2. This creates a 'double hit' in LQT2 patients: reduced IKr from genetic mutation + further pharmacological IKr blockade → prolonged QT → torsades de pointes → VF → SCD. The CredibleMeds website (www.crediblemeds.org) categorises drugs by QT risk. Key QT-prolonging drug categories to avoid in LQTS: macrolides (erythromycin, clarithromycin — azithromycin has less risk but still caution), fluoroquinolones, antipsychotics, metoclopramide, ondansetron, antifungals, class IA/III antiarrhythmics. Amoxicillin/doxycycline are generally QT-safe alternatives for respiratory infections.

Reference: ESC (2022): VA/SCD Guidelines; CredibleMeds