Long QT syndrome
Formal Definition
A congenital or acquired disorder of cardiac repolarization characterized by prolonged ventricular myocyte repolarization (QT interval >470ms in males, >480ms in females), predisposing to the characteristic polymorphic ventricular tachycardia Torsades de Pointes, which can degenerate into ventricular fibrillation and cause sudden cardiac death; congenital forms (Romano-Ward, Jervell-Lange-Nielsen) involve ion channel mutations; acquired forms are commonly drug-induced (fluoroquinolones, macrolides, antiarrhythmics, antipsychotics) or from hypokalemia and bradycardia.
How It's Used on the Ward
"Long QT" — the heart muscle takes too long to repolarize, which sets the stage for a specific type of polymorphic VT called Torsades de Pointes that looks like a twisting bundle of electrical activity on the monitor; the most important thing is to stop any QT-prolonging drugs and check the electrolytes.
Example
""35-year-old woman presents with palpitations and near-syncope. ECG: QTc 520ms, baseline was 410ms. She started ciprofloxacin 3 days ago for a UTI. Monitor shows runs of polymorphic VT with wide complexes that are twisting around the baseline — Torsades de Pointes. Diagnosis: drug-induced (ciprofloxacin) long QT syndrome with Torsades. All QT-prolonging meds stopped immediately. IV magnesium 2g given (2mg/kg, suppresses Torsades even if magnesium levels normal). Potassium repleted to 4.5. Temporary pacing wire placed (atrial overdrive pacing to shorten QT). Torsades suppressed, QTc recovered to 415ms over 48h.""
Clinical Context
Congenital LQTS: most common are KCNQ1 (LQT1 — triggered by exercise/swimming) and KCNH2 (LQT2 — triggered by auditory stimuli/alarm clocks). Beta-blockers first-line for congenital LQTS. ICD for high-risk (prior cardiac arrest, syncope on beta-blockers). Acquired LQTS: most common triggers — antiarrhythmics (sotalol, quinidine, dofetilide, ibutilide), antibiotics (macrolides, fluoroquinolones, pentamidine), antipsychotics (haloperidol, ziprasidone, quetiapine), antiemetics (ondansetron), antihistamines (diphenhydramine), hypokalemia, hypomagnesemia, bradycardia. Management of Torsades: stop offending drug, IV magnesium (even if levels normal — suppresses early afterdepolarizations), correct electrolytes (K >4.5, Mg >2), temporary pacing (overdrive pacing shortens QT), isoproterenol. Drug safety databases (CredibleMeds) should be checked before prescribing QT-prolonging drugs to any patient.