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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
Key points
- QTc prolongation: >470 ms (men), >480 ms (women), or >500 ms (any sex = high risk). Use Bazett formula: QTc = QT / sqrt(RR)
- Congenital: autosomal dominant ion channelopathies. LQT1 (KCNQ1, exercise-triggered, especially swimming), LQT2 (KCNH2/hERG, auditory stimuli), LQT3 (SCN5A, events at rest/sleep)
- Acquired: drugs (most common cause overall), hypokalemia, hypomagnesemia, hypothermia, bradycardia, structural heart disease
- Torsades de Pointes: polymorphic VT with characteristic "twisting of the points" morphology. Treat with IV magnesium, overdrive pacing. Amiodarone CONTRAINDICATED
- Congenital LQTS: beta-blocker (nadolol preferred) is first-line. ICD for cardiac arrest survivors, syncope on beta-blocker, or QTc >500 with high-risk genotype
Overview
Long QT syndrome encompasses congenital and acquired conditions that prolong ventricular repolarization, creating a substrate for the polymorphic ventricular tachycardia Torsades de Pointes (TdP) and sudden cardiac death. Congenital LQTS affects ~1 in 2,000 people and is caused by mutations in cardiac ion channel genes (most commonly potassium and sodium channels). Acquired LQTS is far more common, primarily drug-induced, and usually reversible by removing the offending agent and correcting electrolytes.
Clinical Features
Symptoms
Syncope (most common presentation — from self-terminating TdP episodes)
Palpitations, near-syncope
Sudden cardiac death (may be first manifestation, especially in undiagnosed congenital LQTS)
Seizures (misdiagnosed epilepsy — actually from cerebral hypoperfusion during TdP)
Congenital deafness + syncope = Jervell and Lange-Nielsen syndrome (autosomal recessive, severe LQTS)
Signs
Often entirely normal examination between events
May detect bradycardia (especially LQT3)
Investigations
First-line
12-lead ECGMeasure QTc: Bazett formula QTc = QT / sqrt(RR interval). Prolonged if >470 ms (men) or >480 ms (women). QTc >500 ms = high risk for TdP. T-wave morphology may suggest subtype: broad T (LQT1), notched/bifid T (LQT2), late-appearing peaked T with long ST segment (LQT3)
ElectrolytesK+, Mg2+, Ca2+ — hypokalemia and hypomagnesemia are the most common metabolic causes of acquired QT prolongation
Medication reviewCheck all medications against CredibleMeds.org QT drug list. Common offenders: antiarrhythmics (sotalol, dofetilide, procainamide), antibiotics (macrolides, fluoroquinolones, TMP-SMX), antipsychotics (haloperidol, ziprasidone, droperidol), antiemetics (ondansetron high-dose), methadone
Second-line
Continuous ECG monitoringTelemetry or Holter: capture TdP episodes, assess QTc dynamically, detect T-wave alternans (marker of instability)
Exercise stress testQTc that fails to shorten (or paradoxically prolongs) with exercise suggests LQTS. LQT1 characteristically triggered by exercise
Specialist
Genetic testingRecommended when congenital LQTS suspected: guides subtype identification, family screening, and management (e.g., LQT3 may respond to mexiletine). Pathogenic variant found in ~75% of clinically diagnosed LQTS
1
Acute — Torsades de Pointes
- IV magnesium sulfate 2 g bolus (even if Mg level normal) — first-line
- If pulseless: defibrillation (unsynchronized)
- Overdrive pacing (temporary transvenous) at 90-110 bpm to shorten QT and suppress pauses
- Isoproterenol IV infusion as bridge to pacing (increases HR, shortens repolarization)
- STOP all QT-prolonging drugs immediately
- Correct hypokalemia (target K+ 4.5-5.0) and hypomagnesemia
- DO NOT give amiodarone, procainamide, or sotalol — they prolong QT and worsen TdP
2
Congenital LQTS — chronic management
- Beta-blocker: nadolol (preferred, longest-acting) or propranolol. Effective for LQT1 and LQT2. Less effective for LQT3
- Avoid QT-prolonging drugs (strict lifelong avoidance) — provide drug list to patient
- LQT1: avoid competitive swimming and strenuous exercise
- LQT2: avoid sudden auditory stimuli (alarm clocks, phone ringing) — use vibrating alarms
- LQT3: mexiletine (sodium channel blocker) may shorten QT; ICD threshold is lower
- ICD: cardiac arrest survivors (Class I), syncope on beta-blocker (Class I), QTc >500 with risk factors or high-risk genotype (Class IIa)
- Left cardiac sympathetic denervation: for patients with recurrent events despite beta-blocker + ICD or when ICD is contraindicated
3
Acquired LQTS
- Remove offending drug — QTc usually normalizes within days
- Correct electrolyte abnormalities
- Maintain K+ >4.0 and Mg2+ >2.0 while on any QT-prolonging drug that cannot be stopped
- If drug is essential (e.g., chemotherapy): initiate under telemetry monitoring with serial QTc checks
USMLE Step 2 CK Exam Tips
- 1QTc >500 ms = high risk for Torsades. Any QTc prolongation + polymorphic VT = Torsades until proven otherwise
- 2Torsades treatment: IV magnesium + overdrive pacing + stop QT drugs. Amiodarone is WRONG (prolongs QT)
- 3Know the QT-prolonging drug list: sotalol, haloperidol, erythromycin/azithromycin, fluoroquinolones, methadone, ondansetron (high dose)
- 4Congenital LQTS + deafness = Jervell and Lange-Nielsen syndrome (autosomal recessive, severe)
- 5LQT1 = exercise triggers (swimming). LQT2 = auditory stimuli (alarm clocks). LQT3 = rest/sleep. This is high-yield
- 6Beta-blocker is first-line for congenital LQTS — reduces SCD risk significantly
- 7Young person with "seizures" + family history of sudden death + normal EEG: think LQTS with misdiagnosed TdP episodes — get ECG
practicetest your knowledge on Long QT SyndromeApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — Cardiovascular and beyond.
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