Long QT Syndrome and Torsades de Pointes
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Direct answer
Long QT syndrome is a disorder of ventricular repolarisation in which the QT interval is prolonged, predisposing to torsades de pointes, a polymorphic ventricular tachycardia whose QRS complexes twist around the baseline and can degenerate into ventricular fibrillation. It has congenital forms caused by ion channel mutations and acquired forms, most often from drugs that block the hERG potassium channel. Acute torsades is treated with intravenous magnesium sulfate, even when serum magnesium looks normal.
What you must remember
- QTc thresholds: above about 450 ms in men and 460 ms in women is prolonged, and values over 500 ms carry the highest risk of torsades.
- Genotypes: LQT1 (KCNQ1) is triggered by exercise and swimming, LQT2 (KCNH2) by emotional stress or sudden noise, and LQT3 (SCN5A) during rest and sleep.
- Inheritance: Romano-Ward syndrome is autosomal dominant with normal hearing; Jervell and Lange-Nielsen syndrome is autosomal recessive with congenital deafness.
- Drug triggers: class IA and class III antiarrhythmics, macrolides, fluoroquinolones, methadone, haloperidol and ondansetron all prolong the QT.
- Electrolytes: hypokalaemia, hypomagnesaemia and hypocalcaemia prolong repolarisation, and bradycardia adds to the risk.
- Acute torsades: stop the drug, give magnesium sulfate, correct potassium, and use overdrive pacing or isoprenaline when pauses trigger the arrhythmia.
- Prevention: beta-blockers for LQT1 and LQT2, mexiletine for selected LQT3 patients, and an ICD for cardiac arrest survivors or high-risk genotypes.
Working through a prolonged QT on the ward
A 34-year-old woman with pneumonia is started on azithromycin, and haloperidol is added for agitation. On the third day she feels a flutter, then collapses, and the monitor shows a rotating ventricular tachycardia with the QRS amplitude waxing and waning around the baseline.
- Measure the QTc: The sequence examiners expect begins with the ECG, so the QTc is measured first.
- List the drugs: Every QT-prolonging drug on the chart is listed, and potassium, magnesium and calcium are then checked.
- Stop and give magnesium: The offending drugs are stopped and magnesium sulfate is given by rapid intravenous bolus, even when the serum level looks normal, because magnesium is given empirically in torsades.
- Shorten the pauses: If pauses trigger the arrhythmia, overdrive pacing or isoprenaline shortens them, and defibrillation is reserved for instability.
- Avoid the wrong drugs: Antiarrhythmics that prolong the QT, such as procainamide or amiodarone, are avoided.
- Consider congenital disease: It is considered in a young patient with a family history of sudden death, or when the QTc stays long after the drugs are gone.
- Guide prevention by genotype: Beta-blockade suits LQT1 and LQT2, while LQT3 responds better to mexiletine.
Where the examiner sets the trap
Four traps recur in examiner questions on this topic.
- Trap: treating torsades as ordinary ventricular tachycardia Correct: monomorphic VT is handled with amiodarone or cardioversion, but in torsades those drugs can worsen the arrhythmia, so the twisting morphology and long baseline QT must be recognised first.
- Trap: memorising only the classic drug list Correct: ondansetron, domperidone, citalopram and azole antifungals also prolong the QT, and a stem listing a sedated patient on several of these is testing whether you check the chart.
- Trap: giving magnesium without correcting potassium Correct: examiners also reward the answer that corrects potassium along with giving magnesium.
- Trap: ignoring the word congenital in a stem Correct: it points to the genotype table: exercise for LQT1, auditory triggers for LQT2, rest for LQT3, and deafness for Jervell and Lange-Nielsen syndrome.
Frequently asked questions
Which drugs most often cause torsades de pointes?
Class IA and class III antiarrhythmics such as quinidine, procainamide and sotalol, along with macrolides, fluoroquinolones, methadone and haloperidol, are the classic culprits. Checking the QTc before adding any of them is the simplest preventive step.
What is the first-line treatment for acute torsades de pointes?
Acute torsades de pointes is treated first with intravenous magnesium sulfate, given as a rapid bolus even when serum magnesium looks normal. The offending drug is stopped and potassium corrected at the same time.
How do the congenital genotypes differ clinically?
LQT1 is triggered by exercise and swimming, LQT2 by emotional stress or sudden noise such as an alarm clock, and LQT3 by rest and sleep. The T wave shape also differs, with broad-based T waves in LQT1, bifid T waves in LQT2 and a long isoelectric ST segment in LQT3.
What is the long-term treatment for congenital long QT syndrome?
Beta-blockers such as propranolol or nadolol are first-line and work best in LQT1 and LQT2. Mexiletine is used in selected LQT3 patients, and an implantable cardioverter-defibrillator is reserved for cardiac arrest survivors and high-risk genotypes.
Why is the QT interval corrected for heart rate?
Repolarisation shortens as the heart beats faster, so the raw QT overstates prolongation at high rates. Bazett's formula divides by the square root of the RR interval and overcorrects at fast rates, which is why Fridericia's cube-root formula is often preferred there.