Cardiovascular · Topic 20 of 48

Long QT & Torsades de Pointes

Cardiovascular · QT prolongation

Long QT & Torsades de Pointes

A QTc above 500 ms breeds torsades. Stop the culprit drug, give IV magnesium 2 g, correct potassium, and defibrillate if unstable.

Key numbers

QTc above 500 ms: high torsades riskQTc > 500 ms

Stop culprit drug: QTc >500 or +60 msΔ QTc ≥ 60 ms

Magnesium sulfate 2 g IVMgSO₄ 2 g IV

01 · The numberQTc above 500 ms: high torsades risk

QTc is prolonged above about 450 ms in men and 460–470 ms in women; a QTc > 500 ms in either sex is highly abnormal and carries a 2- to 3-fold higher torsades risk.

02 · GiveMagnesium sulfate 2 g IV

Give magnesium sulfate 2 g IV over 1–2 minutes for torsades regardless of the serum magnesium; repeat 2 g if runs persist, then infuse 3–20 mg/min if renal function allows.

03 · DoUnstable torsades: unsynchronized shock

Torsades that is sustained, unstable or degenerates into VF needs an immediate unsynchronized (defibrillation-dose) shock — the changing QRS cannot be synchronized, and early recurrence is common.

What it is

Core idea

Torsades: twisting polymorphic VT with long QTTorsades de pointes is polymorphic VT whose QRS complexes twist around the baseline in a patient with a long QT; runs often self-terminate (causing syncope) but can degenerate into VF.

Causes & risk factors

Why it happens

Risk factors: female, low K⁺/Mg²⁺, bradycardiaTorsades risk rises with female sex, older age, hypokalemia, hypomagnesemia, hypocalcemia, bradycardia, heart failure or MI, diuretics, renal or hepatic impairment, and more than one QT-prolonging drug.

Presentation

What you'll see

Pause-dependent onset: short–long–short cycleTorsades typically starts after a short–long–short RR sequence (premature beat, pause, then a beat with a very long QT); bradycardia and pauses promote it.

Diagnosis & monitoring

Work-up

QTc (Bazett) = QT ÷ √RRBazett QTc = measured QT ÷ √(RR interval in seconds); it overcorrects at heart rates above about 85 bpm, so use the same lead, method and formula on serial ECGs.

Do right now

In this order

First moves

Unstable torsades: unsynchronized shock

→

Magnesium sulfate 2 g IVMgSO₄ 2 g IV

→

Stop culprit drug: QTc >500 or +60 msΔ QTc ≥ 60 ms

→

Raise the heart rate: pacing or isoproterenol

Why each step

Unstable torsades: unsynchronized shockTorsades that is sustained, unstable or degenerates into VF needs an immediate unsynchronized (defibrillation-dose) shock — the changing QRS cannot be synchronized, and early recurrence is common.

Magnesium sulfate 2 g IVGive magnesium sulfate 2 g IV over 1–2 minutes for torsades regardless of the serum magnesium; repeat 2 g if runs persist, then infuse 3–20 mg/min if renal function allows.

Stop culprit drug: QTc >500 or +60 msAfter starting a QT-prolonging drug, QTc > 500 ms or a rise ≥ 60 ms from baseline calls for action: stop or replace the drug, correct K⁺ and Mg²⁺, review interactions, have a defibrillator ready.

Raise the heart rate: pacing or isoproterenolFor recurrent pause-dependent (acquired) torsades, shorten the QT by raising the rate — temporary transvenous pacing above 70 bpm or IV isoproterenol — with expert consultation.

Management

Orders

Culprits: class Ia/III, antipsychotics, methadone, macrolidesCommon QT-prolonging culprits: class Ia (procainamide, quinidine) and III (sotalol, dofetilide, ibutilide) antiarrhythmics, haloperidol, thioridazine, methadone, erythromycin, clarithromycin, tricyclics and some antifungals — check CredibleMeds.

Replete potassium to 4.5–5 mmol/LCorrect hypokalemia and hypomagnesemia in every patient with torsades; raising potassium to 4.5–5 mmol/L may be considered (Class IIb).

Congenital LQTS: beta-blocker, not isoproterenolIn congenital long QT the mainstay is a beta-blocker (nadolol or long-acting propranolol), not isoproterenol; an ICD is indicated after cardiac arrest or for syncope despite beta-blockers.

Red flags & don'ts

Avoid harm

Avoid class Ia, Ic and III antiarrhythmics avoidDo not treat torsades with class Ia, Ic or III antiarrhythmics, which lengthen the QT further; lidocaine (class Ib) shortens the QT and may help drug-induced torsades.

Torsades or normal-QT polymorphic VT?

Commonly confused

QT between episodes	Prolonged
Usual cause	QT-prolonging drugs, low K⁺ or Mg²⁺, congenital LQTS
Magnesium	First-line drug
Antiarrhythmics	Avoid class Ia, Ic, III; lidocaine may help
Sustained or unstable	Unsynchronized shock

More detail

Reference

Show 7 reference facts

Congenital LQTS triggers by genotype: LQT1 — exertion, especially swimming, or emotion; LQT2 — sudden loud noise such as an alarm clock; LQT3 — rest or sleep.

ECG warnings of impending torsades: T–U wave distortion after a pause, macroscopic T-wave alternans, and new polymorphic ectopy, couplets or nonsustained runs.

Drug-induced torsades may unmask congenital LQTS: review personal and family history, and with unexplained syncope or early sudden death in the family, obtain ECGs in first-degree relatives and consider genetic testing.

Most QT-prolonging drugs block the rapid delayed-rectifier potassium current (IKr, hERG); prolonged repolarization triggers early afterdepolarizations that start torsades.

Methadone guidelines cited by the AHA statement recommend a pretreatment ECG for QTc, a follow-up ECG within 30 days, and then annual ECGs.

In acquired torsades, an acute beta-blocker can precipitate further episodes by causing or worsening bradycardia.

The modified Schwartz score estimates the probability of congenital LQTS (≥ 3.5 high, 1.5–3 intermediate, ≤ 1 low); intermediate- and high-probability patients get genetic testing.

Pearl

Twisting points need magnesium: 2 g IV, fix potassium and magnesium, stop the drug, and speed the heart up.

Sources · Merck Manual Professional — Torsades de Pointes Ventricular Tachycardia · Merck Manual Professional — Long QT Interval Syndromes · Drew BJ et al. Prevention of torsade de pointes in hospital settings: AHA/ACCF scientific statement. Circulation 2010;121:1047–1060 — PMC · 2025 AHA Guidelines for CPR and ECC — Part 9: Adult Advanced Life Support (Circulation 2025; doi 10.1161/CIR.0000000000001376), cpr.heart.org text · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).