What is the most common arrhythmia associated with Long QT (LQT) Syndrome?

Torsades de Pointes is the Most Common Arrhythmia in Long QT Syndrome

Torsades de pointes (TdP) is the characteristic and most common arrhythmia associated with Long QT Syndrome (LQTS). 1 This polymorphic ventricular tachycardia occurs in the setting of a prolonged QT interval and can potentially degenerate into ventricular fibrillation and sudden cardiac death if not promptly treated.

Pathophysiology of Torsades de Pointes in LQTS

Torsades de pointes has several distinctive features:

• Polymorphic ventricular tachycardia with characteristic "twisting of the points" QRS morphology
• Occurs in the setting of prolonged ventricular repolarization (QT interval)
• Often triggered by ventricular premature beats arising after a pause 1
• Typically occurs in self-limiting bursts causing dizziness and syncope 2
• May occasionally progress to ventricular fibrillation and sudden death 2

Risk Factors for Developing Torsades de Pointes

Several factors increase the risk of developing torsades de pointes in LQTS patients:

• Female sex (women are more susceptible than men) 1
• QTc exceeding 500 ms (higher risk threshold) 1
• Specific genetic variants (LQT1, LQT2, and LQT3 have different risk profiles) 1
• Bradycardia or pauses in heart rhythm 3
• Electrolyte abnormalities (particularly hypokalemia, hypomagnesemia, hypocalcemia) 1
• Use of QT-prolonging medications 1
• Specific triggers based on LQTS subtype:
  • LQT1: Physical exertion, especially swimming 1
  • LQT2: Emotional stress, loud noises 1
  • LQT3: Rest or sleep 1

Warning Signs and Precursors to Torsades de Pointes

The American Heart Association guidelines note that several ECG findings may precede torsades de pointes 1:

• Prolonged QT interval
• Pause-dependent increases in U wave amplitude
• Polymorphic ventricular premature beats
• Ventricular bigeminy

These warning signs may appear minutes or even hours before the development of polymorphic couplets, triplets, and eventually longer runs of torsades de pointes.

Acute Management of Torsades de Pointes

When torsades de pointes occurs, immediate treatment includes:

1. Intravenous magnesium sulfate as first-line therapy 2, 4
2. Electrical cardioversion for prolonged or hemodynamically unstable episodes 2
3. Increasing the underlying heart rate using:
   • Isoproterenol (isoprenaline) infusion 1, 2
   • Temporary transvenous pacing 1, 2
4. Correction of electrolyte abnormalities, maintaining potassium in high-normal range 2, 4
5. Discontinuation of any QT-prolonging medications 2, 4

Long-term Management of LQTS to Prevent Torsades de Pointes

The European Society of Cardiology and American College of Cardiology/American Heart Association guidelines recommend:

1. Beta-blockers as first-line therapy for all LQTS patients 1
2. Lifestyle modifications:
   • Avoidance of QT-prolonging drugs 1
   • Correction of electrolyte abnormalities 1
   • Avoidance of genotype-specific triggers 1
3. ICD implantation for high-risk patients:
   • Those with previous cardiac arrest 1
   • Those who experience syncope or VT despite beta-blocker therapy 1
4. Left cardiac sympathetic denervation for patients with recurrent symptoms despite beta-blockers 1
5. Genotype-specific therapies (e.g., sodium channel blockers for LQT3) 1

Clinical Pitfalls and Caveats

• Torsades de pointes can be misdiagnosed as other forms of polymorphic VT; the presence of QT prolongation is key to diagnosis
• Drug-induced LQTS is a common cause of acquired torsades de pointes; always check medication lists 2
• Amiodarone, despite being a QT-prolonging drug, has a relatively lower risk of inducing torsades compared to other Class III antiarrhythmics 3
• Avoid AV nodal blocking agents in patients with pre-excited atrial fibrillation as they may paradoxically increase ventricular response 1
• Patients with LQTS who are resuscitated from SCD have an especially ominous prognosis, with a relative risk of 12.9 for experiencing another cardiac arrest 1

Torsades de pointes represents the hallmark arrhythmia of LQTS and requires prompt recognition and management to prevent progression to ventricular fibrillation and sudden cardiac death.