Torsades de Pointes

Torsades de pointes (TdP) is polymorphic ventricular tachycardia with a twisting QRS axis around the isoelectric line, occurring with a prolonged QT interval.

Mechanism: early afterdepolarisations triggered by prolonged repolarisation, often after a short long short sequence (pause dependent).

Acquired long QT: QT prolonging drugs (class IA and III antiarrhythmics, macrolides, fluoroquinolones, antipsychotics such as haloperidol and thioridazine, methadone, ondansetron, citalopram, azole antifungals, antimalarials, pentamidine), hypokalaemia, hypomagnesaemia, hypocalcaemia, bradycardia, structural heart disease, female sex, subarachnoid haemorrhage, anorexia nervosa.

Congenital long QT syndrome: KCNQ1 (LQT1, exercise or swimming triggers), KCNH2 (LQT2, auditory triggers, postpartum), SCN5A (LQT3, sleep triggers).

QTc over 500 ms (or increase over 60 ms) markedly raises risk.

May degenerate to ventricular fibrillation and sudden death.

• Palpitations, presyncope, recurrent syncope, seizure-like episodes, sudden death.
• Self terminating runs in most; sustained episodes deteriorate to VF.
• Triggers: pauses, bradycardia, drug introduction, hypokalaemia.
• Family history of syncope, sudden death, drowning in congenital forms.

• ECG during tachycardia: polymorphic VT with changing QRS polarity and amplitude, rate 200 to 250 beats per minute.
• Baseline ECG: QTc (Bazett or Fridericia) over 460 ms in women, 450 ms in men, T wave morphology, T wave alternans; Schwartz score for congenital LQTS.
• Electrolytes (potassium, magnesium, calcium), medication review, thyroid, troponin.
• Genetic testing (KCNQ1, KCNH2, SCN5A) in suspected congenital forms; exercise testing (QT response), epinephrine challenge in selected cases.
• Echocardiography to exclude structural disease.

Acute

• Haemodynamically unstable or pulseless: unsynchronised defibrillation.
• Magnesium sulphate 2 g IV over 1 to 2 minutes (repeat once; infusion 3 to 20 mg/min), even with normal levels.
• Replace potassium to 4.5 to 5.0 mmol/L (and magnesium above 2 mmol/L).
• Stop all QT prolonging drugs.
• Increase heart rate to shorten QT: isoproterenol infusion 1 to 4 micrograms/min (not in congenital LQTS), or temporary transvenous overdrive pacing at 90 to 110 beats per minute; atropine in selected bradycardic patients.
• Lidocaine 1 to 1.5 mg/kg may help; avoid class IA, IC and III antiarrhythmics (except in LQT3 where mexiletine is used).

Congenital long QT syndrome

• Beta blockers: nadolol 1 to 2 mg/kg/day (preferred) or propranolol 2 to 4 mg/kg/day, in all patients; avoid metoprolol and atenolol.
• LQT3: mexiletine 150 to 200 mg three times daily; LQT2: potassium supplementation (spironolactone 25 to 50 mg).
• ICD for cardiac arrest survivors and recurrent syncope on beta blocker; left cardiac sympathetic denervation as alternative or adjunct.
• Avoidance of QT prolonging drugs (CredibleMeds list), electrolyte disturbances, specific triggers (LQT1: swimming and strenuous exercise; LQT2: loud noises; LQT3: sleep alarm).
• Genetic counselling and cascade screening.

Acquired

Correct triggers; discontinue offending agents permanently; monitor QTc during high risk therapy (baseline, 3 to 7 days after starting).