Prolonged QT / Torsades de Pointes

See Genetics in Cardiac Disease for the channelopathy framing of congenital long QT, and VT / VF for other ventricular arrhythmias.

What torsades is

Torsades de pointes is a polymorphic ventricular tachycardia in which the QRS axis constantly shifts around the isoelectric line. It sits on a prolonged QT interval, typically one over 500 milliseconds. Most episodes of torsades de pointes are self-limiting. The danger is degeneration into ventricular fibrillation.

The mechanism is delayed ventricular repolarisation. Most culprit drugs block the rapid delayed rectifier potassium channel, which lengthens the action potential and the QT. Delayed repolarisation allows an early after-depolarisation to land on the T wave — the R-on-T phenomenon that starts the arrhythmia.

Recognising it

• ECG is the diagnosis: QRS complexes twisting around the isoelectric line.
• Onset is typically triggered by a premature ventricular contraction falling on the preceding T wave.
• A run of short–long–short R-R intervals often precedes it.
• Around half of patients are asymptomatic between episodes.
• Symptoms when they occur are syncope, palpitations and dizziness.
• Sudden cardiac death is the presenting event in up to 10%.

Defining a long QT

A QTc is conventionally called prolonged above 450 ms in men and 460 ms in women. A QTc over 500 ms carries a 2- to 3-fold increased risk of torsades.

Cut-offs alone are a poor risk tool. A 440 ms cut-off is over-sensitive and flags large numbers of people who have no torsades risk whatsoever.

Measure it manually. Automated readings from standard ECG machines are unreliable, especially in the patients whose QT actually is long. Measure from the start of the Q wave to the end of the T wave, in six leads, and take the median.

Prefer the QT nomogram to a correction formula. Plot the uncorrected QT against heart rate; a pair above the line is prolonged. Bazett’s formula over-corrects at fast heart rates and under-corrects at slow ones; a long QTc in a tachycardic patient is often an artefact of the formula.

Causes

QT prolongation is congenital or acquired. Acquired is far more common and is usually drug-related or bradycardia-related.

Drug classes repeatedly implicated: antiarrhythmics, antipsychotics, antiemetics, antifungals and antimicrobials. Sotalol, amisulpride and citalopram/escitalopram have the best-documented risk. Amiodarone prolongs the QT but rarely if ever causes torsades. Quetiapine, venlafaxine and risperidone have large volumes of normal QT data behind them and a very low torsades risk despite carrying warnings.

Drug interactions matter as much as single agents. Two QT-prolonging drugs together (for example escitalopram plus sotalol) is a pharmacodynamic interaction. A CYP inhibitor raising a QT-prolonging drug’s concentration is a pharmacokinetic one.

Non-drug contributors:

• Electrolyte abnormalities: hypokalaemia, hypocalcaemia, hypomagnesaemia.
• Cardiac ischaemia, cardiomyopathies, hypothyroidism and hypoglycaemia.
• Bradycardia, structural heart disease, age over 65, female sex and diuretic use.

Congenital long QT syndromes: autosomal dominant Romano-Ward syndrome, and the rarer autosomal recessive Jervell and Lange-Nielsen syndrome, which carries congenital sensorineural hearing loss — ask about deafness in a young patient with syncope.

Prescribing a QT-prolonging drug in general practice

• Weigh benefit against harm explicitly before starting.
• Do a baseline ECG before the first dose; more than one recording, or a Holter, if the risk is high or other risk factors are present.
• An abnormal QT off the drug contraindicates starting it.
• Repeat serial ECGs during treatment to check for further QT prolongation.
• Avoid stacking a second QT-prolonging drug, and correct electrolytes.
• Retrieve an old ECG for comparison where one exists.

Management of an episode

First actions, regardless of setting:

• Stop every drug that could be responsible, and optimise the electrolyte profile.
• Check and correct potassium, magnesium and calcium urgently, keeping serum potassium between 4.5 and 5.0 mmol/L.
• Continuous ECG monitoring; do not leave the patient unobserved.

Haemodynamically unstable, with a pulse: synchronised cardioversion. Pulseless: defibrillate and start advanced life support. Arrange urgent transfer.

IV magnesium is first-line drug therapy even when the serum magnesium is normal — magnesium sulfate 50% 4 mL (2 g) IV over 10–15 minutes, then 1–1.5 mL (0.5–0.75 g)/hour for 12–24 hours if required. Evidence for the infusion is limited. Prophylactic magnesium for drug-induced QT prolongation is not well established and does not reliably shorten the QT itself.

Torsades on a background of bradycardia: atropine 0.5 mg IV, repeated after 3–5 minutes if needed, to a maximum of 3 mg.

Refractory or recurrent runs — raise the heart rate. This is a retrieval/inpatient decision, not a GP one: isoprenaline infusion, or overdrive pacing at 90–110/min. Isoprenaline is contraindicated in congenital long QT, where it can paradoxically lengthen the QT.

Avoid amiodarone and sotalol in a patient with torsades — both prolong the QT further.

Congenital long QT — ongoing management

• Beta-blockade: metoprolol tartrate 25 mg PO BD, increasing if required to 100 mg BD; or propranolol 10 mg PO 3–4 times daily, increasing if required to 40 mg 4 times daily.
• Refer to a cardiac electrophysiologist for consideration of an implantable cardioverter-defibrillator.
• Advise a patient with congenital long QT to refrain from strenuous exercise.
• Genetics referral, and cascade screening of first-degree relatives.
• Lifelong medication-list vigilance — every new prescription needs checking against the QT-prolonging list.

Prognosis

Untreated congenital long QT syndrome carries mortality above 50% over 5 years, falling to single digits with treatment. Acquired long QT has a good prognosis once the trigger is identified and withdrawn.