Long-QT syndrome and torsades de pointes

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

The QT interval represents ventricular depolarisation and repolarisation. Rate correction is imperfect: Bazett overcorrects at high rates and undercorrects at low rates, so repeat measurements and clinical context matter.

Long QT can be congenital or acquired. Before labelling an inherited syndrome, review every medicine and supplement, correct electrolytes and repeat the ECG; a concealed inherited predisposition may nevertheless be unmasked by an acquired trigger.

Torsades management differs from other broad-complex tachycardias. Magnesium and removal of the QT trigger are central, while recurrent pause-dependent episodes may need the heart rate raised with pacing or isoprenaline.

Key points

Measure QT from QRS onset to T-wave end in a lead with a clear T wave and correct for heart rate; automated QTc values require manual review.
Repeated QTc of 480 ms or more, a diagnostic score of 3.5 or more, or a pathogenic LQTS variant can establish congenital LQTS after acquired causes are addressed.
Torsades is polymorphic VT whose QRS axis twists around the baseline, usually in the setting of QT prolongation and pause dependence.
Acquired causes include QT-prolonging medicines, hypokalaemia, hypomagnesaemia, hypocalcaemia and bradycardia.
Current RCUK treatment is magnesium 8 mmol IV over 10 minutes, correction of causes, and consideration of isoprenaline or temporary pacing to raise heart rate.
Avoid amiodarone in torsades because it can prolong repolarisation further.
Congenital LQTS requires inherited-cardiac-condition review, trigger and medicine counselling, family screening and usually beta-blocker therapy.
An ICD is for selected high-risk patients; it does not replace beta-blockade, trigger control or correction of acquired factors.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Congenital channel dysfunction

Pathogenic variants that reduce repolarising potassium current or increase inward current prolong ventricular action potentials. Expression varies, so a carrier may have an intermittently normal resting QT.

QT-prolonging medicines

Several prescribed, over-the-counter or recreational drugs delay repolarisation, especially in combination or when metabolic inhibition raises concentrations. Overdose and unrecognised interactions increase risk further.

Electrolyte depletion

Hypokalaemia, hypomagnesaemia and hypocalcaemia reduce repolarisation reserve or prolong repolarisation. Vomiting, diarrhoea and diuretics may cause losses, while renal dysfunction can alter electrolytes and increase exposure to renally cleared QT-prolonging medicines.

Bradycardia and pause dependence

Slow rates, atrioventricular block and short–long–short sequences lengthen repolarisation and favour early afterdepolarisations. This is particularly important in recurrent acquired torsades.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Repolarisation is delayed
Reduced outward potassium current or excessive inward sodium or calcium current prolongs the ventricular action potential. The surface ECG reflects this as a lengthened QT interval.
2
Early afterdepolarisations develop
Prolonged action potentials permit abnormal depolarisation before repolarisation is complete. Bradycardia, pauses, electrolyte depletion and medicines make these triggered beats more likely.
3
Repolarisation becomes heterogeneous
Different myocardial regions recover at different times, creating electrical heterogeneity. A premature ventricular beat arising during prolonged repolarisation can then initiate and sustain a shifting polymorphic rhythm.
4
Torsades begins
In the setting of QT prolongation, polymorphic ventricular tachycardia develops with QRS complexes appearing to twist around the baseline. Episodes may stop spontaneously, recur after another pause or cause immediate haemodynamic collapse.
5
Ventricular fibrillation may follow
Sustained or unstable torsades can degenerate into ventricular fibrillation, eliminating effective cardiac output. Cerebral hypoperfusion causes syncope or seizure-like movement before cardiac arrest and death.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Prolonged QTc

A repeated QTc at or above 480 ms without a drug or electrolyte explanation supports LQTS, but diagnosis can also rest on a validated score or pathogenic variant.

Torsades morphologyRed flag

Polymorphic broad complexes wax and wane in amplitude and appear to twist around the baseline, often after a short-long-short sequence.

Congenital clues

Exertional or emotion-triggered syncope, events after sudden noise, nocturnal events, unexplained seizures and a family history of LQTS or sudden death are important.

Acquired clues

Recent QT-prolonging drugs, overdose, diarrhoea/vomiting, diuretics, renal dysfunction and bradycardia point to a modifiable trigger.

Cardiac arrestRed flag

Unresponsiveness with absent normal breathing and no definite pulse converts the problem from torsades-with-a-pulse to VF/pulseless-VT ALS.

T-wave complexity

Notched, broad or low-amplitude T waves and prominent U waves can make the end of repolarisation hard to define; use a consistent tangent method and expert review.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Repeat manual 12-lead QT measurementFirst step
Why
Confirm that QT prolongation is real and reproducible.
Interpretation and limitations
Measure several beats in a clear lead, exclude a U wave from the T-wave endpoint and compare correction formulae when heart rate is extreme.
02
Medication and interaction review
Why
Identify prescribed, over-the-counter and recreational QT-prolonging agents or metabolic inhibitors.
Interpretation and limitations
Multiple modest QT risks can be additive; stop or substitute the culprit where clinically safe.
03
Potassium, magnesium, calcium and renal function
Why
Detect modifiable pro-arrhythmic abnormalities and guide replacement.
Interpretation and limitations
Correct all deficits; normal serum magnesium does not remove the indication for IV magnesium in torsades.
04
Ambulatory or exercise ECG
Why
Assess dynamic QT behaviour, pauses and symptom-rhythm correlation in specialist evaluation.
Interpretation and limitations
Abnormal recovery-phase QT adaptation can support the phenotype but is not interpreted in isolation.
05
Echocardiogram
Why
Exclude major structural disease and document ventricular function after arrest or recurrent arrhythmia.
Interpretation and limitations
Congenital LQTS usually has a structurally normal heart; an abnormal study broadens the differential.
06
Genetic testing and family evaluation
Why
Confirm a molecular diagnosis and enable cascade testing when clinical criteria are met.
Interpretation and limitations
A pathogenic variant supports diagnosis; a negative panel does not exclude clinically definite LQTS, and a variant of uncertain significance is not diagnostic alone.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Polymorphic VT with a normal QT

Acute myocardial ischaemia can cause polymorphic ventricular tachycardia without preceding QT prolongation. Ischaemic symptoms, dynamic ST-segment change and a QT that is not prolonged outside the arrhythmia point to a different mechanism and management pathway.

Catecholaminergic polymorphic VT

Exercise or emotional stress triggers bidirectional or polymorphic ventricular arrhythmia, often with a normal resting ECG and QT. Exercise testing through a specialist pathway helps discriminate.

Brugada syndrome

Brugada syndrome confers ventricular-fibrillation risk through a spontaneous or provoked type 1 right-precordial ST-segment pattern rather than QT prolongation. Fever or sodium-channel-blocking medicines may reveal the phenotype.

Epileptic seizure

Primary epilepsy can explain recurrent convulsive episodes, but self-terminating torsades also causes cerebral hypoperfusion and tonic movements. Sudden triggers, family history and rhythm correlation are discriminating.

ECG measurement artefact

Prominent U waves, broad or notched T waves and automated mismeasurement may falsely lengthen QTc. Consistent manual measurement in a clear lead and repeat ECG reduce the risk of misdiagnosis.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01arrestPulseless torsadesFirst stepThe patient is unresponsive, not breathing normally and has no definite pulse.

1Start CPR, call the arrest team and attach a defibrillator.
2Treat as VF/pulseless VT with immediate unsynchronised defibrillation under the RCUK ALS algorithm.
3Give ALS drugs at the specified shock points while correcting potassium, magnesium and the precipitating cause.
4After ROSC, continue monitoring and arrange urgent cardiology/inherited-arrhythmia assessment.

02pulseTorsades with a pulsePolymorphic VT occurs with QT prolongation and a pulse is present.

1Assess stability, attach pads and cardiovert immediately if shock, severe hypotension, ischaemia or pulmonary oedema is present.
2Give magnesium sulfate 8 mmol IV over 10 minutes and stop QT-prolonging medicines where safe.
3Correct potassium, magnesium, calcium, hypoxia and bradycardia; avoid amiodarone and other QT-prolonging antiarrhythmics.
4For recurrent pause-dependent episodes, seek expert help for isoprenaline or temporary pacing to increase heart rate.

03acquiredAcquired QT prolongation without torsadesQT is prolonged after medicine exposure, illness or electrolyte loss but no ventricular arrhythmia is present.

1Place high-risk or markedly prolonged cases on monitored care and calculate QTc manually.
2Stop or replace non-essential QT-prolonging medicines and check for interactions that raise drug concentrations.
3Correct potassium, magnesium and calcium and treat renal, endocrine or bradycardic drivers.
4Repeat ECG after correction; if QT remains prolonged or the history suggests inherited disease, refer to an inherited-cardiac-condition service.

04congenitalSuspected congenital LQTSRepeated unexplained QT prolongation, compatible syncope/arrest or a pathogenic family variant.

1Refer to an inherited-cardiac-condition specialist for diagnostic scoring, phenotype assessment and genetic counselling/testing.
2Give a personalised list of medicines and triggers to avoid, and plan hydration/electrolyte management during illness.
3Use a specialist-selected beta-blocker for most patients and assess adherence and genotype-specific issues.
4Consider ICD or left cardiac sympathetic denervation for selected high-risk patients, and offer cascade testing to first-degree relatives.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

First-line pharmacological treatment for torsades with a pulse, alongside cardioversion if unstable and correction of causes.

Magnesium sulfate

RCUK: 8 mmol IV over 10 minutes for polymorphic VT with QT prolongation.

Give with ECG and blood-pressure monitoring; toxicity risk rises in severe renal impairment. This RCUK dose is the resuscitation regimen.

Conditional bridge when rate acceleration is needed and temporary pacing is not immediately established.

Isoprenaline

RCUK adult bradyarrhythmia guidance gives a 5 micrograms/min IV infusion starting dose; in recurrent acquired pause-dependent torsades, titrate only with expert critical-care monitoring to increase heart rate.

Can increase myocardial oxygen demand and provoke arrhythmia; specialist use only. It is generally avoided as a strategy for congenital catecholamine-triggered LQTS unless an expert identifies a specific indication.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Recurrent syncope and injury

Brief self-terminating torsades abruptly reduces cerebral perfusion, causing collapse and sometimes convulsive movement. Falls, driving incidents and repeated misdiagnosis as epilepsy can compound harm.

Ventricular fibrillation and sudden death

Torsades may fail to terminate and degenerate into ventricular fibrillation. Without rapid defibrillation, absent cardiac output causes irreversible neurological injury and sudden cardiac death.

Electrical storm

Persistent QT-prolonging factors and pause dependence can produce closely recurring torsades episodes. Repeated instability requires monitored withdrawal of culprit medicines, correction of electrolytes and treatment of bradycardia, with immediate defibrillation capability.

Iatrogenic worsening

Adding another QT-prolonging medicine or failing to correct electrolyte loss can extend repolarisation further. Some antiarrhythmics may therefore aggravate rather than suppress torsades.

Risk to relatives

Congenital LQTS may remain silent until a trigger causes syncope or arrest. Without specialist family assessment, genetically susceptible relatives may continue avoidable medicine or trigger exposure.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Continuous ECG and defibrillator readiness during torsades or marked acquired QT prolongation.
Serial QTc using the same manual method and lead where possible after each intervention.
Repeat potassium, magnesium, calcium and renal function during replacement and after ongoing losses.
For congenital LQTS, track syncope, seizures, palpitations, beta-blocker adherence and exposure to avoidable QT-prolonging drugs.
Interrogate an ICD after therapy and reassess triggers; a shock does not establish that every future symptom is ventricular arrhythmia.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Manual measurement matters

ESC notes substantial inter-observer variation even among experts. Automated QTc is a prompt to inspect the tracing, not the final diagnosis.

Normal magnesium is not a veto

IV magnesium can suppress torsades even when the measured serum magnesium is not low.

Pause dependence guides treatment

A short-long-short sequence and bradycardia favour recurrent torsades; temporary pacing or isoprenaline reduces pauses while causes are corrected.

Avoid stacking QT risk

MHRA warnings emphasise additive risk from combining QT-prolonging medicines and from hypokalaemia or hypomagnesaemia.

Seizure can be arrhythmia

Brief cerebral hypoperfusion from self-terminating torsades can cause convulsive movements; obtain a cardiac and family history in unexplained seizures.

Family screening is clinical care

A confirmed inherited diagnosis changes assessment of first-degree relatives even when they feel well.

11Common pitfallsFrequent interpretation and management errors.

01
Accepting an automated QTc without checking QRS onset, T-wave end and heart rate.
02
Diagnosing congenital LQTS before correcting drugs and electrolytes.
03
Giving amiodarone to polymorphic VT with QT prolongation.
04
Waiting for a low serum magnesium result before treating torsades.
05
Treating an ICD as a substitute for beta-blocker adherence and trigger avoidance.