Long QT Syndrome
Long QT Syndrome: A Complete Patient Guide
Long QT syndrome (LQTS) is a disorder of the heart’s electrical recharging system. The “QT interval” on an ECG represents the time the heart muscle takes to recharge between beats. When it is prolonged, the heart can develop a dangerous fast rhythm called torsades de pointes, which can cause fainting, seizures or sudden cardiac arrest.
There are two main forms. Congenital (inherited) long QT syndrome is caused by gene changes affecting ion channels in the heart and affects around 1 in 2,000 people. Acquired long QT is caused by medicines, low blood potassium, magnesium or calcium, or other medical conditions, and is far more common.
Many people with a prolonged QT interval have no symptoms and are diagnosed after an ECG or family screening. With the right diagnosis and treatment, which often means simple measures such as beta-blockers and avoiding trigger medicines, the risk of a serious event can be reduced greatly. The 2022 ESC guideline on ventricular arrhythmias and sudden cardiac death sets out diagnosis, risk assessment and treatment.
Long QT syndrome often causes no symptoms at all. When symptoms occur, they come from episodes of dangerous rhythm, which may be triggered by particular situations.
Symptoms
Fainting (syncope), often sudden and with little warning
Seizure-like episodes (which can be mistaken for epilepsy)
Palpitations or a fluttering sensation
Cardiac arrest or unexplained sudden death in the family
Typical triggers depend on the type
Exercise or swimming (especially LQT1)
Sudden loud noises such as an alarm clock or phone ringing, or emotional stress (especially LQT2)
Rest or sleep (especially LQT3)
No symptoms
Many people are identified through an ECG done for another reason, or because a relative has been diagnosed. A prolonged QT should still be taken seriously.
Long QT may be inherited, acquired, or a combination in which a mild inherited tendency is unmasked by a medicine or low potassium.
Inherited long QT
Caused by variants in genes such as KCNQ1 (LQT1), KCNH2 (LQT2) and SCN5A (LQT3)
Usually autosomal dominant, so each child of an affected parent has a 50% chance of inheriting the gene
Genetic testing identifies the cause in around 75% of people with a clear clinical diagnosis
Acquired long QT
Medicines: some antibiotics (such as macrolides and fluoroquinolones), anti-sickness drugs (ondansetron), antipsychotics, some antidepressants, methadone and certain heart rhythm drugs. A regularly updated list is maintained at crediblemeds.org
Low potassium, magnesium or calcium, from diarrhoea, vomiting, diuretics or poor intake
Slow heart rate, heart failure, thyroid disease and eating disorders
Female sex and other factors
Women are more likely to develop dangerous rhythms, particularly around childbirth, and genetic risk interacts with environmental triggers.
Fainting without a clear cause, especially with exercise, emotion or noise, always needs a heart rhythm assessment.
Arrange a prompt assessment if you have:
Unexplained fainting or blackouts, particularly during exercise, swimming or after sudden noise or fright
Seizure-like episodes that have not been explained, or a diagnosis of epilepsy that is not responding to treatment
A relative with long QT syndrome, unexplained sudden death under 40, or drowning or car accidents of unknown cause
A prolonged QT interval on an ECG
A medicine you take that may prolong the QT interval, with new palpitations or fainting
Call 999 immediately if you have:
Collapse or loss of consciousness, particularly with exercise
A seizure-like episode with no known cause
Fast palpitations with feeling faint or very unwell
If someone has collapsed and is not breathing normally, start CPR and use a defibrillator if available
Diagnosis combines the ECG, symptoms, family history and sometimes genetic testing.
1. Resting ECG
A 12-lead ECG measures the corrected QT interval (QTc). Values above about 480 ms (or above 460 ms with symptoms or family history) raise suspicion, and a score is used to confirm the diagnosis. Expert reporting matters, because QT measurement is easily mistaken.
2. Exercise and stress testing
An exercise ECG shows how the QT interval behaves during exertion and recovery, which can distinguish types of long QT. Some people also need medicines testing or a standing test.
3. Rhythm monitoring
Cardiac monitoring records symptoms and rhythm over days or weeks to look for abnormal rhythms.
4. Blood tests and echocardiogram
Potassium, magnesium, calcium, thyroid function and kidney function are checked, and an echocardiogram excludes structural heart disease.
5. Genetic testing and family screening
Genetic testing confirms the diagnosis and its subtype, and allows cascade testing of relatives, who are offered ECG and genetic testing. Genetic counselling is recommended.
Treatment aims to prevent dangerous rhythms. For many people it is simple and highly effective.
Lifestyle and avoiding triggers
Avoid medicines known to prolong the QT interval and check every new prescription with a pharmacist or doctor
Keep potassium and magnesium normal, and treat vomiting or diarrhoea promptly
Avoid specific triggers according to your type, such as competitive swimming in LQT1 or loud sudden noises (alarm clocks, phones) in LQT2
Medicines
Beta-blockers (such as nadolol or propranolol) are the mainstay, and are particularly effective in LQT1 and LQT2
Mexiletine may be added, especially in LQT3
Potassium supplements and potassium-sparing medicines can be used for selected types
Procedures and devices
Left cardiac sympathetic denervation is a surgery that reduces adrenaline effects on the heart for those who continue to have events despite beta-blockers or cannot take them
An implantable defibrillator (ICD) is considered after cardiac arrest, or in high-risk people who continue to have events despite treatment
Everyone should know
Family and friends should learn CPR, and there are advantages in having access to a defibrillator at home or in the workplace for high-risk individuals.
Take your beta-blocker every day and never stop it suddenly. Missed doses increase risk.
Carry a list of medicines to avoid and show it to every doctor, dentist and pharmacist. Check crediblemeds.org.
Stay hydrated and treat vomiting and diarrhoea early, because they lower potassium.
Exercise and sport: the 2020 and 2022 guidance supports shared decision-making. Many people can exercise safely on treatment, but some activities need adaptation. Discuss your own plan first.
Pregnancy and the postpartum period: require specialist care, because the risk is higher after delivery. Most women can have a safe pregnancy.
Family: relatives should be tested. Genetic counselling is available.
Driving: DVLA rules may apply, particularly after fainting or an ICD.
Support is available from SADS UK and the British Heart Foundation.
With diagnosis and treatment, most people with long QT syndrome live a normal lifespan and the risk of serious events falls markedly.
Risk is highest in people who have already fainted or had a cardiac arrest, those with a very long QT interval, and in some genetic types, so these people need closer follow-up.
Untreated long QT with symptoms carries a significant risk of sudden cardiac death, which is why diagnosis matters.
Risk often changes over time and with age and sex, so regular review is needed.
Acquired long QT usually resolves once the trigger medicine or electrolyte problem is corrected.
Long QT syndrome is an electrical disorder that can cause dangerous heart rhythms.
It may be inherited (around 1 in 2,000 people) or acquired through medicines or low potassium.
Many people have no symptoms, but fainting, seizure-like episodes or sudden death in the family are warning signs.
It is diagnosed with a carefully reported ECG, exercise testing, monitoring and genetic tests.
Beta-blockers are the main treatment, with avoidance of trigger medicines.
ICDs and sympathetic denervation are for high-risk individuals.
Relatives should be tested.
Carry a list of medicines to avoid, and correct potassium problems early.
With treatment, most people have a normal lifespan.
Guidelines and sources
This page follows current European guidance:
Zeppenfeld K et al. 2022 ESC Guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death. European Heart Journal 2022;43:3997-4126.
DVLA: Cardiovascular disorders: assessing fitness to drive (GOV.UK).
These are the questions patients most often ask about long QT syndrome.
What is long QT syndrome?
A heart rhythm condition in which the heart takes longer than normal to recharge between beats, which can trigger dangerous fast rhythms causing fainting or, rarely, cardiac arrest.
What are the symptoms of long QT syndrome?
Fainting, seizure-like episodes and palpitations, often triggered by exercise, emotion or sudden noise. Many people have no symptoms.
Is long QT syndrome hereditary?
Often, yes. The inherited form is typically passed on with a 50% chance to each child. Acquired long QT is caused by medicines or low potassium. Relatives of someone with inherited long QT should be tested.
How is long QT syndrome diagnosed?
By an ECG that measures the QT interval, along with symptoms, family history, exercise testing, rhythm monitoring and genetic testing.
Which medicines should I avoid with long QT?
Some antibiotics, anti-sickness medicines, antipsychotics, antidepressants, methadone and some heart rhythm drugs can prolong the QT interval. A regularly updated list is at crediblemeds.org, and any new medicine should be checked with a pharmacist or doctor.
Can I exercise with long QT syndrome?
Many people can, with treatment and tailored advice. Some activities, such as competitive swimming in LQT1, need extra care. Decisions are individual, so seek specialist advice.
Is long QT syndrome dangerous?
It can be, but the risk can be reduced greatly with treatment. Most people who are diagnosed and treated have a normal lifespan.
Do I need a defibrillator?
Only some people do, usually after cardiac arrest or if fainting continues despite beta-blockers. Most people are managed with medicines and avoiding triggers.
Can I have a private ECG review or assessment for long QT?
Yes. Expert ECG reporting is available across the UK, and a telephone, video or face-to-face consultation in Peterborough or Corby can assess symptoms and family history, with cardiac monitoring and exercise ECG arranged as needed.
- Which type of long QT do I have, and what is my risk?
- Do I need genetic testing, and should my family be tested?
- Which medicines must I avoid?
- Do I need a beta-blocker, another medicine or a device?
- How much exercise and sport is safe for me?
- What should I do if I faint?
- Is pregnancy safe for me?
- How often will my ECG be rechecked?
- Which triggers should I avoid for my type of long QT, such as swimming or sudden loud noises?
- Should I carry a medicine card or wear a medical alert?
ECG in long QT syndrome: the QT interval (start of the QRS to the end of the T wave) is prolonged.
Image: Michael Rosengarten, CardioNetworks ECGpedia, E315 (CardioNetworks ECGpedia), Wikimedia Commons. Licensed under CC BY-SA 3.0.
Torsades de pointes: the dangerous rhythm that can complicate long QT syndrome.
Image: CardioNetworks ECGpedia, Rhythm torsade (CardioNetworks ECGpedia), Wikimedia Commons. Licensed under CC BY-SA 3.0.
The QT interval
The ECG: each heartbeat has a P wave, a QRS complex and a T wave. The QT interval is measured from the start of the QRS to the end of the T wave.
Normal QT: the heart recharges within the usual time.
Long QT: recharging takes longer, which can allow a chaotic rhythm (torsades de pointes) to start.
Why measurement matters: the QT is corrected for heart rate (QTc), and accurate measurement needs experienced ECG reporting.
If you have unexplained fainting, a prolonged QT interval on an ECG or a family history of long QT syndrome, you may benefit from a telephone consultation or video consultation, or a face-to-face consultation in Peterborough or Corby for a more comprehensive assessment.
Investigations such as ECG, cardiac monitoring and exercise ECG may be recommended.
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Medically reviewed by Dr Salam Salloum, Consultant Cardiologist. Last updated: . This page is for general information and does not replace personal medical advice.