Acquired long QT syndrome: medicines, low potassium and heart rhythm risk

The British Heart Foundation (BHF) explains that long QT syndrome (LQTS) affects the way electricity travels through the heart. Between each heartbeat the electrical system recharges, and in LQTS the heart takes longer than usual to do so. On an ECG, which records the heart's electrical activity, this delay shows up as a prolonged or long QT interval.

If the condition is present from birth because of an inherited gene, it is called congenital LQTS. The BHF says that LQTS is sometimes caused by certain medicines taken for other conditions, which is called acquired or drug-induced LQTS. The NHS describes acquired causes more broadly: medicines, some other heart problems, a bleed in the brain or stroke, and conditions that cause very low levels of potassium, magnesium or calcium.

Several things can combine. The NHS lists some types of antibiotics, antihistamines, antidepressants and heart medicines as possible causes, and the BHF extends the list to antipsychotics, antiarrhythmics and diuretics. A diuretic, or water tablet, can lower potassium levels, and the BHF notes that prolonged or severe vomiting or diarrhoea can affect sodium and potassium levels.

The risk is usually greatest when more than one factor is present, for example a QT-prolonging medicine together with low potassium after a stomach bug, or a new prescription added to existing ones. Not everyone who takes such a medicine develops a problem, and for most people the medicines are safe and valuable. This is why prescribers and pharmacists check interactions and, where there is a concern, request an ECG and blood tests.

• Certain antibiotics, antihistamines, antidepressants, antipsychotics, antiarrhythmics and diuretics.

• Low blood potassium, magnesium or calcium.

• Severe or prolonged vomiting or diarrhoea.

• Other heart problems, bleeding in the brain or stroke.

• An unrecognised inherited tendency, which may be revealed by a medicine.

The NHS says long QT syndrome can cause heart palpitations, which are a sudden feeling of the heart beating irregularly or faster than usual. The heart rhythm usually goes back to normal on its own, but if it continues there may be fainting, a seizure or fit, or a cardiac arrest. Many people have no symptoms at all.

The NHS says to call 999 if someone is not breathing normally and is not moving or responding, is having a seizure, or has fainted and cannot be woken within 1 minute. It says to ask for an urgent GP appointment or get help from NHS 111 if you have fainted and recovered, have palpitations that keep coming back, last longer than a few minutes or are getting worse, or have had a seizure and recovered. The BHF adds that if someone is unconscious and not breathing, you should call 999 and start CPR until medical help takes over, and ask someone nearby to find a public access defibrillator.

Diagnosis is made using an electrocardiogram (ECG), where sensors are put on the chest. The NHS says the ECG shows a pattern called a prolonged QT interval, and you may also wear a heart monitor for 24 hours or have an ECG while exercising. The BHF says the doctor will ask about family history and medicines you have been taking, and that sometimes LQTS does not show up on a standard ECG so the test may need to be repeated in different ways.

If the QT interval is long, doctors look for reversible causes: a list of medicines is reviewed and blood tests check potassium, magnesium and calcium. Genetic testing may be offered if an inherited cause is suspected, and the NHS says that if an inherited form is diagnosed the immediate family should also be offered testing.

For the acquired form, treatment starts with removing the trigger. The NHS says that where long QT syndrome is caused by another health condition or medicine, treating the condition or stopping the medicine can improve symptoms. This should be done by a doctor, as stopping some medicines suddenly is itself risky. Low potassium, magnesium or calcium is corrected, and the NHS and BHF mention potassium-rich foods such as bananas, vegetables and pulses, with supplements if the doctor recommends them.

Medicines called beta blockers are used for most people with long QT syndrome, according to the NHS, and an implantable cardioverter defibrillator may be recommended for people at high risk. The NHS says treatment depends on the cause, test results, symptoms and whether you are at high risk of serious problems, so the plan for an acquired case is individual to the person.

The NHS advises checking with a doctor or pharmacist before taking any medicines or supplements, avoiding dehydration, which may need rehydration solutions if you are being sick or have diarrhoea, avoiding energy drinks with a lot of caffeine, managing stress and anxiety, and avoiding strenuous exercise if you have been advised to. The BHF says that prolonged or severe vomiting or diarrhoea can affect sodium and potassium levels and recommends telling medical staff you have LQTS.

If an acquired cause is found and removed and the ECG returns to normal, the heart rhythm risk often falls, but the doctor may still note that you are sensitive to QT-prolonging medicines, and it is sensible to keep a record of this to show future prescribers.

ECGs, blood tests and medicine reviews are usually organised by your GP or hospital team within the NHS, with referral to a cardiologist when the QT interval is markedly long or an inherited form is suspected. Private cardiology clinics can provide ECGs and specialist opinions, but the key steps, identifying and removing the trigger, are the same.