Long QT syndrome is an inherited heart rhythm problem where the heart muscle takes longer than normal to recharge between beats. In some people, this can cause fainting or fits (seizures).

Long QT syndrome is uncommon, affecting about 1 in every 2,000 people.

Symptoms of long QT syndrome

Some people with long QT syndrome don't have any symptoms. They may only become aware of their condition after having an electrocardiogram (ECG) for another reason.

Those who do have symptoms usually experience:

• blackouts or fainting, because the heart has stopped pumping blood properly and the brain is temporarily starved of oxygen – the heart's rhythm returns to normal within a minute or two and the person regains consciousness
• seizures, which sometimes happen instead of a blackout when the brain is starved of oxygen
• heart palpitations, when the heart is beating in a fast or unpredictable way

These symptoms can start unexpectedly and may be triggered by:

• stress
• a sudden noise – such as an alarm
• strenuous exercise – particularly swimming
• a slow heart rate during sleep

Risk of death

The heart usually returns to its normal rhythm after it's been beating abnormally.

But if it continues to beat abnormally and isn't treated in time with a defibrillator, the heart won't pump and the person will die. This is called cardiac arrest, and the heart rarely returns to normal if it isn't electrically corrected with shock.

Dial 999 to ask for an ambulance if someone with long QT syndrome suddenly collapses and doesn't regain consciousness. If you're able to, perform cardiopulmonary resuscitation (CPR) until medical help arrives.

Long QT syndrome is a leading cause of sudden cardiac death in young, otherwise healthy, people. It can also be an underlying cause of sudden infant death syndrome (SIDS).

Causes of long QT syndrome

Long QT syndrome is usually caused by a faulty gene inherited from a parent.

The abnormal gene affects the proteins that make up the ion channels regulating electricity in the heart. The ion channels may not work well, or there may not be enough of them, which disrupts the heart's electrical activity.

Certain medicines can also trigger long QT syndrome, including some types of:

• antibiotics
• antihistamines
• antidepressants
• antipsychotics
• diuretics
• heart medications

But drug-induced long QT syndrome tends to only affect people who already have a tendency to develop the condition.

Cardiac Risk in the Young has published a list of medications that people with long QT syndrome should avoid.

The Sudden Arrhythmia Death Syndromes (SADS) Foundation has also produced a guide about acquired, drug-induced long QT syndrome (PDF, 158kb).

Diagnosing long QT syndrome

If your GP thinks you have long QT syndrome, if you have blackouts during exercise, or if you have a family history of unexpected and unexplained sudden death, they may recommend you have an ECG and refer you to a heart specialist (cardiologist).

An ECG records your heart's rhythm and electrical activity. If you have long QT syndrome, the trace of the QT section (showing part of the heartbeat) will be longer than normal. Sometimes an exercise ECG will be needed to confirm the diagnosis.

Genetic testing may be needed to identify the defective gene causing long QT syndrome. It can also help identify other family members who may have inherited the defective gene and need clinical assessment.

Treating long QT syndrome 

Most people with inherited long QT syndrome will need treatment with medicines. Beta-blockers, such as propranolol or nadolol, may be prescribed to help control irregular heartbeats and slow down your heart rate.

If your symptoms are frequent or severe, and you have a high risk of having a life-threatening arrhythmia, you may need to have a pacemaker or implantable cardioverter defibrillator (ICD) fitted.

In some cases of long QT syndrome, surgery may be needed to control the flow of chemicals into the heart. This is known as a sympathectomy.

If medication is causing long QT syndrome, your medication will be reviewed and an alternative may be prescribed.

Living with long QT syndrome

With the right treatment, it should be possible to lead a relatively normal life. However, you may need to make some lifestyle adjustments to reduce your risk of having blackouts.

For example, you may not be able to exercise strenuously or play competitive sports, and may need to try to avoid startling noises, such as alarm clocks. Avoiding stressful situations may also be recommended.

Rehydrating properly after a tummy upset is also essential, usually with supplements containing salt and sugar.

Your doctor may prescribe potassium supplements or suggest increasing the amount of potassium-rich foods in your diet. Good sources of potassium include:

• bananas
• vegetables
• pulses
• nuts and seeds
• milk
• fish
• shellfish
• beef
• chicken
• turkey
• bread

Always tell medical staff that you have long QT syndrome. Any new medication, whether prescription or over the counter, will need to be carefully checked to see if it's suitable for you.

The British Heart Foundation has produced a booklet called Life with Inherited Abnormal Heart Rhythms, which has more information and advice about living with long QT syndrome and other inherited heart conditions.