Long QT syndrome is an uncommon but serious heart condition. It can cause fainting and episodes of an abnormally rapid heart rhythm, called arrhythmia.
This happens because the electrical activity of the heart is disrupted – a problem that is either inherited or acquired later in life (see What is the cause? below).
Not everyone with long QT syndrome will experience arrhythmia, but when it does happen it can be fatal.
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What are the symptoms of long QT syndrome?
Some people with long QT syndrome will find that, at times, their heart suddenly beats in an abnormally fast, uncontrollable way. Doctors refer to this type of arrhythmia as 'torsade de pointes'.
When this happens, the heart cannot pump blood properly and the brain is temporarily starved of oxygen, causing the person to temporarily pass out.
The torsade de pointes rhythm usually returns to normal within a minute, and the person regains consciousness.
These episodes can start at any age and may be triggered by certain things such as:
- a sudden noise, such as an alarm bell
- exercise (especially swimming)
- a slow heart rate during sleep
However, there is not always a trigger.
How can it be fatal?
Sometimes, the abnormal heart rhythm persists, leading to ventricular fibrillation (a rapid, uncoordinated series of contractions). This rarely reverts to normal without medication and, if not electrically corrected, usually causes the heart to stop pumping (cardiac arrest) and death.
Long QT syndrome is a leading cause of sudden cardiac death in young, otherwise healthy people, and is often thought to be an underlying cause of sudden infant death syndrome.
What is the underlying cause of long QT syndrome?
To understand the underlying cause of long QT syndrome, it's important to know first how the heart cells work.
On the surface of each heart muscle cell are tiny pores, or ion channels. These open and close to let electrically charged sodium, calcium and potassium atoms (ions) flow into and out of the cells.
This passage of ions generates the heart's electrical activity. An electrical signal spreads from the top of the heart to the bottom, causing the heart to contract and pump blood. The box, above left, explains in more detail how each heartbeat happens.
Inherited long QT syndrome
People with congenital long QT syndrome will have inherited a mutated (altered) gene from one of their parents. This altered gene will affect the proteins that make up the ion channels in the heart cells.
Having altered versions of these genes means the ion channels may not work well, or there may not be enough of them, which disrupts the heart's electrical activity.
Drug-induced long QT syndrome
Long QT syndrome is not always inherited. It can be brought on by a number of different medications, including certain antibiotics and antihistamines.
According to the charity SADS Foundation, more than 50 commonly prescribed medications can lead to drug-induced long QT syndrome and arrhythmia, usually in people who have inherited a higher risk of developing the condition.
Learn more about drug-induced long QT syndrome by reading this factsheet from the SADS Foundation (PDF, 158kb).
What does 'long QT' mean?
As explained above, every time your heart beats, it produces tiny electrical signals. An electrocardiogram (ECG) machine traces these signals on paper – a typical pattern is shown below.
As the graph shows, each heartbeat is mapped as five distinct electrical waves – P, Q, R, S and T. The part of the pattern from Q to T represents the electrical activity of the heart's lower chambers, or ventricles.
In people with long QT syndrome, this Q-T interval lasts abnormally long. In other words, it takes longer for the heart cells in the ventricles to recharge after each heartbeat.
This can upset the careful timing of the heartbeat and may trigger an abnormally fast heart rhythm.
How is it diagnosed?
If your doctor thinks you have long QT syndrome after assessing your symptoms, they may ask you to have an ECG and refer you to a heart specialist (cardiologist).
In particular, if blackouts have occurred during exercise, or if there is a family history of sudden cardiac death below the age of 40, specialist assessment is needed.
An ECG, mentioned above, is a test that records the rhythm and electrical activity of your heart. In long QT syndrome, the trace of the QT section will be longer than normal.
Small stickers, called electrodes, are stuck on your arms, legs and chest and connected by wires to an ECG machine. The ECG may need to be taken while you exercise on a treadmill, as well as during rest.
Genetic testing may also be carried out to identify the defective gene that may be causing long QT syndrome. This can help determine which members of the family may have inherited the tendency and so need clinical assessment. Learn more about genetic testing.
How is it treated?
Lifestyle changes and medication are very effective in preventing people with long QT syndrome from suffering arrhythmia. Treatment should always begin with beta-blockers, and some people may also need to have a medical device implanted.
These treatments are explained below.
Long QT syndrome is treated first of all with a beta-blocker such as propranolol or nadolol.
Beta-blockers control irregular heartbeats by blocking the electrical nerve impulses that stimulate the heart. This decreases the activity of the heart and can slow heart rate.
Read more general information about beta-blockers.
If beta-blockers are not controlling your abnormal heart rhythm and fainting episodes, you may need to have a small device called an implantable cardioverter defibrillator (ICD) fitted.
An ICD is similar to a pacemaker. If the ICD senses that the heart is beating at a potentially dangerous abnormal rate, it will deliver an electrical shock, which can help return the heart to a normal rhythm or get it pumping again.
Your doctor may advise you to avoid strenuous activity or startling noise, if these tend to be triggers for your arrhythmia.
For example, you may need to tell people close to you never to wake you from sleep or rest with a loud noise, and remove telephones and alarm clocks from your bedroom.
Other activities, such as swimming alone in open water, may have to be avoided.
You will be advised to check carefully before starting any new medicines (either prescribed or over the counter).
You may also be advised to take potassium supplements or include more potassium in your diet. Good sources of potassium include:
- fruit, such as bananas
- nuts and seeds