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Long QT Interval Syndromes


L. Brent Mitchell

, MD, Libin Cardiovascular Institute of Alberta, University of Calgary

Reviewed/Revised Jan 2023
Topic Resources

The long QT interval syndromes are disorders of the heart's electrical activity that predispose people to dangerous heart rhythms and sudden death.

The QT interval refers to the time between two events on the electrocardiogram (ECG)—from the beginning of the Q wave to the end of the T wave. Any abnormality that prolongs the QT interval increases the risk of a rapid, dangerous heart rhythm called torsades de pointes ventricular tachycardia Torsades de Pointes Ventricular Tachycardia Torsades de pointes ventricular tachycardia is a specific type of ventricular tachycardia that occurs in people who have a particular disorder of the heart's electrical activity called a long... read more (torsades). Ventricular tachycardia is a heart rhythm that originates in the ventricles (lower chambers of the heart) and produces a rapid heart rate. Torsades often turns into ventricular fibrillation Ventricular Fibrillation Ventricular fibrillation is a potentially fatal, uncoordinated series of very rapid, ineffective contractions of the ventricles (lower chambers of the heart) caused by many chaotic electrical... read more Ventricular Fibrillation , in which the heart stops beating, which is rapidly fatal.

ECG: Reading the Waves

ECG: Reading the Waves

An electrocardiogram (ECG) represents the electrical current moving through the heart during a heartbeat. The current's movement is divided into parts, and each part is given an alphabetic designation in the ECG.

Each heartbeat begins with an impulse from the heart's pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers of the heart (atria). The P wave represents activation of the atria.

Next, the electrical current flows down to the lower chambers of the heart (ventricles). The QRS complex represents activation of the ventricles.

The ventricles must then undergo an electrical change to get ready for the next heart beat. This electrical activity is called the recovery wave, which is represented by the T wave.

Many kinds of abnormalities can often be seen on an ECG. They include a previous heart attack (myocardial infarction), an abnormal heart rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart (ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.

Certain abnormalities seen on an ECG can also suggest bulges (aneurysms) that develop in weaker areas of the heart's walls. Aneurysms may result from a heart attack. If the rhythm is abnormal (too fast, too slow, or irregular), the ECG may also indicate where in the heart the abnormal rhythm starts. Such information helps doctors begin to determine the cause.

Did You Know...

  • The term "long QT" refers to an abnormal pattern of electrical activity that is seen on an electrocardiogram (ECG).

Causes of Long QT Interval Syndromes

Some people are born with a genetic abnormality that causes a long QT interval (called long QT syndrome). These inherited disorders are one type of heart disorder called a channelopathy Cardiac Channelopathies Cardiac channelopathies are abnormalities in heart cell proteins that control heart electrical activity and thus can cause heart rhythm disturbances. (See also Overview of Abnormal Heart Rhythms... read more .

In other people, the long QT interval is not inherited but results from disorders such as low serum levels of potassium (hypokalemia Hypokalemia (Low Level of Potassium in the Blood) In hypokalemia, the level of potassium in blood is too low. A low potassium level has many causes but usually results from vomiting, diarrhea, adrenal gland disorders, or use of diuretics. A... read more ), a very slow heart rhythm, or a drug. Often, drugs used to treat abnormal heart rhythms cause a long QT interval, but certain antidepressants and certain antiviral and antifungal drugs also can cause it.

The inherited long QT interval syndromes are classified based on the specific gene that has mutated. More than 15 forms of long QT syndrome have been identified. Many of these are inherited in an autosomal dominant pattern Non–X-Linked (Autosomal) Inheritance Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body or code for functional RNA molecules.... read more , in which a person with the disorder has a 50% chance of passing it on to each child they have. However, a specific genetic abnormality is found in only 50 to 75% of people. The likelihood of detecting an abnormality varies depending on the person's symptoms and ECG findings.


Sometimes, exercise brings on the ventricular tachycardia (see Sudden Cardiac Death in Athletes Sudden Cardiac Death in Athletes An estimated 1 to 3 per 100,000 apparently healthy young athletes develop an abrupt-onset heart rhythm abnormality and die suddenly during exercise. Males are affected up to 10 times more often... read more ). Other factors that increase the risk of torsades include female sex, older age, an underactive thyroid gland (hypothyroidism), brain disorders such as a stroke, and certain types of heart disease such as a heart attack or inflamed heart (myocarditis).

Symptoms of Long QT Interval Syndromes

The long QT interval syndromes do not cause symptoms unless torsades de pointes ventricular tachycardia occurs, which can cause palpitations (awareness of heart beats), light-headedness, or fainting. Torsades de pointes ventricular tachycardia may turn into ventricular fibrillation and cause cardiac arrest and sudden death.

Diagnosis of Long QT Interval Syndromes

  • Electrocardiography (ECG)

  • Usually exercise testing and ambulatory ECG monitoring

  • Sometimes genetic testing

A long QT interval may be found when an ECG is done for other reasons. People with unexplained cardiac arrest or fainting or a family history of such when the affected people do not have other heart disease are evaluated for long QT interval syndromes. Doctors typically do an ECG along with an exercise test and also have the person wear a heart monitor at home for several days or longer.

Not all people with a long QT interval have congenital long QT syndrome, and not all people with a congenital long QT syndrome have a long QT interval on any given ECG. Thus, doctors use a scoring system based on the results of the tests and the person's symptoms to predict how likely the diagnosis is. The score helps determine which people need genetic testing or provocative testing. With provocative testing, people are given a drug to try to cause the characteristic long QT interval on the ECG. People with a low likelihood of having a congenital long QT interval syndrome do not need genetic testing, but when the likelihood is intermediate or high, genetic testing is needed.

Close family members of a person with congenital long QT syndrome should be examined by a doctor to detect symptoms that may suggest an abnormal heart rhythm and should undergo ECG. Genetic testing and exercise testing also are sometimes needed.

Treatment of Long QT Interval Syndromes

  • Converting dangerous abnormal heartbeats to normal rhythm by giving an electric shock (defibrillation)

  • Eliminating predisposing causes and triggers, especially electrolyte abnormalities and use of certain medications

  • Usually beta-blockers

  • Sometimes an implantable cardioverter-defibrillator (ICD)

If people are experiencing episodes of ventricular tachycardia or ventricular fibrillation, treatment includes

Measures to prevent sudden death include avoiding specific triggers, including strenuous exercise. When possible, any medications that are likely to cause abnormal heart rhythms are stopped.

Beta-blockers are given to some people, such as those who have had episodes of fainting, cardiac arrest, or ventricular fibrillation or ventricular tachycardia.

A permanent pacemaker may be needed reduce the likelihood of recurrent torsades de pointes ventricular tachycardia. People who have been resuscitated after cardiac arrest and certain others need an implantable cardioverter-defibrillator Destroying Abnormal Heart Tissue (Ablation) There are many causes of abnormal heart rhythms (arrhythmias). Some arrhythmias are harmless and do not need treatment. Sometimes arrhythmias stop on their own or with changes in lifestyle,... read more (ICD).

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

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