The QT interval refers to the time between two events on the electrocardiogram—from the beginning of the Q wave to the end of the T wave.
Some people are born with long QT syndrome. In other people, the disorder is caused by low serum levels of potassium, a very slow heart rhythm, or a drug. Often, drugs used to treat abnormal heart rhythms cause long QT syndrome, but certain antidepressants and certain antiviral and antifungal drugs can also cause it.
People with long QT syndrome may develop torsades de pointes ventricular tachycardia, and sometimes ventricular fibrillation. Sometimes, exercise brings on symptoms (see Sudden Cardiac Death in Athletes).
People with long QT syndrome may have palpitations (awareness of heartbeats) if ventricular tachycardia develops. Torsades de pointes runs of ventricular tachycardia usually stop on their own but frequently recur. People may also feel very light-headed or faint. Ventricular fibrillation causes cardiac arrest and sudden collapse.
Electrocardiography (ECG) is used to detect long QT syndrome.
Because some forms of long QT syndrome are inherited, people who have a family history of the disorder or have relatives who have died unexpectedly because of a heart problem may be tested for long QT syndrome.
ECG: Reading the Waves
Defibrillation is needed if ventricular fibrillation develops. Sometimes doctors also give magnesium sulfate.
If a drug is the cause, it is stopped.
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
American Heart Association: Arrhythmia: Information to help people understand their risks of arrhythmias as well as information on diagnosis and treatment