Long QT syndrome (LQTS) is a disorder of the heart's electrical activity. It may cause you to develop a sudden, uncontrollable, and dangerous heart rhythm called an arrhythmia (ah-RITH-me-ah) in response to exercise or stress.
Arrhythmias also can develop for no known reason in people who have LQTS. Not everyone who has LQTS develops dangerous heart rhythms. However, if one does occur, it may be fatal.
The term "long QT" refers to an abnormal pattern seen on an EKG (electrocardiogram). An EKG is a test that detects and records the heart's electrical activity. The QT interval, recorded on the EKG, corresponds to the time during which the lower chambers of your heart are triggered to contract and then build the potential to contract again. These chambers are called ventricles (VEN-trih-kuls).
The timing of the heartbeat's electrical activity is complex, and the body carefully controls it. Normally the QT interval of the heartbeat lasts about a third of each heartbeat cycle on the EKG.
However, in people who have LQTS, the QT interval usually lasts longer than normal. This can upset the careful timing of the heartbeat and trigger a dangerous, abnormal rhythm.
This answer from the National Heart, Lung and Blood Institute has been reviewed and/or edited by Dr. William D. Knopf.