Long QT syndrome (LQTS) is a rare, usually inherited and sometimes fatal disorder of the heart’s electrical system. It is estimated to occur in about 1 in 10,000 people, and most often is diagnosed in children.

LQTS affects the way the heart responds to the electrical signals that stimulate the heartbeat. In a healthy heart, electrical impulses are generated from a specialized area in the upper right atrium known as the sinoatrial node. The signal first stimulates the upper chambers of the heart, the atria, to contract. Then it travels along a specific conduction pathway to another specialized area called the atrioventricular (AV) node, which lies between the upper and lower portions of the heart. From there, the impulse moves down into the lower half of the heart, where it stimulates the ventricles to contract.

These signals are generated in a regular pattern that changes in response to the body’s demand for oxygen-rich blood. When more oxygen is demanded, during exercise or stress for example, the heart rate is increased. However, when this steady stream of impulses is interrupted, or the regular conduction pathway is compromised, the result is an irregular heartbeat. The heart may beat too slowly, too quickly or in a disorganized fashion. An abnormal heartbeat is known as an arrhythmia.

The electrocardiogram (EKG) is a painless test that measures these electrical impulses as they travel through the heart. The EKG detects five distinct waves in each complete cycle. These five waves are labeled P, Q, R, S and T. The P wave is associated with activity in the heart’s upper atria. The other waves reflect activity in the heart’s lower chambers (ventricles).

Long QT syndrome (LQTS) is a disease in which the Q-T interval is longer than normal (all of these times are measured in fractions of a second). The interval between Q and T corresponds to the time it takes for the ventricles to contract and then rest before beginning the contraction cycle again. A patient with LQTS is at risk for arrhythmias as a result of the lengthened interval. Arrhythmias may be triggered by stress or other external factors such as thunder, alarm clocks, waking up suddenly or swimming. Additionally, exercise may trigger arrhythmias because the Q-T interval does not shorten as it should when the heartbeat increases.

There are several types of arrhythmias that may be caused by LQTS. They include:

Many patients with LQTS do not experience symptoms, making it difficult to determine exactly how common the condition is. People who do experience symptoms may faint (syncope), experience a galloping heartbeat (palpitations) or cardiac arrest. In many cases, these symptoms are the first indication that a person has LQTS, and some researchers have speculated that some cases of sudden infant death syndrome are caused by LQTS.