Long QT syndrome (LQTS) is a rare, usually inherited and sometimes fatal disorder of the heart’s electrical system. LQTS patients often do not notice any symptoms until they are under some kind of physical or emotional stress. At that point, they could experience fainting (syncope) or more dangerous heart conditions such as ventricular tachycardia (a very fast heartbeat originating in the heart's lower chambers). In the most severe cases, LQTS patients may suffer sudden cardiac death. Women are more prone to sudden cardiac death as a result of LQTS.

Patients may or may not have symptoms, but many physicians will choose to treat all patients with confirmed LQTS because of the risk of sudden cardiac death. The most common treatment is medication (e.g., beta blockers), and patients are often recommended to limit their physical activity. If symptoms persist, surgery to implant an artificial pacemaker or an implantable cardioverter defibrillator (ICD) may also be recommended.