- Patients with diagnosed long QT syndrome (Schwartz score >3 or QTc >500 ms).
- Patients under clinical suspicion of long QT syndrome: Asymptomatic subjects with serial ECGs showing QTc values >460 ms (prepubescent) or >480 ms (adults). Subjects with a personal or family history of sudden death. Subjects with a history of syncope of unknown origin. Subjects with ventricular fibrillation of unknown origin.
- Relatives of patients with genetic diagnosis of long QT syndrome.