Long QT Syndrome

The QT interval represents ventricular depolarization and repolarization. In long QT syndrome (LQTS), ventricular repolarization is delayed, creating a longer vulnerable period during which an abnormal beat can initiate polymorphic ventricular tachycardia called torsades de pointes. Torsades may terminate spontaneously and cause abrupt syncope, or deteriorate into ventricular fibrillation and sudden cardiac death. Congenital LQTS is usually caused by a pathogenic variant in a cardiac ion-channel gene. The three most commonly encountered forms are: - LQT1: A KCNQ1 variant reduces the slow delayed-rectifier potassium current, IKs. Adrenergic stimulation during exertion, particularly swimming or diving, is a typical trigger. - LQT2: A KCNH2 variant reduces the rapid delayed-rectifier potassium current, IKr. Sudden auditory stimuli, emotional stress, and the postpartum period are characteristic risk settings. - LQT3: An SCN5A variant increases the late inward sodium current. Events more often occur during rest or sleep. The genotype changes the clinical question from “Why did this person faint?” to “What triggered a malignant ventricular rhythm?” Acquired QT prolongation can arise from medications, electrolyte depletion, bradycardia, or drug interactions. It may also expose previously silent...