The long QT syndrome (LQTS) is a disorder of cardiac ion channels that affect repolarization. The characteristic manifestations are prolongation of the QT interval and T-wave abnormalities on the ECG and exercise or emotion precipitation of syncope and sudden death, resulting from the ventricular tachyarrhythmia torsade de pointes (TDP) (Figure 1). The ion-channel dysfunction may be acquired or inherited. The acquired form is more common, usually caused by administration of QT-prolonging drugs (Table 1, Table 2), which for the most part impair the function of the I_Kr delayed rectifier channel. The inherited form is caused by mutations of genes that encode for cardiac ion channels, principally the I_Kr and I_Ks delayed rectifier potassium channels, with a minority of cases caused by mutations of the gene that encodes for the cardiac sodium channel.

Inherited LQTS has become a particularly important entity for several reasons. It is estimated to be present in 1 in 7000 persons in the United States and thus is not a rare disorder. It may cause as many as 3000 unexpected deaths in children and young adults per year. Further, recent discoveries concerning the molecular genetics and pathophysiology of LQTS have provided important insights into the mechanisms of arrhythmias, not only in LQTS but in general. These findings may provide molecular strategies for arrhythmia prevention in some of the estimated 300,000 to 400,000 sudden cardiac deaths<sup>100 Zipes D.P., Wellens H.J. Sudden cardiac death Circulation 1998 ;  98 : 2334-2351

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