Clinical characteristics and treatment of short QT syndrome.
Short QT syndrome is a new inherited disorder associated with familial atrial fibrillation and/or sudden death or syncope. To date, three different mutations in genes encoding cardiac ion channels (KCNH2, KCNQ1 and KCNJ2) have been identified as causing short QT syndrome. All mutations lead to a gain in function of the affected current (IK(r), IK(s )and IK(1)). The syndrome is characterized in the few patients identified so far by a shortened QT interval of less than 300-325 ms after correction for heart rate at rates below 80 beats per minute. However, no boundary or limit for the QT interval can yet be determined, as more knowledge about this disease is still restricted to a small patient population. Furthermore, the QT interval lacks adaptation to heart rate. The majority of patients exhibit shortened atrial and ventricular effective refractory periods and inducibility of ventricular fibrillation. Death already occurs in newborns, so the short QT syndrome may also account for deaths classified as sudden infant death syndrome. The therapy of choice in families with a history of sudden death or syncope seems to be the implantable cardioverter-defibrillator. Whether patients without a family history of sudden death or symptoms need a defibrillator cannot yet be answered, and requires further investigation. Pharmacologic treatment has only been investigated in patients with a mutation in KCNH2 (HERG), and it could be demonstrated that the mutant currents may be insufficiently suppressed by drugs that are targeted to block the specific current (e.g., sotalol or ibutilide) in patients with a mutation in the IK(r-)coding gene KCNH2 (HERG). Interestingly, in this specific patient population, quinidine proved to be efficient in prolonging the QT interval and normalizing the effective refractory periods. Implantable cardioverter-defibrillator therapy is associated with an increased risk of inappropriate therapies for T-wave oversensing, although this risk can be resolved by reprogramming implantable cardioverter-defibrillator detection algorithms.
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