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Received Oct 31, 2011; Accepted Dec 22, 2011
1. Introduction
Acquired long QT syndrome is a disorder of cardiac repolarization most often due to specific drugs, hypokalemia, or hypomagnesemia that may precipitate torsade de pointes and cause sudden cardiac death. Selzer and Wray first reported QT prolongation and ventricular fibrillation as a response to quinidine in 1964 [1]. Two years later, Dessertenne [2] described torsades de pointes, a polymorphic ventricular tachycardia where QRS complexes twist around an isoelectric line in a sinusoidal fashion in an elderly woman with complete atrioventricular block and syncopal attacks (Figure 1). Torsade de pointes is usually self-limited but may degenerate into ventricular fibrillation. The incidence of acquired long QT syndrome is difficult to be estimated. Although the chances of provoking torsades de pointes by a noncardiac medication are generally lower than antiarrhythmic medications, a number of noncardiovascular drugs have been recently withdrawn from market because of unexpected sudden cardiac deaths associated with prolongation of QT interval and torsades de pointes [3]. The frequency of drug-induced long QT syndrome and our inability to predict the risk for a given individual, makes long QT syndrome an important issue for clinicians. This paper focuses on mechanisms underlying QT prolongation, risk factors for torsades de pointes and describes the short- and long-term treatment of acquired long QT syndrome.
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2. QT Interval Measurement
QT interval on the surface electrocardiogram describes the manifestation of ventricular depolarization and repolarization. It is measured from the beginning of QRS complex to T wave termination and averaged over 3 to 5 beats in a single lead. Longest QT intervals are usually measured in precordial leads and V3 or V4 leads appear more reliable for assessing QT prolongation [4]. Prominent U waves should be included in the measurement if they merge into the T wave. QT interval is influenced by heart rate. The RR interval preceding the QT interval should be measured for rate correction. Several formulae have been proposed for heart rate correction of the QT interval. The most commonly used formulae are Fridericia’s cube root formula (QTc = QT/RR1/3) and Bazett’s square root formula (QTc = QT/RR1/2). Although there is no consensus on best QTc method, Bazett’s formula is considered the gold standard, even...