A 42-year-old woman was referred for assessment of asymptomatic QT prolongation. Her resting ECG demonstrated a corrected QT interval (QTc) of 480 ms (Fig. 1). During gradual bicycle exercise testing macrovolt T-wave altemans (TWA) (Figs. 1 and 2) was noted at intermediate heart rates. Subsequent genetic testing revealed a transmem-brane mutation in the KCNQ1 gene (Leu273Phe). Long-QT syndrome is characterized by a prolonged QT interval and an associated risk of ventricular arrhythmias and sudden cardiac death. Macrovolt TWA is defined as beat-to-beat variability in the amplitude and polarity of the T-wave and is thought to be due to spatial dispersion of repolarization between layers of the ventricular myocardium that may result in functional conduction block and reentrant ventricular arrhythmias. A beta-blocker was prescribed because of the high reported efficacy ofbeta-blockers in type 1 Long QT syndrome (LQT1). An implantable cardiac defibrillator (ICD) was discussed but not recommended because her other "risk factors" including her asymptomatic status and QTc < 500 ms in conjunction with the efficacy of beta-blockers in LQT1 were felt to place her at low risk despite the TWA.
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