Stored endocardial electrogram derived from the ICD memory showing an episode of primary ventricular fibrillation in short QT syndrome. The ventricular tachyarrhythmia was initiated by a short coupled premature ventricular beat (VS encircled; coupling interval, 180 milliseconds) (upper electrogram strip, shock electrogram; lower electrogram strip, rate electrogram; ICD markers: FS, fibrillation sensing; FD, fibrillation detection; VS, ventricular sensing).