This document discusses, among other things, detection of a sudden onset of a tachyarrhythmia. A sudden onset of tachyarrhythmia is determined by monitoring changes in intrinsic ventricular rate, such as by using one or more sensing channels in the ICD. A lowest tachyarrhythmia rate threshold is accompanied by a slightly lower “hysteresis tachyarrhythmia rate threshold.” If a sudden onset of tachyarrhythmia is declared, the sudden onset status is not reset by the ventricular rate falling below the lowest tachyarrhythmia rate threshold, but is instead reset by the ventricular rate falling below the slightly lower hysteresis tachyarrhythmia rate threshold.