An apparatus example comprises a cardiac signal sensing circuit configured to provide a sensed cardiac signal representative of cardiac depolarization events of a subject, a sampling circuit coupled to the cardiac signal sensing circuit, a therapy circuit, and a controller communicatively coupled to the sampling circuit and the therapy circuit. The controller includes a detection module configured to detect tachyarrhythmia using the cardiac signal and a signal analysis module configured to establish significant points (SPs) of the sampled cardiac signal, estimate heart rate during the tachyarrhythmia using the SPs, and provide an indication of whether noise is present in the cardiac signal using the SPs. The controller is configured to select a therapy for delivery by the therapy circuit in response to the tachyarrhythmia detection and to modify the selected therapy according to the heart rate estimation and the noise indication.
