The disclosure includes methods and systems for treating ventricular arrhythmias. Embodiments include an implantable cardiac device or system including a determining module that determines a value of a parameter indicative of a rate of an intrinsic pacemaker of a heart of a patient experiencing fast ventricular arrhythmia (FVA) and a delivery module, programmed to deliver therapy for ventricular arrhythmias to a patient. Some methods include determining a value of a parameter indicative of a rate of an intrinsic pacemaker of a heart of a patient experiencing an FVA; if the value indicates the rate is about equal to or higher than a threshold, delivering a first therapy to the patient for terminating the FVA, and if the value indicates the rate is lower than the threshold, delivering a second therapy, different from the first therapy, to the patient for terminating the FVA.