The disclosure includes methods and systems for treating cardiac arrhythmias. Some embodiments provide an implantable cardiac device, for treating an abnormal heart rhythm. The device includes a determining module that determines a change in a parameter indicative of a sinus node cycle length of a heart of a patient between a time prior to the abnormal heart rhythm and a time during the abnormal heart rhythm. The device also includes a delivery module, programmed to deliver a first therapy for treating the abnormal heart rhythm to the patient when the change in the parameter is within a first range, and to deliver a second therapy for treating the abnormal heart rhythm, different from the first therapy, to the patient when the change in the parameter is within a second range.
