The disclosure includes methods and systems for treating cardiac arrhythmias. Some methods for treating an abnormal heart rhythm include determining a change in 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; when the change is within a first range, delivering a first therapy to the patient for treating the abnormal heart rhythm; and when the change is within a second range, delivering a second therapy to the patient for treating the abnormal heart rhythm, wherein the first therapy is different from the second therapy. In some embodiments, the first therapy may include shock therapy and the second therapy may include anti-tachycardia pacing.