Certain aspects of the present disclosure provide methods and apparatus for determining a precise localization of an arrhythmia origin or exit site in a heart of a subject using internal electrocardiograph (ECG) signals sensed and stored by an implantable device implanted in the subject. One example method of analyzing an arrhythmia in a subject generally includes reading, from an implantable device implanted in the subject, a plurality of internal ECG signals sensed and stored by the implantable device while the subject was experiencing an arrhythmia event (e.g., at any time, including while the subject was ambulatory)； performing an analysis of the read internal ECG signals； and determining a localization of the arrhythmia associated with the arrhythmia event, based on the analysis.
