An apparatus comprises an arrhythmia detection circuit configured to: detect atrial arrhythmia in a first portion of a sensed cardiac signal using a first arrhythmia detection criteria, wherein the sensed cardiac signal is representative of cardiac activity of a subject； and upon detection of the atrial arrhythmia, analyze a second portion of the cardiac signal that is prior in time to the first portion using a second different arrhythmia detection criteria to detect the presence or absence of the atrial arrhythmia in the second portion of the cardiac signal.
