A method and medical device for detecting a cardiac event that includes sensing a cardiac signal, identifying R-waves in the cardiac signal attendant ventricular depolarizations, determining RR-intervals between successive R-waves in response to the sensed cardiac signal, detecting an atrial tachyarrhythmia based on an analysis of the RR-intervals, iteratively sensing groups of a predetermined number of P-waves attendant atrial depolarizations in response to detecting the atrial tachyarrhythmia, and confirming the atrial tachyarrhythmia based on an analysis of the iteratively sensed groups of P-waves.
