An implantable medical device and method for determining an atrial arrhythmia event that includes a cardiac sensing device comprising a housing having circuitry positioned therein, a plurality of electrodes electrically coupled to the circuitry to sense a cardiac signal, and a processor configured to generate an initial detection of an atrial arrhythmia event in response to an atrial arrhythmia threshold, determine whether a P-wave occurs during the initial detection, determine an adaptive threshold in response to the P-wave being detected, adjust the atrial arrhythmia threshold in response to the adaptive threshold, and generate a subsequent initial detection of an atrial arrhythmia event using the adjusted atrial arrhythmia threshold.