A method and implantable medical device for determining an atrial arrhythmia event that includes sensing a cardiac signal, determining an atrial arrhythmia score for identifying the arrhythmia event in response to the sensed cardiac signal, determining a sensing window in response to the sensed cardiac signal, the sensing window having a first portion and a second portion, determining signal characteristics of the sensed cardiac signal within the first portion and within the second portion, determining whether the sensed cardiac signal within the first portion and within the second portion corresponds to a P-wave in response to the determined signal characteristics, determining whether a signal to noise ratio of the sensed cardiac signal within the first portion and the second portion of the sensing window is satisfied, determining whether to update the arrhythmia score in response to the determined P-wave and the determined signal to noise ratio, and determining whether to delivery an arrhythmia therapy in response to the updated arrhythmia score.