A medical device performs a method for detecting an atrial tachyarrhythmia by determining RR intervals between successive R-waves of a cardiac electrical signal and determining classification factors from the R-waves identified over a predetermined time period by determining at least a first classification factor correlated to variability of the RR intervals and a second classification factor indicating a presence of a ventricular tachyarrhythmia. The device classifies the cardiac electrical signal of the predetermined time period as unclassified, atrial tachyarrhythmia or non-atrial tachyarrhythmia by comparing the determined classification factors to classification criteria. The predetermined time period is classified as unclassified when the second classification factor indicates the presence of a ventricular tachyarrhythmia.