1. A method of detecting an atrial tachyarrhythmia episode in a medical device, comprising: sensing a cardiac signal; identifying R-waves in the cardiac signal attendant ventricular depolarizations; determining classification factors from the R-waves identified over a first predetermined time period; classifying the first predetermined time period as atrial tachyarrhythmia based on comparing the determined classification factors to classification criteria; adjusting a classification criterion of the classification criteria from a first classification criterion to a second classification criterion after classifying the first time period as atrial tachyarrhythmia; classifying at least one subsequent time period as atrial tachyarrhythmia by comparing classification factors determined over the subsequent time period to the adjusted classification criterion; and detecting an atrial tachyarrhythmia episode in response to at least one subsequent time period being classified as atrial tachyarrhythmia based on the adjusted classification criteria.