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 to 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 after adjusting the classification criterion and in response to both of the first predetermined time period being classified as atrial tachyarrhythmia based on the first classification criterion and the at least one subsequent time period being classified as atrial tachyarrhythmia based on the adjusted classification criterion.