A medical device system for detecting an atrial tachyarrhythmia, comprising: sensing circuitry configured to sense a first cardiac electrical signal from a first sensing electrode vector; a processor; and memory accessible by the processor, the processor configured to: identify R-waves in the first cardiac electrical signal attendant ventricular depolarizations; determine RR intervals between successive R-waves in the first cardiac electrical signal; determine classification factors from the R-waves identified over a first 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; classify the first predetermined time period as one of unclassified, atrial tachyarrhythmia and non-atrial tachyarrhythmia by comparing the determined first classification factor and the determined second classification factor to classification criteria, wherein the first predetermined time period is classified as unclassified in response to the second classification factor indicating the presence of a ventricular tachyarrhythmia; detect an atrial tachyarrhythmia episode in response to the first time period being classified as unclassified and at least one preceding time period and at least one subsequent time period being classified as atrial tachyarrhythmia; and in response to detecting the atrial tachyarrhythmia episode, storing episode data determined from at least the first cardiac electrical signal in the memory for transmission to an external device. sensing circuitry configured to sense a first cardiac electrical signal from a first sensing electrode vector; a processor; and memory accessible by the processor, the processor configured to: identify R-waves in the first cardiac electrical signal attendant ventricular depolarizations; determine RR intervals between successive R-waves in the first cardiac electrical signal; determine classification factors from the R-waves identified over a first 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; classify the first predetermined time period as one of unclassified, atrial tachyarrhythmia and non-atrial tachyarrhythmia by comparing the determined first classification factor and the determined second classification factor to classification criteria, wherein the first predetermined time period is classified as unclassified in response to the second classification factor indicating the presence of a ventricular tachyarrhythmia; detect an atrial tachyarrhythmia episode in response to the first time period being classified as unclassified and at least one preceding time period and at least one subsequent time period being classified as atrial tachyarrhythmia; and in response to detecting the atrial tachyarrhythmia episode, storing episode data determined from at least the first cardiac electrical signal in the memory for transmission to an external device. identify R-waves in the first cardiac electrical signal attendant ventricular depolarizations; determine RR intervals between successive R-waves in the first cardiac electrical signal; determine classification factors from the R-waves identified over a first 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; classify the first predetermined time period as one of unclassified, atrial tachyarrhythmia and non-atrial tachyarrhythmia by comparing the determined first classification factor and the determined second classification factor to classification criteria, wherein the first predetermined time period is classified as unclassified in response to the second classification factor indicating the presence of a ventricular tachyarrhythmia; detect an atrial tachyarrhythmia episode in response to the first time period being classified as unclassified and at least one preceding time period and at least one subsequent time period being classified as atrial tachyarrhythmia; and in response to detecting the atrial tachyarrhythmia episode, storing episode data determined from at least the first cardiac electrical signal in the memory for transmission to an external device.