Systems and methods for detecting cardiac arrhythmias such as atrial tachyarrhythmia (AT) are discussed. An exemplary system includes a ventricular beat analyzer circuit to detect ventricular beats and assess ventricular activity, such as to evaluate a ventricular rate stability. The system includes an arrhythmia detector circuit to detect respective AT indications in distinct time periods using portions of received physiologic information during the distinct time periods. A control circuit can monitor the ventricular beats on a beat-by-beat basis, in response to the detected ventricular beats satisfying an instability condition, trigger AT detections during the distinct time periods and withhold AT detection in a subsequent time period if no AT is detected in the present time period. An AT characteristic may be generated using the detected AT indications. A therapy may be delivered in accordance with the AT characteristic.
