Systems and methods for detecting cardiac arrhythmia are discussed. An exemplary arrhythmia detection system can receive physiologic information of the patient, measure a first signal metric using a first portion of the received physiologic information, and determine an arrhythmia detection duration using a comparison between the measured first signal metric and a reference signal metric value. The system includes an arrhythmia detector to detect an AT episode using a second portion of the physiologic information corresponding to the determined arrhythmia detection duration.
