Systems and methods for detecting cardiac arrhythmia are discussed. An exemplary medical-device system includes an arrhythmia detector circuit that receives physiologic information, including respiration and heart beat information a patient, and determines whether a respiratory sinus arrhythmia (RSA) is present or absent using the respiration and the heart beat information. An indication of the presence or absence of RSA may be stored in a memory. The arrhythmia detector circuit can detect an AT episode using the indication of RSA.