Systems and methods for detecting slow and persistent rhythms, such as indicative of ventricular response to atrial tachyarrhythmia (AT), are described herein. An arrhythmia detection system monitors patient ventricular heart rate, and identifies slow heart beats with corresponding heart rates falling below a rate threshold during a detection period. The system identifies one or more sustained slow beat (SSB) sequences each including two or more slow heart beats. The system determines a first prevalence indicator of the identified slow heart beats, and a second prevalence indicator of the identified SSB sequences during the detection period. An arrhythmia detector circuit detects a slow and persistent rhythm using the first and second prevalence indicators.
