An apparatus may include a sensing circuit and an arrhythmia detection circuit. The sensing circuit is configured to generate a sensed physiological signal representative of cardiac activity of a subject. The arrhythmia detection circuit is configured to monitor ventricular depolarization (V-V) intervals using the sensed physiological signal, detect when at least a portion of the V-V intervals satisfies an arrhythmia detection threshold interval, calculate a value of variability of the V-V intervals and calculate a value of variability of a systolic portion of the V-V intervals in response to the detection, and generate an indication of atrial fibrillation (AF) according to a comparison including the value of variability of the V-V intervals and the value of variability of the systolic portion of the V-V intervals and provide the indication to at least one of a user or process.