A cardiac rhythm management system senses a cardiac signal indicative of heartbeats and an acoustic signal indicative of heart sounds and detects atrial tachyarrhythmia based on the sensed cardiac and acoustic signals. In various embodiments, the system senses the cardiac and acoustic signals without using an atrial lead, thus allowing for, for example, monitoring atrial fibrillation burden in a heart failure patient who does not wear an implantable device with an atrial lead. In various embodiments, the system detects heartbeats and heart sounds, measures parameters associated with the detected heartbeats and heart sounds, and detects one or more specified types of atrial tachyarrhythmia using the measured parameters. In various embodiments, the measured parameters are selected from heart rate, heart sound amplitude, cycle length variability, and systolic and diastolic intervals.
