An implantable medical device which senses atrial electrograms, detects atrial tachyarrhythmias and delivers atrial cardioversion/defibrillation pulses in response thereto, which employ a correlation analysis to assist in detection of arrhythmias and/or timing of delivery of cardioversion/defibrillation shocks. The device includes an electrogram recorder which stores atrial electrogram segments associated with R--R intervals extending over multiple atrial depolarizations and a correlator which measures correlation of successive stored atrial electrogram segments. The device may use the measured corellation meeting a defined threshold value to detect the occurrence of a defined tachyarrhythmia such as atrial fibrillation or allow delivery of a cardioversion/defibrillation pulse.