An implantable cardiac stimulator includes a cardioversion/defibrillation unit connected to at least one electrode pair for generation and delivery of cardioversion or defibrillation shocks； an atrial sensing unit detecting atrial contraction, and outputting an atrial sensing signal indicating a atrial event when an atrial contraction is detected； a ventricular sensing unit detecting ventricular contraction, and outputting a ventricular sensing signal when a ventricular contraction is detected； a tachycardia detection unit connected to the atrial and ventricular sensing units and detecting a tachycardia, and classifying it as a ventricular tachycardia (VT) or as a supraventricular tachycardia (SVT)； and a treatment control unit designed to trigger at least one atrial cardioversion shock when a ventricular rhythm detected by the ventricular sensing unit is faster than a programmed frequency limit, and the tachycardia detection unit classifies an SVT as an atrial fibrillation (AFib).
