The invention relates to an implantable ventricular heart stimulator (10), comprising an at least partially electrically conductive housing (12), an electrode line terminal for connection of a ventricular defibrillation electrode line (20) and a terminal for a defibrillation electrode (26). According to the invention, the heart stimulator has a far-field electrocardiogram detection unit (70) which has a first input connected to the terminal for the ventricular defibrillation electrode and has a second input connected to the electrically conductive housing and designed or configured to generate a far-field electrocardiogram on the basis of the electric potentials applied to the two inputs during operation. The inventive heart stimulator also has a far-field electrocardiogram evaluation unit (72) which is connected to the far-field electrocardiogram detection unit and is designed to detect signal features characteristic of atrial depolarizations in a far-field electrocardiogram generated by the far-field electrocardiogram detection unit.
