The implantable heart stimulator (10) has a right-atrial sensing unit connected to a right-atrial electrode for recording electrical potentials of the myocardium of a right atrium. The heart stimulator also has a left-ventricular sensing unit connected to a left-ventricular electrode for recording electrical potentials of the myocardium of a left ventricle of a heart (18). The left-ventricular sensing unit is implemented to detect signal features in the electrical signal recorded through the left-ventricular electrode which characterize a left-atrial contraction. An independent claim is included for a heart stimulation system with a telemetry unit.
