An implantable cardiac stimulator includes a cardioversion/defibrillation unit connectable to at least one ventricular sensing electrode and one ventricular defibrillation electrode, and is designed to generate and deliver cardioversion or defibrillation shocks. A ventricular sensing unit having automatic threshold adaptation is connectable to the ventricular sensing electrode, and is designed to process the signals of the sensing electrode and detect a chamber contraction, and if a chamber contraction is detected, to output a ventricular sensing signal. The ventricular sensing unit processes the signals of the sensing electrode with at least two switchable sensing thresholds wherein after every sense, a VF detection window is started at a first lower sensing threshold once the VF detection window has passed, a T wave blanking window is activated at an upper second sensing threshold and once the T wave blanking window has passed, sensing at a second lower threshold is started.
