An apparatus for terminating an atrial fibrillation of an atrium of a heart comprises: (i) at least one sensor configured to provide an electric signal representative of the present electric state of the atrium； (ii) a determination unit configured to determine from the electrical signal whether the present electric state of the biological tissue is an atrial fibrillation； (iii) an electric pulse generator for generating at least one series of electric low energy anti-fibrillation pacing pulses at intervals depending on the electrical signal； (iv) at least one electrode connected to the pulse generator configured to apply the low energy anti-fibrillation pacing pulses to the atrium； (v) at least one further sensor configured to provide a further electric signal representative of an ventricular action potential of a ventricle of the heart； (vi) a further electric pulse generator configured to generate electric pacing pulses； (vii) at least one further electrode connected to the further pulse generator and configured to apply the electric pacing pulses to the ventricle； and (viii) a synchronization unit configured to synchronize the at least one series of low energy anti-fibrillation pacing pulses with the electric pacing pulses such that no low energy anti-fibrillation pacing pulse of the at least one series of low energy anti-fibrillation pacing pulses is applied in a vulnerable window during which the ventricle is susceptible to shock-induced ventricular fibrillation.
