An implantable cardiac electrostimulator includes an atrial sensing channel generating an atrial sense event signal upon detection of atrial activity, a ventricular sensing channel generating a ventricular sense event signal upon detection of ventricular activity, a VES detector detecting ventricular extrasystoles, atrial and ventricular stimulation pulse generators, and a stimulation control unit. The stimulation control unit determines scheduled delivery times of atrial stimulation pulses (T(A)) and/or ventricular stimulation pulses (T(V)), and triggers delivery if no atrial sense event signal arises before the end of an atrial escape interval (VAI) timing out at T(A), or if no ventricular sense event signal arises before the end of a ventricular escape interval (VEI) timing out at T(V). T(A) and T(V) depend upon detection of a ventricular extrasystole, and proper atrioventricular synchrony is maintained by setting a physiologically adequate A-V-delay between T(A) and T(V).