A heart stimulator for biventricular pacing comprises at least one stimulation pulse generator connected or connectable to a right ventricular or atrial stimulation electrode lead and a left ventricular or atrial electrode lead, respectively. The heart stimulator is adapted to generate right chamber stimulation pulses for a right atrium or ventricle and left chamber stimulation pulses for a left ventricle or atrium. A high rate protection unit connected to said stimulation pulse generator is adapted to trigger, upon any right or left chamber stimulation pulse, a protection time window having a predetermined duration in time and comprising an early part immediately followed by a late part. The protection unit is further adapted to only allow atrial or ventricular stimulation pulses in said early part of said protection time window and to suppress any atrial or ventricular stimulation pulse in said late part.