An implantable cardiac stimulator (10), configured to switch mode of operation between at least one right ventricular stimulation mode in which no control signals triggering left ventricular stimulation pulses are delivered to the left ventricular stimulation unit and a biventricular stimulation mode in alternation. Switching takes place as a function of duration of prevailing QRS signal interval, such that the cardiac stimulator switches to biventricular stimulation mode when comparison of the duration of a prevailing QRS signal interval with a first comparison value reveals the duration of the prevailing QRS signal interval is longer than a first reference value represented by the first comparison value and switches to right ventricular stimulation mode when comparison of the duration of a prevailing QRS signal interval with a second comparison value reveals the duration of the prevailing QRS signal interval is shorter than a second reference value represented by the second comparison value.
