The invention concerns a cardiac stimulating device (2) for biventricular stimulation (3,4). The V-V interval between the stimulation pulses to the right and left ventricles is variable. A farfield ECG obtained from measuring electrodes (6) located outside of the heart is used for monitoring the mechanical synchronization between the right and left ventricles. The mechanical syncronization is optimized by an automatical adjustment of the V-V interval until the QRS duration is minimized.
