A biventricular (BiV) implantable cardiac stimulator contains a stimulation control unit, one or more stimulation units, an impedance measurement unit and an impedance evaluation unit. The stimulation control unit is operatively connected to one or more stimulation units to control delivery of stimulation pulses by the one or more stimulation units. The stimulation control unit is configured to assess ventricular contractility based on an impedance signal generated by the impedance evaluation unit and to switch between at least a univentricular left ventricular stimulation mode and a biventricular stimulation mode and to evaluate the ventricular contractility in relation to the respective ventricular stimulation mode.
