The invention refers to a biventricular (BiV) implantable cardiac stimulator that comprises a stimulation control unit (62), one or more stimulation units, an impedance measurement unit (70) and an impedance evaluation unit (78) wherein the stimulation control unit (62) is operatively connected to one or more stimulation units to control delivery of stimulation pulses by said one or more stimulation units. The stimulation control unit (62) is configured to assess ventricular contractility based on an impedance signal generated by the impedance evaluation unit (78) 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.
