Implantable heart stimulator connectable to an electrode arrangement has a pulse generator adapted to deliver stimulation pulses to a heart of a subject； an impedance measurement unit adapted monitor at least one heart chamber of the heart of the subject to measure the impedance in the at least one monitored heart chamber for generating an impedance signal corresponding to the measured impedance. The impedance signal is applied to a processor where the signal is processed, according to specified criteria, and a fractionation index value is determined represented by the curve length of the impedance signal during a predetermined measurement period. The fractionation index value is a measure of different degrees of mechanical dyssynchrony of the heart.
