In an implantable medical device and a method for monitoring ventricular synchronicity of a heart, impedance signals are measured that reflect septal wall movements and impedance amplitude peaks in the impedance signal are detected. A synchronicity index indicating a degree of synchronicity is determined based on detected impedance peaks, with at least two impedance peaks detected within a predetermined time window including a cardiac cycle or a part of a cardiac cycle indicating an increased dyssynchronicity in the ventricular contractions.
