In an implantable medical device and a method for monitoring ventricular synchronicity of a heart. In particular, impedance signals are measured and an occurrence of a notch is detected in the impedance signal coincident with a period including a change from rapid to slow filling of a ventricle. The notch is indicated by a first positive slope change in a negative slope in a predetermined time window during a diastolic phase of a cardiac cycle. A degree of synchronicity is determined based on the notch feature, wherein a decreasing notch feature indicates an increased degree of synchronicity in the filling phase of the ventricles.
