1. An implantable medical device for delivering cardiac therapy to a heart of a subject comprising: an electrode connector electrically connectable to multiple electrodes of which at least one is arranged on a cardiac lead electrically connected to said electrode connector; a signal generator connected to said electrode connector and configured to generate an electric signal applicable over two electrodes of said multiple electrodes; a signal sensing unit connected to the electrode connector and configured to sense a resulting electric signal over two electrodes of said multiple electrodes; an impedance processor configured to determine a cardiogenic impedance signal for at least a portion of a cardiac cycle based on said electric signal generated by said signal generator and said resulting electric signal sensed by said signal sensing unit; a moment processor configured to calculate, based on said cardiogenic impedance signal determined by said impedance processor, a moment parameter value representative of a weighed sum of impedance amplitudes within a time window encompassed by said at least a portion of said cardiac cycle and centered at a defined time instance within said at least a portion of said cardiac cycle, wherein each respective weight is dependent on a length in time between said defined time instance and the point in time for the impedance amplitude associated with said each respective weight; and an arrhythmia classifier configured to classify an arrhythmia of said heart based on said moment parameter value calculated by said moment processor.