An implantable device for analyzing a high frequency (HF) electrogram signal including an implantable electrode, a signal pickup configured to pick up an electrogram signal including a HF component, a signal filter connected to the signal pickup and configured to measure a HF component from the signal only during a specific portion of a cardiac cycle, and an analyzer for analyzing the HF component, wherein the signal pickup, the signal filter and the analyzer are included within an implantable container, and the analyzer is configured to analyze at least one time-varying parameter of the HF component, and the signal filter is configured to measure the signal by using a signal picked up from at least one electrode selected from a group consisting of (a) intracardiac, (b) subcutaneous, (c) a can of the implanted device, (d) a combination of two of the above. Related apparatus and methods are also described.