According to the invention, a heart monitoring and/or therapy device for detecting and quantifying fusion beats is proposed, wherein the device comprises- an IEGM signal sensing channel that puts out a recorded IEGM signal showing at least prominent ventricular components and- an evaluation unit operatively connected to said IEGM signal sensing channel and being configured to detect fusion beats.The evaluation unit comprises or is connected to at least one template memory comprising at least a first template electrogram signal representing an intrinsic beat and a second template electrogram signal representing a captured (stimulated) beat. The evaluation unit is further configured to compare the recorded IEGM signal with both, the first and the second template electrogram signal and to determine two degrees of similarity for the recorded IEGM signal:- a first degree of similarity reflecting the similarities between the recorded electrogram signal and the first template electrogram signal and- a second degree of similarity reflecting the similarities between the recorded electrogram signal and the second template electrogram signal.The evaluation unit is further configured to calculate a fusion index FI from the at least two degrees of similarity.
