A heart monitoring and/or therapy device for detecting and quantifying fusion beats, wherein the device includes an IEGM signal sensing channel that puts out a recorded IEGM signal and an evaluation unit operatively connected to the IEGM signal sensing channel that detects fusion beats. The evaluation unit comprises or is connected to a template memory including at least a first template electrogram signal and a second template electrogram signal. The evaluation unit compares the recorded IEGM signal with both, the first and the second template electrogram signal and determines at least 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 then calculates a fusion index (FI) from the at least two degrees of similarity.