Here is a Pacemaker ECG with no signs of PM malfunction: Beat A is an intrinsic beat (atrial fibrillation). Beat B is a pseudofusion beat. Beat C is a fully paced beat. Beat D is a fusion beat. Ventricular fusion is the electrical summation of an intrinsic beat of the heart and depolarization from a pacing stimulus. The morphology lies between a fully paced beat and a complete intrinsic beat. Beat B is a pseudofusion beat: the pacemaker spike falls on top of an intrinsic beat, but the pacemaker stimulus does not contribute to the depolarization of the heart.