This is a tracing from a patient in the pacemaker clinic. The pacemaker was set to VDD mode. There is group beating. The rhythm begins with a ventricular paced beat which occurs at the pacing interval as set by the programmed rate. There is then a retrograde P that is sensed by the atrial channel of the pacemaker and a pacemaker mediated tachycardia begins. The tachycardia stops probably because a retrograde P was blocked. Lack of capture of the atrium either seen with the VDD mode of pacing or with failure of the atrial pacing lead to capture is a mechanism for the initiation of pacemaker mediated tachycardia.