"Invisible" Pseudo-Pseudofusion Beats

Three simultaneous leads (I, II and VI) were recorded in a patient with a normally functioning bipolar AV sequential (DVI) pacemaker. Superimposition of the ineffectual atrial spikes upon the early part of the QRS complexes resulted in pseudo-pseudofusion beats, clearly identified in lead II, However, the small spikes were hardly seen in lead I and not seen at all in lead VI. Therefore, the pseudo-pseudofusion beats were "almost invisible" and "invisible," respectively, in these leads. Delivery of atrial spikes within the beginning of the QRS complexes was possible because sensing of the ventricular electrogram occurred after the onset of ventricular depolarization in the surface leads. (PACE, Vol. 5. March-April, 1982)