Figure 16 o

Surface electrocardiogram with marker channels of a patient with a dual-chamber pacemaker in the DDD mode. Note atrial pacing of aLl complexes. The first, second, and last complexes have paced ventricular outputs in the middle of the QRS complex. Compared to the third, fourth, and fifth complexes, which do not have these ventricular pacing spikes, there is no change in the QRS complex. This is consistent with pseudofusion" of the QRS complex. This is due to the fact that the ventricular depolarization is not sensed by the pacemaker until the middle or end of the QRS complex in this particular patient. Subtle variations in the conducted PR interval account for the fact that some of the complexes have a paced ventricular complex and some do not.

Mitrani,R.D. and others,Cardiac Pacemakers,Hurst's The Heart,10th edition,Vol.1,pp.963-992