Figure 16 q

Surface leads I, II, and III are shown at paper speed of 50 mm/s. Note that there is complete undersensing of the P wave. The pacemaker therefore tends to pace at the Lower rate interval, which corresponds with an intrinsic QRS complex. This QRS complex is sensed within the ventricular safety period, triggering ventricular safety pacing. For this particular pace maker, ventricular safety pacing occurs at 110 ms after atrial pacing in orde to avoid ventricular pacing during the I wave.

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