Figure 16 l

Schematic diagram of the pacemaker timing cycles during VVI pacing. After a ventricuLar paced or ventricular sensed event, the pacemaker begins a ventricular refractory period (VRP). This is a programmable value usually between 250 and 400 ms. During this time, a sensed ventricular event will not initiate a new timing interval and will not reset the pacemaker. The Lower rate interval (LRI) of the pacemaker corresponds to the programmed lower rate. If this interval expires and there is no sensed ventricular event following the end of the ventricular refractory period, the pacemaker stimulates the ventricle (second and fourth complex) and the VRP and LRI begin anew. If there is an intrinsic ventricular depolarization between the end of the VRP and during the LRI, then ventricuLar output is inhibited and the VRP and LRI begin anew.

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

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