Figure 16 n

Schematic diagram of DDD pacing with selected timing cycles and refractory periods. After a paced atriaL complex, the paced atrioventricular interval (PAy) begins. If there is no ventricular depolarization before this interval expires, the pacemaker response is to output a ventricular impulse. After a paced ventricular output, several refractory periods and timing cycles are initiated. The ventricular refractory period (VRP) is the time during which a ventricular event will not reset the timing intervals. The postventricular refractory period (PVARP) represents the time during which an atrial event will not be sensed or will not reset the timing intervals. The upper rate interval represents the shortest interval (maximum rate) that a pacemaker will ventricular pace corresponding to the programmed upper tracking rate. The ventricular-atrial escape intervaL (VA) represents the time during which, there is no sensed atrial electrogram, atrial pacing occurs. The programmed ower rate corresponds to the AV interval and the VA interval. During the First two complexes, there were no sensed atrial complexes; therefore, the VA interval expired and atriaL pacing occurred. During the third and fourth complexes, there were sensed atrial electrograms following the PVARP and before the VA interval expired (shaded area in the VA bar). Note that atriaL sensing usually occurs after the start of the P wave, representing the atrial conduction time to the atrial electrodes. The programmed AV interval following a sensed atrial compLex (SAV) may be programmed at a value lower than the PAV to obtain equivalent PR intervals.

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