Increased Atrial Pacing Rate Requires Progressive Shortening of AV delay. After optimization AV delay at atrial pacing rate of 70 bpm in the same patient as shown in Figure 4, paced atrial rate was then increased to 80 beats/min at the AV delay of 120 ms. Marked E and A fusion developed again (A). Progressive shortening of AV delay to 90 ms (B), and then 80 ms (C) caused mitral inflow E and A separation. Further shortening of AV delay to 70 ms (D) led to shortening of mitral inflow A-wave and premature closure of mitral valve. Hence, optimal AV delay at a paced atrial rate of 80 bpm was 80 ms.