Increased atrial pacing rate requires progressive shortening of AV delay. After baseline AV delay optimization shown in Figure 1, atrial pacing rate was increased to 70 beats/min, and AV delay was kept fixed at 190 ms. Marked E and A fusion developed (A). AV delay was then progressively shortened to 120 ms until E and A separation occurred (C). Hence, optimal AV delay at a paced atrial rate of 70 beats/min was 120 ms.