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Figure 4 Consequences of optimization of atrioventricular (AV) delay during biventricular pacing at stable heart rate. The QRS complex resulting from P1 is wide due to apical right ventricular pacing (165 ms). The aortic pre-ejection time interval (Pre-Ao1) is long; the aortic systolic phase is also long due to the wide QRS complex. The second QRS complex resulting from P2 is narrowed due to biventricular pacing leading to a shorter aortic pre-ejection time interval (Pre-Ao2) compared with Pre-Ao1. Consequently, time duration of the aortic systolic phase is reduced, and the E-wave corresponding to P3 occurs earlier (compared to P1 and P2) with a greater amplitude, indicating a better LV filling phase. Pre-Ao3 is even shorter than Pre-Ao2 due to the addition of an AV delay optimization during P3, resulting in a greater cardiac output (CO) during P3 compared with the one obtained during P2, in which biventricular pacing was delivered without AV delay optimization.