Figure 1

The upper limit of the band was set at an AV interval (AV1) 40 ms shorter than the intrinsic conduction time (time between atrial and right ventricular sensing), a choice that gives LBBB morphology but with a slight variation from that of the native QRS. The lower limit was set at the AV interval (AV4) that produced an RBBB-like morphology, but not complete RBBB. The time between the limits was divided into three intervals, thus identifying two intermediate AV intervals: AV2 and AV3.