The Physiologic Mechanisms of Paced QRS Narrowing During Left Bundle Branch Pacing in Right Bundle Branch Block Patients

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  FIGURE 1. (A) The QRS morphologies of intrinsic right bundle branch block (RBBB) under different pacing configurations of left bundle branch pacing (LBBP) at speed of 50 mm/s. QRS duration is shortened from intrinsic 148–122 ms at low output unipolar LBBP, and is further shortened to 102 ms at high output. The paced QRS morphology does not change significantly in unipolar pacing with low or high outputs, both presented a Qr pattern in lead V1. At low output bipolar LBBP, the paced QRS duration is 123 ms, which was similar to the performance at low output unipolar LBBP. However, at high output bipolar LBBP, the paced QRS duration is shortened to 84ms, and the paced QRS morphology changes significantly, showing that the r’ wave at the end of the QRS in lead V1 disappeared, suggesting the RV delay was compensated. (B) Sheath angiography of 3830 lead after LBBP completion. (C1–C6) Schematic diagram of different pacing modes of LBBP performed on RBBB. (C1–C3) Low output unipolar LBBP only captures the left bundle branch (LBB) and a small portion of the surrounding myocardium to partially shorten the RBBB. High-output unipolar LBBP captures LBB and more surrounding myocardium, further shortening…