Abstract 15078: Non-selective but Not Selective His Bundle Pacing Uniquely Resolves Acute and Chronic Right Bundle Branch Block

Introduction: While acute (Ac) right bundle branch block (RBBB) may be in proximal HB (lead fixation injury) and chronic (Ch) RBBB in distal HB, they both cause terminal QRS delay. Both types of RBBB may be corrected with non-selective (NS) His bundle pacing (HBP), however the mechanism is unknown. Hypothesis: To study the role of high (5V) and low (1V) pacing voltage as well as presence of pre-excitation (in NS-HBP) in resolving Ac and chronic Ch-RBBB. Methods: Of the thirty-nine patients (27 ChRBBB and 12 AcRBBB), 25 ChRBBB and 10 AcRBBB patients showed NS-HBP at 5V and 11/25 ChRBBB and 6/12 AcRBBB patients transitioned to selective (S)-HBP at 1V. Four patients showed S-HBP at 5V and 1V. Results: 1)During NS-HBP at 5V: (In 25 ChRBBB and 10 Ac-RBBB) Complete resolution occurred 12/25ChRBBB and in all 10 AcRBBB. A partial resolution of ChRBBB occurred 13/25 ChRBBB.2. During NS-HBP at 1V: (6/14 ChRBBB and 2/4 AcRBBB showed complete resolution at 1V, remainder showed incomplete RBBB3. During S-HBP at 5V : (2 ChRBBB 2 AcRBBB) only 1 AcRBBB showed resolution of RBBB. 4. During S-HBP at 1V : All 13 ChRBBB and 8 AcRBBB did not show resolution os RBBB including 11 Ch-RBBB and 6 Ac-RBBB RBBB which resolved during 5V NS-HBP, abruptly recurred on transition to S-HBP at a mean of 2.4±0.8V. Conclusions: 1)The lack of RBBB resolution with S-HBP suggests that pacing site was proximal to site of block, however, from the same site NS-HBP either completely or incompletely resolved both Ac- and Ch-RBBB. 2) Partial resolution of RBBB in NS-HBP may be explained by right ventricular free wall pre-excitation when site of Ch-RBBB is distal.3) Complete resolution of both acute and Ch-RBBB in NS-HBP, more so at high pacing voltage, suggest that peri-Hisian tissues may behave more like a specialized conduction tract which uniquely resolves conduction block.