Effect of His Bundle Pacing on Right Bundle Branch Block Located Distal to Site of Pacing.
Rehan Mahmud MD,
McLaren Bay Region,
1900 Columbus Ave, Bay City MI 48708, USA
Corresponding author : drmahmud@msn.com
Shakeel Jamal, MD
McLaren Bay Region,
1900 Columbus Ave, Bay City MI 48708 USA
jamal1sm@cmich.edu
Brenda Harris RN
McLaren Bay Region,
1900 Columbus Ave, Bay City MI 48708, USA
drmahmud@msn.com
Abstract
Aims: it is generally accepted that bundle branch block (BBB) may be corrected simply by pacing (P) the His bundle (HB) distal to site of block. This hypothesis, based on observations with percutaneous catheters, assumes that conduction block is in proximal HB. However, these postulations have not been systematically studied following active fixation of HB pacing lead.
We analyzed role of pacing voltage and capture thresholds in selective (S) and non-selective (NS) HBP in patients with right (R) BBB.
Methods: In thirty-nine patients with RBBB, 4 showed S-HBP, 18 showed NS-HBP, and 17 showed NS-HBP at >2.4±0.8 V and S-HBP at lower voltage (NS-S HBP group).
Results
  1. During S-HBP there was no resolution of RBBB.
  2. NS-HBP completely or partially resolved RBBB along with a decrease in QRS activation time (91±9ms from 98±6ms).
  3. NS-HBP group with capture threshold of 1.3±0.5V completely resolved RBBB in 9/14 vs 3/11 patients in NS-S HBP group with higher capture threshold of 2.4±0.8V.
  4. During NS-HBP higher voltage caused complete resolution of RBBB in 22/39 patients vs 10/39 at lower voltage.
Conclusions:
  1. Lack of correction with S-HBP suggests that RBBB was distal to site of HBP and yet was corrected with NS-HBP.
  2. Voltage dependent properties in NS-HBP suggests that conduction via a specialized parallel pathway maintains normal ventricular activation time.
  3. Correction of RBBB in all patients with NS-HBP, suggests that conduction block was either bypassed or right ventricular free wall pre-excited by conduction via a parallel pathway.
Introduction
In His bundle pacing (S-HBP) it is generally assumed that conduction block is in proximal His bundle and pacing distal to site of block results in normal QRS complex (1-4).
However, the site of His bundle pacing is predicated on recording a His bundle electrogram and achieving acceptable pacing threshold and moving the lead distally along the His bundle is not always an option. It would seem plausible therefore that correction of HPS conduction block may be due to mechanisms other than serendipitously pacing from a site distal to site of block.
Right bundle branch block (RBBB) presents a unique opportunity to study the mechanisms involved as acute RBBB seen following lead fixation may well be in close proximity yet ‘distal’ to pacing lead tip location, whereas chronic RBBB may be in either proximal or distal.
Furthermore, in RBBB the conduction delay primarily affects terminal QRS vectors and is less likely to be directly affected by NS-HBP which alters the initial portion of QRS.
Methods
Definitions: Selective (S) and non-Selective (NS) His bundle pacing (HBP) have been defined before (2). In general S-HBP had an isoelectric stimulus to QRS interval and NS-HBP had a delta wave like abnormality following the stimulus.
Acute RBBB was defined as right bundle branch block which occurred during active fixation of the lead. A pre-existing RBBB was labeled as chronic RBBB .
Patient population: An ethical informed consent approved by the relevant hospital institution was obtained from each patient along with the patient’s consent to publish de-identified data. All analysis was done on stored data routinely obtained during pacemaker implant.
An audit of past 300 consecutive patients who had undergone His bundle pacing revealed 27 patients with chronic RBBB and 12 patients who developed acute RBBB which persisted throughout the implant procedure. All patients had undergone a standardized pacing threshold protocol. Using 1.0 ms bipolar pulse width, starting at 5V with stepwise decrements to 1V. A 12 lead electrocardiogram (ECG) at each pacing voltage was labeled and stored for review.
The patients were further divided into 3 groups based on whether observable conduction was occurring via a) His bundle, a parallel pathway or both:
  1. Both NS-HBP and S-HBP was observed (NS-S HBP group)
  2. Only NS-HBP was observed (NS-HBP group):
  3. Only S-HBP was observed (S-HBP group):
We did not identify any patient with S-HBP at high voltage with transition to NS-HBP at lower voltage.
Method of implantation of the Medtronic 3830 HBP lead has been previously described (5).
Site of implantation: In brief, we required that post lead fixation H-V interval was > 35 msec and that the stimulus to peak ventricular activation time at 5V was not > 10 msec longer than baseline His- peak ventricular activation time, otherwise the lead was relocated.