• CLINICAL VIGNETTE

Misleading transition: How His-bundle pacing imitated left bundle branch pacing

Maciej Fularz1, Przemysław Mitkowski2

1Department of Cardiology, University of Zielona Gora, Zielona Góra, Poland

21st Department of Cardiology, Poznan University of Medical Sciences, Poznań, Poland

Transitions of the paced QRS complex are pi­votal in confirming conduction system pacing. Non-selective capture is usually observed at higher pacing output. A decrease in pulse amplitude could lead to selective capture of either the myocardium or conduction system. Identifying this phenomenon in most cases of His-bundle pacing (HBP) is relatively simple. Selective HBP usually provides QRS complexes identical to the intrinsic rhythm. Myocardial-only capture in the para-Hisian site produces broader QRS with left bundle branch block morphology. Appropriate diagnosis of transitions in left bundle branch pacing (LBBP) may be more challenging as differences between distinct modalities could be subtle. Jastrzębski et al. [1] provided useful criteria. During transition to selective LBBP, the V6–V1 interpeak interval increases due to loss of myocardial capture and, in consequence, more delayed right ventricle activation, while R-wave peak time in lead V6 remains unchanged. During transition to myocardial capture, R-wave peak time in lead V6 increases because left ventricle activation is less rapid. An isoelectric interval may also be useful as it occurs during selective HBP and selective LBBP [2].

An 84-year-old female patient with permanent atrial fibrillation and bradycardia was qualified for implantation of a single-chamber pacemaker. The intrinsic QRS complex was narrow. The lead was positioned in the basal septum to perform LBBP. An intraprocedural transition of paced QRS complex was demonstrated and identified as a transition from non-selective to selective LBBP (Figure 1A). Echocardiography confirmed that the lead was deployed deep in the basal septum (Figure 1B). The bipolar electrical parameters on the day after implantation were: R-wave 2.8–4.0 mV, pacing threshold 0.5 V × 0.4 ms, and impedance 552 ohms. Postprocedural chest radiograms are shown in Supplementary material, Figure S1. During a follow-up outpatient visit, a gradual output decrease test was performed. Surprisingly, an additional transition was found. In the first step, non-selective capture transformed to selective HBP and after a further decrease in pulse amplitude, selective HBP with right bundle branch block (RBBB) occurred (Figure 1C).

Transition to selective HBP was absent during the procedure probably due to temporarily equal capture thresholds of the myocardium and right bundle branch fibers. In consequence, a direct transition from non-selective capture to QRS complex with RBBB pattern occurred, and it was initially diagnosed as selective LBBP. Later demonstration of transition to selective HBP during the control visit revealed that the paced QRS complex with RBBB morphology was actually selective HBP with RBBB. Reachability of this type of capture could be explained by the longitudinal dissociation of His-bundle [3, 4]. According to this theory, fibers predestined to form right and left bundle branches are isolated from each other within His-bundle. Hence, their capture thresholds may differ, and selective recruitment of left bundle branch fibers inside His-bundle is achievable. The final diagnosis of HBP was also supported by a potential to QRS complex interval of 40 ms (Figure 1D) and relatively low R-wave sensing.

The three main lessons learned from the presented case are (1) His-bundle can be captured in deep septum [5], which may result in terminal R-wave during non-selective capture due to a left-sided myocardial component, resembling non-selective LBBP; (2) Left bundle branch fibers may be selectively captured inside the distal His-bundle, which may imitate selective LBBP; (3) Some transitions may not be demonstrable due to temporarily equal capture thresholds of two structures and may appear later.

Supplementary material

Supplementary material is available at https://journals.viamedica.pl/kardiologia_polska.

Article information

Conflict of interest: None declared.

Funding: None.

Open access: This article is available in open access under Creative Common Attribution-Non-Commercial-No Derivatives 4.0 International (CC BY-NC-ND 4.0) license, which allows downloading and sharing articles with others as long as they credit the authors and the publisher, but without permission to change them in any way or use them commercially. For commercial use, please contact the journal office at kardiologiapolska@ptkardio.pl

REFERENCES

1. Jastrzębski M, Burri H, Kiełbasa G, et al. The V6-V1 interpeak interval: a novel criterion for the diagnosis of left bundle branch capture. Europace. 2022; 24(1): 40–47, doi: 10.1093/europace/euab164, indexed in Pubmed: 34255038.
2. Wu H, Jiang L, Shen J. Recording an isoelectric interval as an endpoint of left bundle branch pacing with continuous paced intracardiac electrogram monitoring. Kardiol Pol. 2022; 80(6): 664–671, doi: 10.33963/KP.a2022.0094, indexed in Pubmed: 35380007.
3. Jastrzebski M, Kukla P, Czarnecka D. His bundle capture proximal to the site of bundle branch block: A novel pitfall of the para-Hisian pacing maneuver. HeartRhythm Case Rep. 2018; 4(1): 22–25, doi: 10.1016/j.hrcr.2017.09.002, indexed in Pubmed: 29379721.
4. Teng AE, Massoud L, Ajijola OA. Physiological mechanisms of QRS narrowing in bundle branch block patients undergoing permanent His bundle pacing. J Electrocardiol. 2016; 49(5): 644–648, doi: 10.1016/j.jelectrocard.2016.07.013, indexed in Pubmed: 27485351.
5. Vijayaraman P. Deep septal, distal His bundle pacing for cardiac resynchronization therapy. HeartRhythm Case Rep. 2020; 6(10): 791–793, doi: 10.1016/j.hrcr.2020.07.024, indexed in Pubmed: 33101957.