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Permanent left posterior fascicular area pacing through the interventricular septum in a patient with infra-Hisian block

Right ventricular apical (RVA) pacing is associated with a high incidence of heart failure and atrial fibrillation related to ventricular dyssynchrony.1, 2 Permanent His bundle pacing (HBP) has emerged as an alternative approach that has proven safe and has yielded better clinical outcomes compared with RVA pacing in patients requiring permanent pacing.3 However, Vijayaraman and colleagues4 reported that HBP was not successful in 24% of patients presenting with infranodal atrioventricular (AV) block. The optimal pacing site in these patients has yet to be determined. We describe a case of a patient with infranodal AV block, which was successfully corrected by permanent left posterior fascicular (LPF) area pacing via the interventricular septum (IVS).


Introduction
Right ventricular apical (RVA) pacing is associated with a high incidence of heart failure and atrial fibrillation related to ventricular dyssynchrony. 1,2 Permanent His bundle pacing (HBP) has emerged as an alternative approach that has proven safe and has yielded better clinical outcomes compared with RVA pacing in patients requiring permanent pacing. 3 However, Vijayaraman and colleagues 4 reported that HBP was not successful in 24% of patients presenting with infranodal atrioventricular (AV) block. The optimal pacing site in these patients has yet to be determined. We describe a case of a patient with infranodal AV block, which was successfully corrected by permanent left posterior fascicular (LPF) area pacing via the interventricular septum (IVS).

Case report
A previously healthy 37-year-old man presented with a 2week history of syncope and bradycardia (36 beats/min) and was referred to our institution for pacemaker implantation. The baseline electrocardiography (ECG) showed 2:1 AV block with right bundle branch block (RBBB) ( Figure 1A). The results of the physical examination and echocardiography were unremarkable. Given the young age and the deleterious effects of long-term RVA pacing, we attempted HBP. A SelectSecure 3830 lead (Medtronic, Minneapolis, MN) was placed through a C315 sheath (Medtronic) and guided to the AV septum for mapping and pacing. His bundle recordings revealed a prolonged HV interval of 242 ms of conducted sinus beats and atrio-His block of nonconducted sinus beats, suggesting both AV nodal block and infra-Hisian conduction delay ( Figure 1B). Even at a high output of 5 V/1 ms, HBP resulted in antegrade block ( Figure 1C). Therefore, we attempted to pace the proximal left fascicle by screwing the lead deeply into the IVS, starting approximately 15 mm anterior and 5 mm inferior to the His region ( Figure 2B). Mapping and pacing was performed alternately while screwing the 3830 lead, to obtain a qR pattern in lead V 1 and a Purkinje potential before the local V wave. The depth of the 3830 lead in the septum was confirmed by the angiography performed using the C315 sheath ( Figure 2A). The lead was screwed in by means of 15-20 clockwise rotations before a sharp Purkinje potential followed by a ventricular electrogram were recorded ( Figure 1D). We decided to fix the lead in this particular position. Unipolar pacing during the procedure yielded a narrow QRS (94 ms) with a threshold of 0.75 V/0.42 ms. The atrial lead was placed in the right atrial appendage. Postprocedural echocardiography suggested that the tip of the 3830 lead was positioned just beneath the left ventricular endocardium ( Figure 2C). The 12-lead pacing ECG obtained after the procedure showed left axis deviation and right ventricular activation

KEY TEACHING POINTS
In patients with infranodal atrioventricular block and failed His bundle pacing, the optimal pacing site has yet to be determined, and pacing distal to the site of block is a clinically achievable option.
The left posterior fascicular area is composed of multiple fibers dispersed over a relatively broad area, providing extensive opportunity for successful capture via the interventricular septum.
The 3 most useful indicators that guide lead insertion are the V 1 morphology during the intermittent pacing test, the appearance of a Purkinje potential before the local V wave, and the depth of the lead in the septum. delays during unipolar pacing, which indicated left anterior fascicular block (LAFB) and RBBB ( Figure 3A). Bipolar pacing corrected the RBBB, leaving only the LAFB ( Figure 3B). There was no change in QRS morphology during threshold testing. After 9 months of follow-up, this patient was free of any symptoms. The 12-lead ECG and the ventricular pacing threshold were unchanged during the follow-up period.

Discussion
Left bundle branch area pacing through the IVS was first reported in treating a patient with heart failure and left bundle branch block in whom this novel pacing strategy corrected the left bundle branch block with accompanying RBBB on the electrocardiogram. 5 In our case, the intermittent atrio-His conduction suggested AV nodal block, and the prolonged HV interval and failure of His bundle capture suggested the existence of infra-Hisian block. Based on a combination of intracardiac electrograms, fluoroscopy, and postprocedural 12-lead ECG features, we inferred that the 3830 lead has been capturing the LPF area and possibly the adjacent myocardium in this patient. The infra-Hisian conduction delay site occurred between the His bundle and the LPF area. This observation also explains why pacing distal to the site of the block would correct AV block in this patient. Notably, the LPF area is composed of multiple fibers dispersed over a relatively broad area, 6 providing extensive opportunity for successful capture. In this case, however, we inferred that the pacing site was the LPF trunk in this patient based on the following 2 findings: (1) a large Purkinje potential was recorded at this site; and (2) the intraprocedural ECG suggested selective pacing during low output (not shown). As the LPF area was captured first, the left anterior fascicular area was activated slightly later, resulting in LAFB manifestation on the surface ECG. In conclusion, proximal LPF area pacing through the IVS provides a more stable, low-threshold, physiological approach to pacing in patients with infra-Hisian block.