Recruitment of bundle branches with permanent His bundle pacing in a patient with advanced conduction system disease: What is the mechanism?

Abstract

The patient was a 69-year-old man with coronary artery disease who presented with syncope. His electrocardiogram showed sinus rhythm with a first-degree atrioventricular block, right bundle branch block (RBBB), and left anterior fascicular block (LAFB), and he was referred for an electrophysiology (EP) study. Baseline electrograms suggested intra-Hisian conduction disease, with split His potentials, H′ and H, and an intra-Hisian interval of 78 ms (Figure 1A). It was difficult to maintain His catheter stability during the EP study, and only the distal His signal (H) was consistent, with an His-ventricular (HV) interval of 70 ms (Figure 1A). Decremental atrial pacing demonstrated progressive rate-related HV prolongation up to 280 ms, beyond which HV Wenckebach block occurred (Figure 1B). No arrhythmias were induced with programmed atrial or ventricular stimulation. Given the evidence of infranodal AV conduction disease with probable intra-Hisian disease, we decided to implant a pacemaker. Pacing from the proximal His bundle (HB) position with the His catheter resulted in a wide QRS complex (duration 164 ms) with RBBB/LAFB and a long stimulus-QRS interval, whereas pacing at the distal HB resulted in a narrower QRS using a pacing output of 5 mA at 2 ms (threshold). Given narrowing of the QRS with distal HB pacing, we decided to implant the ventricular lead in the HB region. Unipolar recordings from the tip electrode of a 3830 pace-sense lead (3830, Medtronic Inc., Minneapolis, MN) were used to map the HB region. As seen in Figure 2, recordings from the proximal HB reveal a large A with an HV interval of 64 ms, while recordings from the distal HB reveal a far-field atrial electrogram (A) with a shorter HV interval of 46 ms. Pacing at the distal HB (distal to the site of block/delay resulting in RBBB/LAFB) resulted in para-Hisian (nonselective HB) capture (His + V), evidenced by a narrow QRS of duration 124 ms and presence of a slurred upstroke indicating local myocardial capture. The threshold of para-Hisian capture was 0.5 V at 0.5 ms. The lead was implanted in the distal HB location. What is the mechanism of narrowing of the QRS complex by permanent HB pacing in this patient with conduction disease characterized by RBBB and LAFB? Figure 2Proximal and distal His bundle (HB) recordings (paper speed 50 mm/s) and paced QRS morphology using permanent His bundle lead (3830, Medtronic). A = atrial electrogram; H = His electrogram; V = local ventricular electrogram. View Large Image Figure Viewer Download Hi-res image