By Joshua D. Moss, MD
Associate Professor of Clinical Medicine, Cardiac Electrophysiology, Division of Cardiology, University of California, San Francisco
Dr. Moss reports no financial relationships relevant to this field of study.
SYNOPSIS: When used as either a primary alternative to biventricular pacing or a rescue therapy for failed biventricular pacing, permanent His-bundle pacing was associated with significant QRS narrowing, an increase in left ventricular ejection fraction, and an improvement in New York Heart Association functional class.
SOURCE: Sharma PS, Dandamudi G, Herweg B, et al. Permanent His-bundle pacing as an alternative to biventricular pacing for cardiac resynchronization therapy: A multicenter experience. Heart Rhythm 2018;15:413-420.
Although the concept of permanent His-bundle pacing (HBP) is not new, the last several years have seen rapidly growing interest in and research on its potential clinical applications. Sharma et al sought to assess the feasibility of and outcomes related to HBP in various patients for whom biventricular pacing traditionally has been used as first-line therapy.
A total of 106 patients with baseline LV ejection fraction less than or equal to 50% and New York Heart Association (NYHA) class II-IV heart failure symptoms at five centers underwent attempted HBP for cardiac resynchronization therapy (CRT). In 33 patients, traditional biventricular pacing (BVP) for CRT had failed already, either due to unsuccessful LV lead implantation (n = 25) or lack of clinical response to BVP (n = 8). In the remaining 73 patients, HBP was used as first-line CRT instead of BVP for elevated right ventricular (RV) pacing burden (n = 31), bundle branch block (n = 27), or AV block or AV junction ablation (n = 15). The same pacing lead was used for all patients, with the implant procedure limited to either five attempts or 20 minutes of fluoroscopy for successful lead positioning for HBP. In patients for whom HBP was unsuccessful, an LV lead was implanted via traditional coronary venous approach (if not attempted previously).
“Selective” HBP was defined as ventricular activation achieved exclusively via the His-Purkinje system, with an isoelectric segment between the pacing stimulus and QRS onset. “Nonselective” HBP was achieved when pacing resulted in capture of both the His-bundle and the basal ventricular septum.
Successful HBP was achieved in 95 patients, with half classified as selective HBP and half nonselective. Mean baseline QRS duration (either native or via RV pacing) decreased significantly, from 157 msec to 118 msec with HBP; narrowing of at least 20% was achieved in 44 of 48 patients with underlying bundle branch block. There was significant improvement in LV function with HBP, with 73% of patients demonstrating a > 5% improvement in LV ejection fraction.
Among 72 patients with baseline EF less than or equal to 35%, mean EF improved from 25% to 40%, and 27 of those patients were “super-responders,” with absolute improvement in EF ≥ 20%. Six of eight patients who were prior non-responders to BVP demonstrated a mean increase in EF of 30% to 38% (an improvement that did not reach statistical significance), and seven of eight exhibited a clinical response (with significant improvement in NYHA class from 2.8 to 1.8). No immediate procedure-related complications were noted, although one patient required system explant for pocket infection at six months.
The authors concluded that HBP used as either a primary alternative to BVP or a rescue therapy for failed BVP was associated with significant QRS narrowing, an increase in left ventricular ejection fraction, and an improvement in NYHA functional class.
In permanent HBP, a pacemaker lead is fixated directly into or immediately adjacent to the penetrating bundle of His rather than other commonly used pacing sites in the right ventricle, such as the apex, septum, or outflow tract. By directly stimulating the bundle of His, depolarization of the ventricles can be achieved via the native conduction system, thereby reproducing a “normal” narrow-complex QRS. Narrow-paced QRS complexes often can be achieved even in patients with an underlying bundle branch block, effectively correcting the native wide-complex QRS. This somewhat counterintuitive phenomenon is most often explained by the concept of “longitudinal dissociation,” whereby the bundle of His actually is composed of fibers already pre-destined for the right bundle and left bundle. If pacing capture can be achieved in the bundle of His distal to a site of block in these pre-destined fibers, a bundle branch block can be bypassed electrically.
Pacing capture thresholds often are higher for HBP, particularly when trying to overcome an underlying bundle branch block, resulting (on average) in shorter pacemaker battery life. In 7.4% of patients in this study, there also was significant late increase in HB capture threshold — three of those patients required a repeat procedure for HBP lead extraction and replacement with an LV lead, and the other four required high-pacing outputs to be programmed. Thus, the risk of unanticipated loss of HBP capture, plus the risks associated with more frequent operative procedures, is not trivial. Practically, some operators still prefer to implant a “backup” traditional RV pacing lead in addition to the HBP lead in patients who are pacemaker-dependent, although complete loss of RV pacing capture does not typically accompany loss of His-bundle capture.
Interest in using HBP as an alternative to both BVP for CRT and RV pacing for AV block is growing in the electrophysiology community, given the close reproduction of normal electrical physiology that apparently can be achieved. Sharma et al add substantial data to support that interest. HBP proved to be a successful “rescue” strategy in patients for whom BVP could not be achieved or who did not respond to BVP, with an overall success rate of 91%. It also was feasible, safe, and effective as first-line therapy in patients with cardiomyopathy, heart failure, and an indication for CRT (complete AV block, anticipated ventricular pacing burden > 40%, or bundle branch block).
Randomized studies comparing HBP to BVP for CRT, as well as HBP vs. RV pacing for standard pacing indications, are ongoing. Some degree of experience is required for operators to achieve consistently successful HBP, and procedural times are likely to remain longer on average, which may delay widespread adoption. However, HBP ultimately may become a standard of care in pacing therapy as efforts continue to restore physiology the way nature intended.