Tuesday, October 23, 2018

His Bundle Pacing


His bundle pacing in humans was first described in 1970 by Narula et al. They demonstrated that it was possible to stimulate the His bundle to produce normal physiological ventricular activation via the His-Purkinje system. However, the first report of permanent His bundle pacing, by Deshmukh et al., did not occur until 2000. In that study, His pacing was performed in a series of patients with impaired left ventricular systolic function and AF prior to atrioventricular (AV) node ablation.

The lack of dedicated tools for implantation initially hampered enthusiasm; however, the development of specially designed sheaths and leads for delivering permanent His bundle pacing has led to a renewed interest. The potential role of His pacing in heart failure is large: it may prevent the development of pacing-induced cardiomyopathy; it may be used as an alternative to biventricular pacing in patients with heart failure and left bundle branch block (LBBB); and it may extend pacing therapy in heart failure to patients with narrow QRS and PR prolongation by providing AV synchrony without inducing ventricular dyssynchrony.






His Bundle Pacing: Conduction System and Outcomes (A) Schematic representation of the His-Purkinje conduction system. The membranous septum is indicated in yellow. Image courtesy of K. Shivkumar, MD, PhD, UCLA Cardiac Arrhythmia Center, Wallace A. McAlpine MD collection. Reproduced with permission. (B) Clinical outcomes of HBP. Kaplan-Meyer survival curves demonstrating a statistically significant reduction in the primary endpoint (composite endpoint of all-cause mortality, HFH, or upgrade to biventricular pacing) with His bundle pacing (HBP) compared with right ventricular pacing (RVP) in all patients and in patients with ventricular pacing (VP) >20%. Reprinted from Abdelrahman et al. (62). AVN = atrioventricular node; CS = coronary sinus; HB = His bundle; IVC = inferior vena cava; LBB = left bundle branch; LV = left ventricle; PA = pulmonary artery; RA = right atrium; RBB = right bundle branch; SVC = superior vena cava

Anatomy of the His Bundle and Implant Technique


The bundle of His extends from the compact AV node to the membranous interventricular septum, and measures approximately 20 mm in length. The bundle is a cord-like structure made up of multiple strands, which, even before the branching, are predestined to become the right or left bundle branches. His bundle pacing can be achieved by placing the lead at the atrial portion against the septum.

The most commonly used lead for His bundle pacing is the 69 cm Select Secure™ 3830 (Medtronic). This is a non-stylet-driven active fixation lead. Importantly, the screw forms part of the tip electrode allowing penetration of the capsule of the bundle of His and, therefore, direct stimulation of the His bundle fibres. The lead can be delivered to the His bundle region using either the specially-designed non-deflectable His delivery sheath (C315 43 cm; Medtronic) or a deflectable sheath (C304 69 cm; Medtronic). Unlike traditional lead placement that primarily uses fluoroscopic guidance, His lead placement primarily uses electrical mapping. An electrogram from the lead tip is displayed using placement via either a lab electrophysiology system or a standard pacing system analyser.

A His signal is targeted, aiming for the local ventricular electrogram to be approximately twice the amplitude of the atrial signal . To confirm successful His capture, a 12-lead ECG is used to assess the QRS morphology with pacing. Criteria used to establish whether His capture has occurred are well described.Recently-published data suggests thresholds of <2.5 at 1 ms should be achieved. An increase in pacing threshold is observed in ~10 % of patients, leading to shorter battery duration. There is also a higher rate of lead revisions (6.7 %) due to loss of capture or increased threshold.

Anatomic Variations of the His Bundle

 (A) Type 1: The His bundle (AVB) runs under the membranous part of the interventricular septum (MS). (B) The type II His bundle runs within the muscular part of the interventricular muscle apart from the lower border of the membranous part of the interventricular septum. (C) The type III His bundle (arrow) is naked running beneath the endocardium with no surrounding myocardial fibers. AT = attachment of septal tricuspid leaflet; AVB = atrioventricular bundle; AVN = atrioventricular node; CS = coronary sinus

Despite recent advances and interest in HBP, several unanswered questions and concerns remain . Although permanent HBP may be an attractive option for physiological pacing in several groups of patients, its reliability and long-term performance are yet to be fully validated in large prospective studies. Particularly relevant are patients with infranodal, intra-Hisian AV block and BBB, where long-term safety of HBP has not been well studied. In such patients, should a backup RV lead be placed with HBP? What happens to the His bundle when it is traumatized by the screw on the tip of the lead in the long term? Can a second His Bundle pacing lead be placed successfully if the earlier lead fails in the long run? Considerable effort needs to go into improving the design and structure of the lead and the delivery tools to allow for easier implantation and stabilization of the lead. Beyond implant, what are the implications of extracting a chronic HBP lead? And beyond pacing hemodynamics, what is the impact of HBP on arrhythmia? Does HBP reduce the risk of ventricular tachyarrhythmias in the presence of myocardial scar? These and other questions remain.

What is certain is that this technique holds potential and requires further validation in larger studies with longer follow-up. It is also clear that collective and collaborative efforts from physician scientists, industry partners, scientific societies, and regulatory authorities will be required to successfully develop this technology and advance our understanding of the physiology of pacing.

Conclusions

HBP is an attractive mode of physiological pacing with significant promise for future applications in patients who are traditional candidates for RV pacing as well as CRT. Widespread adaptation of this technique is dependent on the improvement of tools and further validation of its efficacy in large randomized clinical trials.

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