His Bundle Pacing: In Pursuit of the “Sweet Spot”

In the world of audiophile, there is a “sweet spot” where the listener sits; the music flows with a true-to-life realism that can be bettered only by a live performance. In the world of pacing, there is a similar spot: when the heart is being paced, the flow of cardiac conduction is the way nature intended. The spot is at the bundle of His. Traditional ventricular pacing was developed to support heart rhythm either because of symptomatic sinus bradycardia or atrioventricular (AV) nodal block. Right ventricular (RV) apical pacing has been the standard for the past half-century because of the simplicity of implant techniques, safety, and overall stability. Not until recently have we recognized that RV apical pacing can induce iatrogenic dysynchrony and risks cardiomyopathy and arrhythmia. Alternate pacing approaches have been tried but none have so far demonstrated consistent benefit over the standard RV apical approach.1 In theory, His bundle pacing (HBP) should preserve native ventricular activation without bypass of the normal conduction system, particularly when the conduction system is intact distal to the AV junction. The bundle of His can be paced in one of two ways. The first method is direct HBP, where the lead is deployed deep into the His bundle and direct His bundle capture is inferred by the observation of higher far-field myocardial capture threshold than the local His bundle threshold. The paced QRS complex narrows as the pacing output decreases below myocardium pacing threshold; in addition, the pacing stimulus to QRS onset delay is similar to the intrinsic His-ventricular interval. The second method is para-Hisian Pacing (PHP), where the lead is either deployed immediately adjacent to the His bundle or is only superficially penetrating it. The location is inferred by the observation of a lower local myocardial capture threshold than the far-field His bundle threshold. The paced QRS widens as the pacing output drops below the His Bundle threshold, and there is no pacing stimulus to QRS onset delay seen. Since the inception of HBP with the report from Deshmukh et al.,2 studies using HBP have consistently shown benefit over conventional RV pacing hemodynamically and clinically.3, 4 Despite this, only a few operators worldwide have adopted HBP. This may in part be because of the somewhat cumbersome technique in the early days requiring an inferior approach to the His bundle for mapping in order to guide superior implantation of the lead. With the advent of the Medtronic SelectSite™ (Medtronic Inc., Minneapolis, MN, USA) deflectable delivery system, Model C304, and SelectSecure® 4Fr pacing lead, Model 3830, which allows a superior-only approach combining mapping and implantation, we are now seeing renewed interest in HBP. However, although the C304 system helps tremendously with respect to His Bundle mapping, the parallel orientation of the lead tip with respect to the septum at the AV junction results in an oblique lead tip-tissue fixation angle when approached from above the annulus. Although a retrograde approach from below the annulus might partially correct this angle mismatch, it is more difficult to manipulate the delivery system and deploy the lead because of tricuspid leaflet movement and the interference from subvalvular chordae apparatus. Thus, the penetrating depth of the helix can become too superficial with a helix of only 1.8 mm in length with this angled approach. It is easy to appreciate the tenuous nature of HBP because of failed implant, high threshold, and vulnerability to micro dislodgement or local tissue fibrosis with consequent late rises in threshold and loss of capture. In this issue of the Journal, Vijayaraman et al.5 continued to report their experience of HBP with outstanding results. Over the course of 7 months, they attempted 67 HBP implants with 60 (90%) cases performed successfully. The reported thresholds acutely and during short-term follow-up are the lowest in the literature reported to date. In fact, the authors are so confident with the implant technique that no patients received backup RV pacing, despite the fact that for 60% of patients implanted, the indication was for AV conduction system disease. This is the first case series report that uses the fixed-curve delivery catheter (C315 HIS, Medtronic Inc.) for HBP. As the authors allude to, the unique design feature of this catheter is the distal curve that directs the lead tip perpendicularly against the septum for a more direct and deeper penetration of the fixation mechanism. This tip-perpendicular design makes sound mechanical sense based on the depth of the His bundle and the absence of this curve was the major shortcoming of the C304 catheter, not withstanding that the fixed primary curve of the C315 could potentially limit its reach and could hamper steering and directional positioning. Despite these limitations, however, the investigators still manage with amazing precision to fix the lead in an appropriate location to achieve HBP in 90% of implants. The procedure time and fluoroscopy time are also in line with other reported series. Perhaps the future delivery catheter design will be some form of hybrid, combining the strengths of both the C315 and C304. This is the first report and evaluation of His bundle injury current (IC) recording. There are two general attributes with respect to IC in pacing. It points to the tissue that is being paced and also is a marker of adequate fixation. In this study, the authors define the fixation IC by manually measuring the unipolar ST-segment elevation following the His bundle signal deflection. The threshold of ST-segment elevation considered His Bundle IC positive was either greater than 0.2 mV above baseline or greater than 25% of His bundle signal amplitude with 0.5–250-Hz electrogram band pass filter using the Medtronic Pacing System analyzer (Model 2290). For 22 (37%) patients of the 60 successful implants, His bundle IC was observed. The IC positive group had a significantly lower His Bundle capture threshold than the IC negative group (1.16 V/0.5 ms vs 1.75 V/0.5 ms). The one-year threshold measurement remained excellent and only marginally higher than that at implant. IC analysis of the 3830 lead has been well studied with RV myocardial pacing.6 Although the tissue is different to the His bundle, the principle is the same in that ST-segment elevation of acute IC is predictive of a lower pacing threshold at implant. There are, however, some methodological uncertainties related to the IC measurement. The modern lead fixation IC assessment that we are accustomed to seeing is in bipolar configuration, as displayed by the stock vendor pacing system analyzers in routine clinical use. The effect on the interpretation of IC measurement with the presentation of a unipolar signal input into an analyzer with a fixed configuration is unknown. The use of the unipolar configuration for measurement is a reasonable choice to minimize signal loss, considering the low-current amplitude of the His bundle signal. However, for increased flexibility, it may be more appropriate to feed the unipolar signal to a electrophysiological recording system so that the band pass filter and gain settings are adjustable for improving signal processing and analysis. Regardless, it is of little surprise that the observed His bundle IC is associated with significantly lower His bundle capture threshold. In Vijayaraman et al.'s5 series, IC is also associated with a greater chance of direct HBP (59%). The PHP (41% of this IC group) still had a respectable threshold outcome with an average His bundle capture threshold of just 1.6 V/0.5 ms. Ultimately, does it matter if it is direct HBP or PHP, if the threshold is low? Both pacing outcomes still use the patient's own conduction system to activate the ventricles with very little conduction system bypassed where PHP is seen. Current literature would suggest that there is no significant clinical difference between direct HBP and PHP. Purists among us would perhaps consider direct HBP to be the ultimate display of skill and manual dexterity in the world of pacing, but the reality is that PHP may in fact offer the added advantage of backup RV pacing, should the implanter opt to minimize hardware burden and not place a separate RV lead. HBP experience is at a relatively early stage of development and this cohort is too small to draw conclusions or change clinical practice. However, these results still provide new understanding to HBP physiology. Should we actively look for His bundle IC to guide HBP implantation? Would improved delivery tools or having a longer helix for deeper penetration improve IC-guided implant outcomes? Does the long-term pacing outcome remain stable in the IC versus no IC cohort? This is the first reported series where HBP was delivered deliberately without any planned RV pacing for backup, even in patients with a definite ventricular pacing requirement. Although the short-term threshold data are encouraging with this new approach, the long-term reliability remains uncertain. Current published data would suggest that a significant portion of patients during long-term follow up would have a substantial late rise in pacing threshold and risk of exit block. As such, HBP without RV backup pacing cannot be categorically endorsed at this time, particularly in patients with significant AV node or intraventricular conduction system disease. In conclusion, the authors are to be congratulated for the success in achieving such low threshold His bundle pacing with their novel approach. One should remember that these results are the representation of a single-center study, which has a long history of excellence in pacing innovations. Generalization of the outcomes will require broader experience in order to be conclusive. Yes, the “sweet spot” is there for us to master in implantation of HBP, and a report like this by Vijayaraman et al.5 is timely and directive. Hopefully, with close industry collaboration and with improving design of novel delivery systems and lead technology, more of us can get to the “sweet spot,” and stay there.