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Tuesday, March 16, 2021

So maybe Bundle of His pacing (HBP) is not the holy grail of pacing after all.

 To understand why EP cardiologists might have considered  His Bundle Pacing (HBP) the holy grail it might help to have a brief survey of some of the history of pacing the heart and then look at why HBP , thought  to be the most physiological may not now be considered to be the best-at least by some EP cardiologists.

By the early 2000s EP cardiologists had not yet determined the best way to pace the heart even though the dual chamber pacemaker had been employed widely since the mid 1980s .

For example, in a patient with sinus node disease and bradycardia should the patient be implanted with a single lead in the  right ventricle or be given dual chamber pacing with a lead in the right atrium as well. At that time right ventricular pacing meant pacing in the apex of the right ventricle as opposed to pacing in the right ventricular septal wall or the right ventricular outflow path or actually accessing the His Purkinje system also now referred to a conduction system pacing. 

While it was known that pacing the ventricle alone could at times lead to something called pacemaker syndrome and it seems that synergy of the atria and ventricles made much more physiologic sense,there had been no clinical trial demonstrating the best approach.Looking back from the vantage of an outsider it seems that as recently as 20 years ago the EP cardiology community was not in agreement about how many leads did a patient with sick sinus syndrome need/

The MOST trial  was designed to settle that issue and results were published in 2002.That trial and the DAVID trial provided less than slam dunk evidence favoring the dual chamber (DDD) mode  ( 20% fewer instances of atrial fibrillation) but subsequent analysis of the data gave another important answer related to the harmful effect of pacing the heart from the right ventricle. That and subsequent analysis indicated that there was a significant risk of heart failure if the right ventricle were pacing more than 20-40% of the time. Consider that-the treatment that could be live saving for irreversible bradycardia could over time cause the patient to develop heart failure. Some EP cardiologists must have thought that there must be a better way. 

It gets worse -patients with pacing induced heart failure respond poorly to the usual heart failure battery of medications and the only fix was another pacemaker, which while  potentially very  helpful, implanting a pacemaker should be considered a big deal . Infection and other complications may occur more frequently when PM are revised or replaced.

The patient would  have to undergo another PM implantation procedure with the attendant risks of infection,pocket hematoma,pericardial  effusion,pneumothorax,etc. This time a lead would be placed from the right atrium through the coronary sinus into  a vein on the epicardial surface of the left ventricle. Then the PM would be programmed to attempt to optimize the timing of the right ventricle (still paced at the apex) with the left ventricle which is now depolarized from epicardium to endocardium  which is the  opposite of the usual route.Two wavefronts would be generated, one from the endocardium of the right ventricle and the other from the epicardium of the LV and the two would  hopefully fuse so as to improve  cardiac output and  even bring about often beneficial remodeling of the heart.

This Rube Goldberg arrangement worked quite well for these cases of pacemaker induced cardiomyopathy as it does for about 70% of patients with medication refractory heart failure   It seemed that those cases whose EKG showed a left bundle branch pattern were most likely to have a good and often impressive improvement, sometimes referred to as "super responders".

By 2013 the value of CRT was firmly established. CRT means cardiac resynchronization therapy and  at that time  was synonymous with biventricular pacing (Bi-V) and randomized clinical trials had proven its value in symptomatic relief,reduction in hospitalizations and  increased survival.As best I can tell, as a non cardiologist looking in, the randomized controlled trial evidence supporting pace maker implantation for CRT is more robust and convincing that for any other pacemaker indication.

A look at the time line regarding recognition and acceptance of  the role of loss of interventricular and intraventricular synchrony as exemplified by left bundle branch block (LBBB) is of interest juxtaposed to the evolution of thought concerning right ventricular apical pacing .

Blanc et al 2005 and Vaillant et al 2013 described cases of heart failure caused by left bundle branch block and treated successfully with CRT. Blanc's cases seem to be the first recognition that lone LBBB could cause heart failure and that this desynchrony  could be fixed by CRT often with dramatic resolution of the heart failure.

The abnormal depolarization and sequence of ventricular contraction in RV apical pacing and that induced by LBBB are very similar as are the EKGs and it should not be a surprise that CRT worked well for both, at least in many cases

The first clinical series of His Bundle pacing was done  in 2000 by Desmuhk  in patients with rate control recalcitrant atrial fibrillation patients who needed an av node ablation but its use did not become widespread due to the complexity of the implantation and lack of tools designed for the task.   

For the next decade there was little clinical application of His Bundle pacing in the US although  work was progressing in Spain and Italy.

Two important papers in 2015 contributed to a growing interest and application of HBP. A case series by Dandimundi and Vijayaramen from the Geisinger Clinic  demonstrated that HBP was not overly difficult after all and that results seemed good. Daniel Lustgarten from Vermont published a proof of concept cross over study  study that compared HBP with BiV pacing .

By 2017 at the Heart Rhythm Society meeting there was considerable enthusiasm regarding HBP and one prominent EP cardiologist referred to HBP as the Holy Grail of pacing and it seemed that   finally physiologic pacing was practical. The alternative, namely right ventricular apical pacing and Bi V pacing could hardly be considered physiologic.By 2020 the same EP cardiologist wondered in left bundle branch area pacing would be the new holy grail. 

In 2019 Dr. Kenneth Ellenbogen gave a presentation that was very optimistic regarding the future of His pacing in part based on a small ( n=20) group with five year followup.

However, by 2020 the bloom was less well fixed on the  His Bundle pacing rose. Though physiologically beautiful HBP had some negatives such as: High initial capture threshold, adequately initial low capture thresholds that subsequently rose to unacceptable levels in some cases,low R waves which may lead to sensing problems and  the technical difficulty of the procedure. Earlier  than anticipated pulse generator replacements were occurring because of the high thresholds. Padala and Ellenbogen claim that 25% of HBP patients will have an increase in capture threshold to 2.V at 1 ms. over time. In Zanon's report of 844 HBP patients the median battery replacement time was 5.8 years .

So When Dr. W Huang  published results of left bundle branch area pacing (LBBAP) which was technically easier,resulted in lower thresholds and higher r waves hence less programming problems  LBBAP seemed to be  very quickly accepted in China and in some US centers.

LBBP may not be quite as physiologic as HBP after all it in effect may cause a right bundle branch block ( it does not always though it should theoretically ). The LBBAP technique is now considered by at least one prominent EP cardiologist as the best even though at this point long term followup is lacking. 

In a YouTube presentation  given on  Feb 2010, by Dr. Santosh Padala from VCU  who  discussed his results with LBBAP and indicated they they now proceed with that modality without trying  His capture first. The reason for this seemed to be that they had seen some cases where the initial His capture threshold was acceptable but within a few months the threshold increased to unacceptable levels. He mentioned they had two out of 59 LBBAP cases with a LV penetration complication but the lead was withdrawn and apparently no significant consequences resulted.

I believe a good  argument can be made for saying that such a move is premature, We now  have fairly robust data on followup on HBP patients.In 2019 Zanon published a multicenter  experience of 844 patients with HBP who were implanted from 2004 to 2014 with a median followup of 3 years. (91.6 % were free of complications. Mean pacing threshold at implant was  1.6  V. and  2.0 at followup. In the second group of 476 in whom the fixed curve sheath was used the complication rate was 4.2 % versus 11.9 in the earlier group of 368 patients in whom a deflectable sheath was used. 

However, LBBAP is relatively new and the medium and long term results have yet to play out.How well a pacing lead inserted 1.5 cm  into the septal will perform  over time remains to be determined. There has been at least one case of a delayed migration of the lead through the septum penetrating into the left ventricle.

Venkatesh, and Sharma from Rush reported their experience with LBBAP regarding 59 patients done from June 2018 to April 2020 most of which were attempted because of unsatisfactory His implantation or unacceptably high His capture voltage.

There were 7 "lead related "complications during the short follow up period of about 6 months with 3 requiring lead revisions.In one patient an interventricular septal perforation occurred two weeks after the initial implantation.Penetratiion during the procedure apparently can be quickly recognized and at least so far  it seems  that no significant harm was done.Late penetration is a different matter. but it seems rare. Capture voltages were low(average 0.62 Volts at 0.4ms)and stable over the short followup period which should predict a longer battery life than is seen with HBP. Improvement was noted in left ventricular performance  in HF patients during the follow up as has typically been the case with HBP.

1) Venkatesh,R et al Pros and Cons of Left Bundle Branch pacing.A single center experience.Cikrculation,arrhythmias and electrophysiology 2020 13 12 

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