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SAN FRANCISCO -- wasn't better than conventional biventricular cardiac resynchronization therapy (BiV CRT) in the first head-to-head pilot trial, although crossover appeared to be a factor.

In the His-SYNC trial, primary outcomes did not significantly favor His pacing over BiV CRT at a median follow-up of 6.2 months:

• Reduction in QRS duration from preimplantation (172 to 144 ms vs 165 to 152 ms, P=0.42)
• Increase in left ventricular ejection fraction (+9.1% vs +5.2%, P=0.33)
• Time to cardiovascular hospitalization or death at 12 months, although few events occurred (six hospitalizations, two deaths)

Roderick Tung, MD, of the University of Chicago Medicine, presented the findings at a late-breaking trial session at the Heart Rhythm Society meeting and simultaneously online in the Journal of the American College of Cardiology.

Other studies at the meeting compared selective versus nonselective His pacing and looked at the feasibility of left bundle branch pacing.

"There's been a considerable amount of enthusiasm for those on Twitter around the hashtag called ," he noted at a press conference. His pacing recruits the intrinsic wiring of the heart and attempts to restore the natural physiology. "People have been very enthusiastic about it, but we lack randomized comparative trials."

His-SYNC is the first randomized trial to compare His and conventional pacing, albeit only as a proof of concept. It included 41 patients randomized to His-CRT or coronary sinus lead for BiV-CRT with routine implantation techniques. Physicians reading echo results were blinded to the treatment group.

Crossover from His-CRT, which was mandated if it did not achieve sufficient QRS narrowing to a width of ≤130 ms or had correction thresholds over 5 V at 1 ms, occurred in 48% of patients.

Crossover from BiV-CRT, which was permitted for those in whom an LV lead could not be placed, occurred in 26% of patients. Lead placement in the anterior interventricular or middle cardiac veins was discouraged.

With such unexpectedly high crossover rates, the study couldn't directly assess treatment efficacy, the group cautioned.

Also, the trial did not select patients based on overt left bundle branch block (LBBB), which other researchers discovered during the course of the trial is unlikely to be corrected by His-CRT, Tung noted.

Per protocol analysis did show significantly greater effects with His pacing for QRS duration reduction and ejection fraction.

"Improved patient selection may decrease crossover rates and larger prospective studies may be useful to assess for smaller differences in effect size between CRT modalities," his group concluded.

Meanwhile, "His-CRT was not suitable as a first-line strategy in an unselected CRT population," Tung said at the section.

Selectivity Not Key?

Another late-breaking study at the conference looked at whether selectively pacing only the His bundle might be better than an unselected strategy that also recruits some of the slower-conducting surrounding myocardium.

"If dyssynchrony was the main reason for us even thinking about His bundle pacing, then is there a chance [that] nonselective His bundle pacing is worse?" noted Dominik Beer, DO, of Geisinger Heart Institute in Wilkes Barre, Pennsylvania, who presented the findings at a press conference.

His group looked at 350 consecutive His bundle pacing patients treated at Geisinger and Rush University for bradyarrhythmic indications who had at least a 20% ventricular pacing burden 3 months after implant. Their QRS morphology was nonselective in 232 cases and selective in 118 at the 3-month follow-up.

Despite heterogeneity between the groups that yielded a sicker population having nonselective His pacing, risk of all-cause mortality or heart failure hospitalization was not significantly higher in the nonselective group (35% vs 19%, HR 1.38, P=0.17). The same was true for the highest risk subgroups with higher pacing burden or lower left ventricular ejection fraction.

"We cannot randomize to who will have selective His bundle pacing, who will not have selective His bundle pacing," Beer noted. "This is dependent on how we're able to capture the His bundle in vivo and in some patients we can't get a certain characteristic. In some cases, we actually want nonselective His bundle pacing, if they have advanced infra-nodal disease, because the RV septal myocardial capture provides us some backup in case there's eventual progression of disease."

"So always in our mind there was the question are we giving these patients something not as good as selective His bundle pacing," he said. "This makes us feel a little better going back to the lab knowing there is no obvious difference in these outcomes between selective and nonselective His bundle pacing."

Further Down the Line

Another strategy in development is to head just downstream to the left bundle branch to put the lead there with intraseptal fixation.

"It's along the same spectrum but different ways to achieve it," Tung noted, and apparently with lower thresholds that might save battery life.

Ragesh Panikkath, MD, also of the Geisinger Heart Institute, presented a feasibility study of LBBB area pacing for patients needing pacing for bradycardia or heart failure but with a failed left ventricular or His bundle pacing lead.

It was feasible using the same hardware as conventional His pacing in 93 of the 100 patients. Among them, LBBB capture was confirmed in 83 patients. Twelve cases couldn't get LBB potentials recorded, 15 had compete heart block without an escape rhythm, and 10 had narrow QRS.

"Echocardiographic evaluation confirmed the pacing lead to traverse the thickness of the interventricular septum," the researchers noted. Tricuspid regurgitation was only modest at 3 months. Lead dislodgement occurred in three cases but there were no late dislodgements or strokes.

LBBB pacing "provides a viable alternative to HBP [His bundle pacing] in challenging implants, high thresholds or conduction disease not corrected by HBP," Panikkath concluded.