Methods
Study population
This was a prospective, single center observational study. Consecutive
patients referred to one electrophysiology team, between October 2019 to
August 2021, for device implantation for bradycardia or cardiac
resynchronization were offered conduction system pacing which was
explained to them as a newer non-standard method which may have
advantages but was associated with a research MRI scan. The study was
approved by the local ethics committee (REC 19/YH/0174). Consenting
patients were enrolled. They underwent a pre-implant cardiac MRI to
which the cardiac catheterization laboratory team were kept blinded.
Cardiac MRI
Cardiovascular magnetic resonance was performed in a 1.5 Tesla (Aera,
Siemens Medical Solutions, Erlangen, Germany) using a standard clinical
scan protocol including LGE imaging following a bolus of 0.1mmol/kg of
Gadobutrol (marketed as Gadovist, Bayer Pharma AG, Berlin, Germany). In
the majority of cases, scar imaging was undertaken using a
free-breathing motion corrected sequence as detailed by Captur et
al8.
(25/35, 71%) also had dark blood late gadolinium enhancement imaging
which increases sensitivity for detecting subendocardial scar9.
Image Analysis
Cardiovascular magnetic resonance imaging analysis was performed using
CVI42 software (Version 5.13.7, Calgary, Canada) blinded to clinical
parameters. The presence and extent of myocardial scar in the basal
septum (defined as the inferoseptal and anteroseptal segments in the
most basal 3 complete slices) was assessed by an experience level 3 CMR
operator. The observer was aware of the hypothesis regarding the basal
septum but had no access to information on the procedural outcome. Scar
burden was quantified either using full width half maximum semiautomated
technique7with manual review, or manually (for dark blood images). In the patients
who had both images, the greater of the two scar quantifications was
used. Scar extent was quantified as the percentage of the amount of
myocardium in those segments.
Left bundle area pacing
LBP was carried out in the electrophysiology laboratory (Hammersmith
Hospital, UK) electrocardiograms were recorded using the
electrophysiology system (Boston Scientific, Natick, Massachusetts).
The C315His fixed curve sheath (Medtronic, Minneapolis, MN) was used to
position the SelectSecure 3830 lead (Medtronic, Minneapolis, MN) onto
the bundle of His. We mapped the His bundle and stored fluoroscopic
image in the right anterior oblique view, and this was used as a
roadmap. The lead was then advanced to basal right ventricular septum
about 2cm below the His bundle. From that position it was deployed deep
into the ventricular septum (Figure 1). A single operator (ZW) assessed
the deliverability of the left bundle lead and the electrical response
while blinded to the results of the cardiac MRI.
During lead advancement, we monitored impedance and time to peak R wave
(RWPT) (time from the stimulus to peak R wave in lead V5 or V6) with
unipolar pacing. Left bundle capture was confirmed by a right bundle
branch block morphology with a terminal R wave in lead V1 and any of: 1-
RWPT time < 90ms, 2-The presence of left bundle branch
potential (15-30ms before the QRS onset), 3-Transistion from
non-selective to selective left bundle capture (Figure 2) or
non-selective left bundle capture to myocardial only capture with change
in pacing output or with programmed
stimulation.10
Lead depth was assessed by placing the end of the sheath onto the septum
and measuring the distance to the tip of the lead or by contrast
injection. Furthermore, in cases where the lead was not advanced or left
bundle was not captured, electrocardiographic monitoring showed left
bundle branch morphology and the RWPT in V5/V6 remained >
80ms.
We defined two modes of failure. Unsuccessful lead delivery was
inability to advance the lead deep enough into the septum to reach the
left bundle area. Unsuccessful left bundle capture was failure to
stimulate the left bundle despite successful lead delivery.
Paced QRS duration during LBAP was used to assess electrical response.
The QRS was optimized by allowing fusion with intrinsic right bundle
conduction if possible.
Statistical analysis
Continuous variables were expressed as mean and standard deviation if
normally distributed and mean (IQR) otherwise. Correlation were assessed
by the Spearman rank correlation coefficient. Comparisons between groups
were performed using the Mann Whitney U test. A p value <0.05
was considered statistically significant. Statistical analysis was
conducted in RStudio using the tidyverse package.