Discussion
This study finds that in patients with less than about 40% basal septal
scar, the LBP lead (3830 SelectSecure) was always successfully advanced;
in those with more than 60% basal septal scar, it was never
successfully advanced. Importantly, once the lead is successfully
advanced, the amount of basal septal scar has no impact on the QRS
duration achieved. Finally, we observed that dark blood imaging is
better able to identify ischemic scar and,
in agreement with previous research9is advisable in research or clinical practice where scar quantification
is important.
Septal scar and failure to advance pacing lead to left
bundle
area
In this study, lead delivery was unsuccessful in 14% of patients. Our
patient group was drawn from a tertiary center where advanced
cardiomyopathy was over-represented. 82% had a form of cardiomyopathy
and 34% had ischemic cardiomyopathy. Our center is often referred
patients with severe disease. For example 88% of our non-ischemic
cardiomyopathy patients had scar, which is a higher rate than other
published cohorts11,
but may be due to selection bias in those being referred on for devices.
Early reports of left bundle area pacing were largely bradycardia pacing
in patients with structurally normal hearts. However, extending its
application to heart failure including ischemic cardiomyopathy
inevitably brought higher failure rates3.
Some authors have discouraged left bundle area pacing in ischemic
cardiomyopathy for this reason12,13,
suspecting that scar in the basal septum might be to blame.
Our high-quality tissue characterization by MRI showed that even when
there was scar, in 80% of cases the left bundle could be paced. What
mattered was the extent of scar in the basal septum. There was a clear
association between extent of scar and failure of lead advancement.
Nevertheless, even at 50% basal septal scar, our data suggests that
70% of cases have successful lead delivery.
Moreover, in this study we used only the (SelectSecure 3830,Medtronic,
Minneapolis, MN). It is possible that stylet driven leads or alternative
tools may help overcome the problem of lead advancement in patients with
extensive septal scar. As has occurred with other pacing modalities,
development of more specific leads and delivery equipment is likely to
improve success rates in the more challenging cases with extensive scar.
Implications of dual blood supply to the conduction
system
Unlike myocardial tissue which generally has a single blood supply, much
of the conduction system is supplied by more than one coronary vessel.
This may explain why even when there is infarction severe enough to lead
to substantial myocardial scarring, the electrical system in the basal
septum can still be captured and achieve narrow complex activation of
the ventricle.
In the patients where we failed to advance the lead due to the severity
of basal septal scarring, we cannot test whether left bundle capture is
possible or achieves a desirable QRS pattern. We should not assume that
the answer is no, because within the patients where the lead could be
advanced there was no significant sign of greater basal septal scar
being associated with a worse electrical response. It is known that in
biventricular pacing non-viability of the entire septum is associated
with non-response14.
but this does not necessarily mean that the conduction system cannot be
captured at the septum to activate the rest of the ventricle.
Extensive myocardial scar might also cause uncoupling of the myocardium
from the conduction system, which could give an activation pattern
matching criteria for left bundle branch block, even when the proximal
conduction system is functioning normally. This may be the explanation
for the finding by Upadhyay et al that 36% of patients with left bundle
branch block had intact conduction in the His-Purkinje fibers15.
Utility of dark blood late gadolinium
sequences
Accurate detection and quantification of the scar extent was a key part
of this study. In addition to bright blood, we used dark blood late
gadolinium enhancemnt. Dark blood sequences allow better delineation of
subendocardial scar so is particularly useful in patients with ischaemic
cardiomyopathy9.
In 7 patients (five of whom had ischaemic scar in the septum), the full
extent of scar was not appreciated on standrad bright blood sequences.
It was only when dark blood late gadolinium enhancement sequences were
utilised that the full extent of scar was appreciated. The key advantage
is better appreciation of the border between the subendocardium and
blood.
The findings from our study suggest that dark blood sequences should be
carefully reviewed in patients who are due to undergo left bundle area
pacing. This will ensure that patients with extensive fibrosis are
reliably identified, which is likley to help with procedural planning.
Clinical implications
This study indicates that Cardiac MRI provides useful information before
attempting left bundle pacing. Operators planning left bundle pacing
should consider the presence and extent of basal septal scar in their
procedural planning: if there is < 50% extent, lead
advancement is very likely to be successful; if there is
>70%, lead advancement is very likely to fail. This can
help the operator decide whether to try left bundle pacing and how
persistent to be in the attempt.
Our research also suggests that patients would benefit from the
development of better tools to penetrate an extensively scarred basal
septum, because once the left bundle area is reached the scar extent may
not impair successful stimulation. The mechanism of preserved simulation
despite extensive scar may be the dual blood supply.