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.