Lateral Mitral Isthmus Line
The lateral mitral isthmus is a common target to treat peri-mitral
flutter owing to its relatively short length, averaging 34.6mm but
ranging from 17-51mm, and ability to clearly define bidirectional block
using the coronary sinus (CS).14 However,
bidirectional block can be difficult to achieve with endocardial
ablation only, owing to several anatomic factors.
Firstly, autopsy studies report a wide variation of myocardial thickness
in this region – on average 3mm at the level of the LIPV (range
1.4-7.7mm), 2.8mm at the mid-isthmus (range 1.2-4.4mm) and 1.2mm at the
mitral annulus (range 0-3.2mm).14 The thickness of the
mitral isthmus has been shown to determine the acute success of ablation
in this region.15 The operator is typically unaware of
individual variability in tissue thickness and generally adopts fixed
output ablation along the isthmus. In recent times, high power short
duration ablation has been adopted. However, when deeper lesions are
required, lower power for longer durations may be
preferable.16
Secondly, the CS lies on the epicardial surface adjacent to the
endocardial aspect of the mitral isthmus as it traverses the inferior
left atrial wall approximately 1cm above the mitral valve annulus and
becomes the great cardiac vein beyond the Valve of Vieussens and/or the
bundle of Marshall.14 The CS limits the achievement of
bidirectional block across the mitral isthmus in two main ways. The CS
has a myocardial sleeve of variable thickness (ranging from 0.3 to
2.5mm) which extends a variable distance from the CS ostium (mean
40±8mm).17 This cuff of muscle can act as an
epicardial bridge, bypassing the endocardial mitral isthmus at the site
adjacent to endocardial ablation. In addition, the CS blood pool can act
as a ‘heat sink’ that reduces conductive heating of the subepicardium
and limits lesion transmurality.18 This idea is
supported by previous studies demonstrating a reduced need for
epicardial CS ablation when CS balloon occlusion is performed prior to
endocardial ablation and that a larger CS diameter is associated with
increased ablation time and the need for epicardial CS ablation to
achieve bidirectional block.19,20
Finally, the bundle of Marshall, a fibromuscular sleeve that surrounds
the vein of Marshall (VOM) as it traverses epicardially along the ridge
between the LAA and left pulmonary veins (PVs), may have variable
connections to the CS musculature and the left atrium (LA), thereby
providing another source of epicardial connection which can prevent
mitral isthmus block.21 As a result of these anatomic
challenges, between 48 and 97% of patients have been reported to
require epicardial ablation within the CS in addition to endocardial
ablation to achieve mitral isthmus block.22-24 More
recently, the convenient anatomic location of the VOM on the epicardial
aspect of the mitral isthmus has been used to advantage. Valderrabano et
al. demonstrated extensive lateral mitral isthmus ablation with the use
of ethanol infusion into the VOM (VOM ETOH).25,26Following VOM ETOH, minimal ablation is then required at the mitral
annular end of the isthmus to achieve bidirectional block.
While the left circumflex artery is separated from the CS by adipose
tissue, this protective cushion is variable and in some patients, it
lies in close proximity to the CS and may be susceptible to injury
during ablation.14 In a single-centre series, the
incidence of symptomatic circumflex artery occlusion post mitral isthmus
ablation was rare at 1 in 499 patients.27 However, in
a separate study, the incidence of asymptomatic coronary artery injury
with mitral isthmus ablation was much higher at
28%.28 Fortunately, all coronary stenoses resolved
with intracoronary nitrates suggesting the injury was thermally-mediated
vasospasm without permanent injury.28 Ablation within
the CS, proximity of the left circumflex artery to the CS and a small
distal left circumflex artery diameter were risk factors for injury in
this study.28 While pulsed field ablation has been
reported to reduce the risk of injury to non-myocardial tissue, left
circumflex artery occlusion due to vasospasm has been reported due to
PFA of the endocardial mitral isthmus line.29