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