Ablation
Ablation is performed using a 3.5mm irrigated force-sensing ablation catheter via a steerable Agilis sheath to ensure catheter stability and adequate tissue contact with ‘point-by-point’ ablation guided by 3D electroanatomic mapping. High frequency low volume ventilation is used to facilitate catheter stability. Ablation should not be performed in AF but rather during flutter or atrial pacing. Our decision regarding the location of our ablation line is determined by the voltage map with the lateral mitral isthmus line generally preferred in the setting of normal atrial voltages. An anterior mitral isthmus line is pursued only in the presence of extensive regions of anterior low voltage. A meta-analysis of peri-mitral flutter ablation studies showed no difference in rates of acute bidirectional mitral isthmus block or ablation time when comparing anterior and lateral mitral isthmus ablation lines but anterior lines were associated with a higher percentage of patients maintaining sinus rhythm during follow-up.30
When performing a lateral mitral isthmus line, ablation commences at a 4 o’clock position on the lateral mitral valve annulus and point by point ablation is completed pursuing the shortest line to the anterior aspect of the LIPV. If the left PVs have not already been isolated then wide antral circumferential ablation is completed first, prior to the lateral mitral line. Given the aim of obtaining deep, transmural lesions, we use 30-35W rather than higher power settings. A contact force of >10g is maintained at all times, targeting an ablation index (AI) or lesion size index (LSI) of 600 or 6, respectively. More often than not, mitral isthmus block is not achieved at the first pass and ablation is extended a variable distance superiorly along the perceived endocardial course of the ligament of Marshall to the junction of the left PVs. The line is then remapped and if there are no obvious ‘gaps’, we proceed to ablate within the CS. When ablating within the CS, the catheter is deflected towards the endocardial line such that atrial electrograms are seen on the distal electrode of the electrode catheter and the impedance closely monitored. Ablation lesions are delivered at 25W, watching closely for rises in impedance. We generally do not utilise CS balloon occlusion or displacement. If mitral isthmus conduction is still present, we return to the LA and map predominantly just superior to the endocardial line and deploy an additional line. We then map, ideally using a high density multipolar mapping catheter, during CS distal pacing to determine the earliest breakthrough. In difficult cases, the ablation may be in close vicinity to the base of the LAA, corresponding to the endocardial aspect of the ligament of Marshall. When available, if mitral isthmus block is not achieved with endocardial ablation and ablation within the CS, we consider the use of VOM alcohol ablation. The approach to the lateral mitral isthmus ablation is summarized in Figure 4.
If opting to perform an anterior mitral isthmus line instead, we utilise 40W of power, aiming for a contact force of >10g and targeting an AI or LSI of 400 or 4-5, respectively. Shorter duration lesions are typically delivered at sites of low tissue voltage and longer duration lesions in the endocardial region adjacent to Bachmann’s bundle. Typically with the assistance of a steerable sheath, ablation would begin at the superior mitral annulus with counter-clockwise rotation releasing the curve to ascend superiorly to the RSPV, LSPV or roof line, depending on the location and extent of the low voltage anteriorly.