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.