Discussion
Our study corroborates that the long-term success of accessory pathway
ablation in the pediatric population can be increased to >
98% after a repeated procedure. It also demonstrates that the main
reasons for failure include inaccurate mapping and/or diagnosis, and
inadequate long-term lesion formation (mainly related to the use of
cryo-energy during the initial procedure).
The most recent expert consensus on management of accessory pathway in
the pediatric age estimated the single procedural success at 94%
(11-13). A significant number of patients will experience failure, thus
requiring multiple procedures to achieve definitive success. A recent
review of risk factors associated with recurrence in the pediatric
population suggests that the position and number of accessory pathways
are related to higher incidence of recurrence following ablation (14).
Our group came to the same results and tried to identify specific
challenges at each position.
The main apparent reasons for the recurrence especially in the left and
right lateral location were related to difficulties with catheter
stability, which can be solved with the introduction of contact force,
the use of 3D electroanatomical mapping and long sheaths, and direct
visualization of the interface catheter-tissue with intracardiac
echography imaging.
It has been long apparent that left lateral pathways are the most
frequent (15). Regardless of the location, a deep understanding
of the anatomic landmarks and a careful mapping seem to be the keys for
success. A right lateral position correlates with more difficult
procedures with higher number of lesions to achieve success, leading to
incorporate long (preferably steerable) sheaths, contact force sensing
catheters and, in rare cases, jugular access to assure better catheter
stability. Likewise, Ebstein’s anomaly is a risk for failed ablation and
recurrence, the difficulties being related to the high prevalence of
multiple pathways and to the lack of catheter stability secondary to the
displacement of the tricuspid valve (16, 17).
Para-hisian and antero-septal pathways’ challenge relates to the
proximity of the AV node. The wisdom of an accurate mapping and catheter
stability cannot be overemphasized. Some centers advocate for the use of
cryoablation to avoid injury to the nodal structures, advocating that
stability is achieved once the ice ball is formed facilitating contact..
However, an important drawback is that the catheter is stiff which
interferes with manipulation in small hearts. Given the high prevalence
of recurrence with cryo-energy (15-20%) (18, 19) it has been our choice
to attempt a cautious radiofrequency ablation protocol including a
conscious delineation of the right atrium, tricuspid valve and right
ventricle anatomy with 3D activation mapping. Rotational angiography of
the right atrium with 3D overlay on live fluoroscopy has been recently
incorporated in many laboratories to help a precise location of the His
position. Because the His bundle is located deeper and protected with a
fibrous tissue envelope in the ventricular septum as compared to the
atrial and superficially located compact AV node, a key point with this
approach is to target the ventricular insertion by searching for
predominant ventricular signals with a small atrial. In the challenging
cases of high risk anterograde only accessory pathways with no inducible
supraventricular tachycardia, mapping is usually performed during fast
atrial pacing in an attempt to maximize ventricular preexcitation. In
the case of bidirectional pathways in this location, both atrial (in
case of inducible orthodromic reciprocating tachycardia) and/or
ventricular mapping during fast atrial pacing or sinus rhythm can be
used. In all cases, we emphasize on confirmation of underlying intact AV
node conduction with differential atrial pacing maneuvres before and
during the ablation. In some cases, the use of contact force technology,
carefully increasing the contact from 5 to above 30 grams allows precise
identification of the sites of both mechanical AV block and mechanical
bumping of accessory pathway conduction. The 3D contact force catheters
currently available in the market are irrigated tip catheters
(ThermoCool SmartTouch catheter, Biosense Webster, California, US; and
TactiCathTM Quartz Contact force ablation catheter,
Abbot, Chicago, Illinois, US). Our strategy has been to set the
irrigation mode low (usually 2 to 7ml/min) during ablation, to use them
as non- or minimally irrigated catheters and thus limit the extension of
the lesion. In selected cases, applications with limited energy delivery
(starting at 5 to 10 watts and escalating by 5 watts until 25-30 watts)
allows highly accurate lesions, which can be stopped immediately in the
case of junctional acceleration and/or AV block (20). However, the
drawback is that this strategy can result in local oedema thus reducing
direct contact between the catheter and the fibre. The use of a long
(preferably steerable) sheath, a superior approach via the jugular vein
and performing the ablation during apnea may also increase catheter
stability and help to limit the risk for AV node injury.
An alternative approach for antero-septal accessory pathways is mapping
and targeting the accessory pathway from the aortic cusps. This approach
can be safely performed with adequate precautions and may be considered
in cases with failed previous procedures and/or high risk of AV block
with the standard right-sided approach (21, 22).
In the case of postero-septal pathway ablations, an irrigated-tip
catheter may help target deep myocardial or epicardial substrates (23),
rarely requiring combined right and left atrial applications, or
intracoronary applaications. Because coronary sinus aneurysms are also
related to long and difficult procedures and high incidence of
recurrence, we advocate for very thorough understanding of the anatomy
using an angiogram, 3D electroanatomical mapping, or intracardiac
echocardiography.