Pietro Delise, Aldo Bonso*, Leonardo Coro, Mauro Fantinel, Gianni Gasparini*, Antonio Raviele*.
Operative Units of Cardiology, Hospitals of Feltre and *Mestre, Italy
|
|
Common atrioventricular (AV) node reentrant tachycardia (AVN RT)
consists of anterograde conduction via a slow pathway and retrograde conduction via a fast pathway. Although
both the fast and the slow pathway can be selectively ablated to control AVN RT, slow pathway ablation is
considered the better at preventing the risk of inadvertent AV block. However, also the slow pathway ablation
has a low but significant risk of inadvertent complete AV block1-4. In
patients with normal or modestly impaired
PR interval (<300 msec) the risk of block is about 0.5-2%. In patients with markedly prolongerd PR interval (>300
msec) such risk may be significantlt higher. As AVN RT generally is a benign arrhythmia the complication of a
complete AV block is difficult to tolerate especially when patients are young.
In recent years many authors tried to find a method able to avoid or to reduce the risk of block. Thakur et al5
reported that a relatively fast rate of junctional tachycardia (JT) caused by radiofrequency associated with loss
of V-A conduction is related to an increased risk of AV block. However, these markers for impending heart block
do not allow the risk of AV block to be determined before radiofrequency (RF) energy delivery. Indeed, AV block
may occur concurrently with the development of retrograde fast pathway block during JT induced by RF.
Hintringer et al6 suggested that a useful electrophysiologic marker to assess the risk of block is represented by
the interval between the atrial component of the His bundle electrogram and the atrial signal of the distal
mapping catheter [A(H) - A(Md) interval]. Indeed, these authors suggest that a short A(H)-A(Md) interval (17±8
msec) should be associated with a high risk of inadvertent AV block. According to the authors the longer the
interval, the greater the distance between the mapping catheter and the compact AV node should be.
The explanation of inadvertent AV block during slow pathway ablation suggested by Hintringer et al however
is not completely convincing and their method to avoid AV block is not completely satisfactory. Indeed, the
causes of inadvertent AV block during slow pathway ablation vary and may not involve a lesion of the compact
AV node. In some cases, the block can be due to an intentional damage of the AVN-His junction due to delivery
of the energy to the anterior aspect of the triangle of Koch in the junction between the His bundle and compact
AV node. In other cases, the block is related to the injury of both fast and slow pathways during serial attempts
at ablation of both pathways4. In other cases, when a markedly prolonged PR interval is present, the block is
related to the lesion of the slow pathway in the absence of the anterograde conduction of the fast pathway7. In
yet other cases the reason of block is unclear because the PR interval is normal and the energy is delivered at
the posterior aspect of the triangle of Koch, far from the usual site of the anterograde fast pathway and far from
the compact AV node. A possible explanation in the latter case is an atypical location of the fast pathway or
again the absence of its anterograde conduction.
Anatomical studies8 have demonstrated that AV node morphology is quite variable in man and reproduces the
electrophysiologic model of the duality in only few cases. Electrophysiologic studies9 have confirmed that the
AV node is a complex and variable structure. For example, multisite simultaneous catheter mapping of Koch’s
triangle have demonstrated that retrograde conducion of the AV node both during ventricular pacing and AVN
RT is heterogeneous, indicating that in most patients multiple retrograde fast pathways are often present even
if not all are used during AVN RT. Indeed, in about two out of three patients, early retrograde breakthrough is
observed in multiple sites of the Koch’s triangle or in a large area of a broad wave front activation.
Furtheremore, it has been suggested that the site of the anteriorly conducting fast pathway is not always
coincident with the posteriorly conducting fast pathway (which is retrogradely utilized during AVN RT). For
example, in some cases, the anterograde fast pathway is located in the anterior aspect of the Koch’s triangle,
while the retrograde fast pathway is located in the posterior aspect of the Koch’s triangle. It has also been
suggested that in some cases the fast pathway is able to conduct only retrogradely while its anterograde
conduction is absent7,10. Such variability explains why in selected cases unexpected AV block can occur
during slow pathway ablation of AVN RT. Indeed, the lesion of the slow pathway can also damage the
anterograde fast pathway if the latter is located close to the slow pathway itself. AV block can also follow
slow pathway ablation in cases in which the anterograde conduction of the fast pathway is absent and AVN
RT has as retrograde limb a fast pathway which is able of retrograde conduction only.
If the mechanism of AV block is an abnormaly located or a non conducting fast pathway, such risk should be
reduced localizing the site of the anterogradely conducting fast pathway. With this purpose, we11 suggested to
localize the site of the anterogradely conducting fast pathway in patients with AVN RT by the pacemapping of
Koch’s triangle.
|
