Case Presentation
An 83 year old lady was highly symptomatic from paroxysmal
Atrial Fibrillation (AF) despite numerous antiarrhythmic and
rate-limiting medications. Given her preserved left ventricular
function, a pace and ablate strategy was pursued. Ablation
of the AV node proved challenging requiring 2 procedures
with apparently stable AV block and regular escape at 40bpm
after the first sitting. Persistent Right Bundle Branch Block
(RBBB) was created prior to AV node block at the first sitting.
Two irrigated-tip ablation catheters (Coolflex, medium
curve, St Jude's Medical, MN, USA) were placed on either side
of the interventricular septum but with the intention to ablate
the AV node from the left. Identification of the His bundle
from the left proved difficult as the patient was now in AF.
Instead the left bundle was targeted and complete AV block
achieved by combined Right Bundle Branch (RBB) and Left
Bundle Branch (LBB) ablation.
Post ablation the escape rhythm was observed to switch from
predominant right bundle branch block-like (RBBB-like)
morphology to occasional left bundle branch block -like
(LBBB-like) morphology (Figure 1A). What is the mechanism?
Comment
After successful AV node ablation, the patient is usually left
with an automatic slow escape beat [1]. In this case the predominant
RBBB-like beat can be thought of as such an escape
probably originating from the left ventricle. To explain
the LBBB-like beats, 2 possible mechanisms should be mentioned
(i) they may represent a second automatic focus akin
to parasystole or (ii) a complex re-entry involving the separately
ablated left and right bundle branches.
Traditionally parasystole has been described as an automatic
focus protected from external influences by "entrance block"
and leading to intermittent ventricular activation by a postulated
"exit block" [2]. Thus classically a parasystolic mechanism
is suspected if (i) there is varying coupling interval of the
ectopic beat (including fusion beats) and (ii) the length of the
interectopic intervals are in simple mathematical relationship
to one another (the "minimum multiple" law) [2]. In the case
presented here the first rule is met; over a 2 minute period, the
coupling interval between the LBBB-like beat and the preceding
RBBB-like beat varied from 698-801ms, with mean coupling
interval = 753±42ms and the mean change in coupling
interval = 55±42ms. Such variation makes a re-entry mechanism
unlikely. However, the second rule of parasystolic foci
was not fulfilled, as over the same period the intervals between
the eight LBBB-like beats were 2.5s, 4.3s, 18.0s, 46.0s, 37.0s
56.2s and 21.9s respectively (i.e. no common denominator).
This, may be explained by the more common type of parasystole
with incomplete entrance block which can be influenced
by external depolarisations (also known as modulated parasystole)
[3].
On manipulation of the RV ablation catheter from the apex to
the septum, there was complete suppression of the LBBB-like
beats in the surface ECG (Figure 1B). Instead there was repeated
detection of a lower amplitude local signal at a similar
coupling interval to the previous LBBB-like beats. Indeed over
a 2 minute period, the coupling interval of this non-propagating
impulse varied between 660-773ms. This unique observation
is highly suggestive of increased exit block of a parasystole
whose local, non-propagating activation is recorded. To confirm
our observations, it would be ideal to pace from this area
in order to get the LBBB beat manifested. However, this was a
retrospective analysis of the case that did not allow for further
manoeuvres.
