Case :
We report the case of a 22 years old man with hypertrophic
cardiomyopathy presenting with paroxysmal narrow QRS-supraventricular
tachycardia. There was no preexcitation at baseline or during atrial
pacing and no dual AV node physiology. Retrograde conduction was
decremental and tachycardia was induced by ventricular extra-stimulus
followed by an increase in VA interval and VAV pattern (fig 1).
Tachycardia displayed 1 :1 AV relationship with relatively long VA
interval (160 ms at the coronary sinus) and a VAV pattern after
entrainment by ventricular fast pacing (fig 1). Ventricular
extra-stimulus during tachycardia at the time the His bundle is
refractory advanced the next atrial depolarization with decremental
properties (fig 2). Tachycardia rate was slower when transient right
bundle branch block was present (150 vs 170 bpm). Thus, reciprocating
tachycardia utilizing a right bypass tract with unidirectional
retrograde slow decremental conduction was diagnosed.
Mechanism was then more deeply investigated using ultra-high density 3D
mapping system (Rhythmia ™, Boston Scientific, Inc). Earliest
activation was mapped in the right atrium (shortest VA interval around
140 ms with large isolectrical interval), with a focal activation at the
lateral part of the right atrium near the tricuspid annulus. A PPI equal
to the tachycardia cycle length was elicited at this spot during
entrainment. Interestingly, there was a potential following ventricular
activation (around 80 ms after) in a relatively large area closer to the
the annulus, which preceded the earliest atrial activation (around 60 ms
before) (fig 2). Reannotating on this potential and then on successive
atrial signals revealed a relatively wide structure at the level of the
annulus, activated from the ventricle and then conducting to the atrium
in a concentric fashion (fig 3). Ablation at the earliest atrial site
did not terminate the tachycardia, but repeated ablation attempts on the
expected location of the intermediate branching structure at the level
of the annulus finally terminated the tachycardia which could never be
induced again, and the patient remained free of any tachycardia at 6
months follow-up.
Discussion :
The tachycardia involved a concealed by-pass tract with relatively long
and decremental retrograde conduction and without anterograde conduction
(acting like purely retrograde « Mahaim » fibers). This extra-anatomical
retrograde conduction was associated with a particular wide structure
located at the tricuspid annulus, harboring specific potential, and
compatible with an accessory atrioventricular (AV) node and conduction
pathway. To our knowledge, this is the first 3D documentation of such
retrograde accessory pathway and of potential accessory AV node and
conduction network.
Atrio-fascicular/ventricular connections (« Mahaim fibers ») usually do
not demonstrate retrograde conduction (1, 2) and are considered to
represent an accessory AV conduction pathway (2) because of decremental
properties and of a specific potential – similar to His potential –
recorded at the tricuspid annulus and further along the right lateral
ventricular free wall (1, 2). It is expected to consist of a proximal
component (similar to the AV node) at or above the tricuspid annulus,
and a distal component (similar to the His bundle) that generates the
specific potential, extending toward the right ventricular apex and
distal components of the right bundle branch (2). Accessory AV node has
been identified as an insulated tract of specialized cardiomyocytes,
piercing the insulating pathways of the atrioventricular junction, and
extending into the right ventricle, thus producing a second
atrioventricular conduction system located on the lateral part of the
tricuspid annulus, representing remnants of atrioventricular ring tissue
(3). Atrial component of decremental accessory pathways, found in the
vestibule of the tricuspid valve, could harbor remnants of the tissues
that give rise to the normal AV node (4). Localization of the area of
recording of a specific potential in our case was consistant with the
location of the vestibule of the tricuspid valve (4) and thus could
appear to be a 3D representation of what has been described in autopsies
(3, 4). The relative wide area of recording was not indicative of common
accessory pathway potentials, neither the long VA interval was evocative
of conduction through an usual accessory pathway where local ventricular
and atrial events are commonly fused. We were not able not depict the
proximal (ventricular) end of the accessory pathway since it could not
be mapped during orthodromic tachycardia (fused with global ventricular
activation) and since there was no anterograde conduction.
3D mapping of decremental AP has been published but only for anterograde
conduction (5-7). Rarely such decremental atrio-fascicular/ventricular
connections demonstrated retrograde conduction (8-10). Concealed
retrograde conduction had been demonstrated through the distal part of a
decremental accessory pathway that blocked at its proximal atrial
insertion (specific potential not followed by atrial activity) (11).
Orthodromic reciprocating tachycardia utilizing such pathways have been
sometimes documented (9, 10, 12), but only in patients with anterograde
conduction and pre-excitation. Thus, it seems to be the first report of
orthodromic reciprocating tachycardia using a purely concealed
decremental accessory pathway with findings compatible with an accessory
AV node.
Some more common by-pass tracts however may also have decremental
properties without being categorized as « Mahaim » fibers : these
« short decrementally conducting AV pathways » may simply be due to
alteration of conduction properties or tortuosity (4, 11). However they
usually do not present with retrograde conduction (2, 12) and large
areas of specific potentials are not described in this setting.
Therefore, this case may be considered as a 3D illustration of
orthodromic tachycardia involving an accessory AV node at the level of
the tricuspid annulus in a patient without pre-excitation.