Discussion
Dual atrioventricular nodal pathways physiology refers to the functional
dissociation of conduction through the atrioventricular node wherein
atrial impulses conduct with two distinct PR intervals (2 “families”
of PR interval). Myriad clinical and ECG presentations are known to
result from dual pathways physiology.1 Most commonly,
dual pathways physiology can manifest as typical atrioventricular nodal
reentrant tachycardia (AVNRT), a form of reentrant tachycardia with
antegrade conduction down the “slow pathway” and retrograde conduction
up the “fast pathway” of the AV node.
An uncommon presentation of dual AV nodal pathways physiology is “dual
atrioventricular nodal non-reentrant tachycardia” (DAVNNRT), in which a
single atrial impulse conducts antegradely down both AV nodal pathways
resulting in two separate ventricular depolarizations, also called
“double fire tachycardia”. For this to occur, several conditions must
be met: 1) A large difference in conduction velocities of each pathway;
2) absent/poor retrograde fast pathway conduction; and 3) refractory
period of the distal common final pathway is short enough to allow
conduction of the second wave front from the slow
pathway.2 There are numerous case reports of such
patients presenting with heart failure due to tachycardia-related
cardiomyopathy, which is typically reversible.
Numerous reports have also been published of multiple antegrade
“jumps” (AH increment of >50 ms with 10 ms decrement in
atrial coupling interval), which have been interpreted as being
reflective of multiple AV nodal pathways.3-5 However,
more than a dual antegrade conduction has only rarely been described.6-9
Our case demonstrates a most unusual presentation: “Triple-fire
tachycardia” due to triple antegrade AV nodal pathways physiology, with
the longest PR interval of 840 ms. This finding was repeatedly
demonstrated throughout the study. The marked difference in conduction
velocities of the second and third AVN pathways (AH2 versus AH3
>250ms) is consistent with the criteria to allow for
multiple, separate ventricular depolarizations. The documentation of a
3rd QRS complex on telemetry excludes catheter-induced
His extra-systoles. As is usually the case, ablation in the posterior
interatrial septum abolished repetitive firing over all but the “fast
pathway”. This case highlights the remarkable variations in human AV
nodal physiology, remarkably slow antegrade conduction in the absence of
retrograde conduction, and the potential to cause cardiac pathology.