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.