A Rare Evidence of a Dual Atrioventricular Nodal Physiology in a Patient with Narrow Complex Tachycardia

The present case details an interesting intracardiac electrogram in a patient who demonstrated recurrent episodes of narrow complex tachycardia and who was subsequently found to have a typical atrioventricular nodal reentrant tachycardia during an electrophysiology study. The patient subsequently underwent slow pathway ablation and was found to be noninducible for any tachycardia after ablation.


Introduction
Atrioventricular (AV) nodal reentry tachycardia (AVNRT) is the most common supraventricular tach ycardia (SVT) in adults. Patients with AVNRT have a dual AV node consisting of slow and fast pathways. Various electrophysiological findings and maneuvers can prove the AVNRT to be a mechanism of tachycar dia during electrophysiology study (EPS). 1,2 The most commonly used definition of a dual AV nodal physio logy is the jump and echo phenomenon noted during an EPS. Jump is defined as an increase in the A-H interval of more than 50 ms, with a 10 ms decrement in an extrastimuli or a 10 ms decrement in an atrial drive train. 2 Rarely, patients can have simultaneous conduc tion down both the slow and fast pathways, resulting in two QRS complexes for a single Pwave. On the electro gram (EGM), this phenomenon is seen as an atrial EGM followed by two ventricular EGMs (ie, two-forone phe nomenon or a " doublefire"). In this case report, a trac ing showing a twoforone phenomenon ("doublefire") in a patient who had recurrent palpitations and AVNRT is presented (Figure 1).

Case history
A 67yearold male with a longstanding history of palpi tations was seen in our arrhythmia clinic and was offered participation in an EPS. The patient was brought to the electrophysiology laboratory. After proper sedation and completion of the usual sterile drape and preparation protocols, electrophysiology catheters were advanced under fluoroscopy to the right ventricular apex, His region, high right atrium, and coronary sinus. EPS was performed and, during atrial extrastimuli, the findings noted in the tracing were consistently reproduced.
Notably, the patient had an extrastimulus followed by two Rwave EGMs. There was an echo beat observed on the second Rwave. The first Rwave resulted from the antegrade conduction over the fast pathway. The second Rwave resulted from the antegrade conduction over the slow pathway way. This phenomenon has been reported previously 3,4 but is more commonly seen in females in the fifth decade of their life.
During EPS, an echo was noted with the retrograde con duction over the fast pathway. However, this phenom enon failed to initiate the tachycardia as the premature ventricular complex (PVC) came in. In the patient, the

Dual AV Nodal Physiology in Narrow Complex Tachycardia
PVC was observed to find the AV node refractory (con cealed conduction from retrograde conduction), as it had just been depolarized by an echo beat and conducted ret rogradely with a long V-A interval over the slow path way. Although this is a known phenomenon, what makes this tracing interesting is the presence of an atrial echo on the second QRS. An atrial echo seen on the second QRS is rare during a doublefire and initiation of AVNRT. There are reports of initiation of nonreentrant tachycardia by simultaneous conduction over fast and slow pathways, and these often lead to cardiomyopathies. [5][6][7] The patient was subsequently induced for an SVT with a very short septal VA interval. Ventricular entrainment during tachycardia resulted in a VAHV response and a long postpacing interval suggestive of an AVNRT as a mechanism of the tachycardia. Slow pathway ablation was performed with complete elimination of any slow pathway conduction and the patient was noninducible for any tachycardia after the ablation.
It is possible that the next echo beat could have been a junctional ectopic. However, this finding was reproduc ible and the patient had inducible AVNRT and, after ablation, this finding could not be reproduced. Thus, it is believed that the second ventricular complex is because of simultaneous conduction over the slow pathway.