Spontaneous Transition of Double Tachycardias with Atrial Fusion in a Patient with Wolff-Parkinson-White Syndrome

Dual atrioventricular (AV) nodal physiology is a common finding during electrophysiologic studies (EPS). Even among patients with Wolff-Parkinson-White (WPW) syndrome, atrioventricular nodal reentrant tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT) can coexist in a single patient. However, direct transition of AVNRT and AVRT is quite rare. Retrograde atrial activation sequences during ventricular pacing and reentrant tachycardia provide important information for the localization of accessory pathways (APs).1) Here, we report a case of double tachycardia with direct transition and eccentric retrograde atrial activation during typical AVNRT.

Multiple supraventricular tachycardias with multiple reentry circuits are relatively common in a patient during EPS. However, direct transition between double tachycardias is rarely observed. Delacretaz et al.2) reported a case of direct transition from AVNRT to AVRT, and Rakovec3) described a patient with WPW syndrome, in whom transition occurred from AVRT into AVNRT. Kuo et al.4) reported 3 cases of double tachycardia with transition. In their report, Kuo et al.4) proposed that similarity in tachycardia CLs may predispose to transition and the cutoff point of 25 msec had 80% positive predictive value for transition between double tachycardias. Usually, transitions were induced by premature beats or block of the AV nodal pathway or APs.

In the present case, tachycardia CLs of the two tachycardias were quite different and the difference was greater than 25 msec. Transition occurred with slight prolongation or shortening of the tachycardia CLs, rather than being induced by premature activities in the atria or ventricles.

Spontaneous tachycardia transition without premature activities is a rarely observed phenomenon. Delacretaz et al.2) reported a case with transition without premature beats. However, an AP was located in the right posterior region relatively proximal to the AV nodal region and the difference in the CL of the tachycardia was less than 25 msec. In contrast, AVRT using the left lateral AP as a retrograde limb of the circuit was far distal to the AV node in this particular patient. Therefore, it is difficult to expect spontaneous transition of AVNRT or atrial fusion with the retrograde fast pathway.

The following factors might have contributed to or may have facilitated the spontaneous transition of tachycardia in the present patient: 1) Anterograde or retrograde AV nodal conduction may be influenced by the preceding cycle length or autonomic tone.5) Intense variation of tachycardia CL and VA intervals was observed around the time of transition. 2) Although spatial distance of the AP was relatively far from the AV node, VA interval in the left free wall region (70 msec) was similar to that in the proximal CS recording (56 msec). 3) Atrial fusion continued during AVNRT. 4) Anterograde conduction of both tachycardias was transmitted through the slow pathway. However, the exact circuit of AVNRT remains unelucidated. Also, it is difficult to postulate the mechanisms underlying tachycardia transition in complicated AVNRT. Further research should be conducted.

The possibilities for unusual CS activation during narrow QRS tachycardia include multiple APs, atypical AVNRT,6) left superior AP,7) and intra-atrial block.8) Eccentric atrial activation sequences during AVNRT, which was an unexpected type of tachycardia in a patient with WPW syndrome, were caused by the left lateral AP in this patient. Also, the retrograde atrial activation sequences were changed with direct transition of tachycardia. These findings were prone to misinterpretation regarding the retrograde sequence of atrial activation and led us away from the correct diagnosis. This instructive case emphasizes the need for careful interpretation of retrograde atrial activation sequences in the CS region in order to correctly identify the presence and location of APs.

In summary, we described a rare case of direct transition between AVNRT and AVRT in a patient with WPW syndrome. Fusion of retrograde activation of the atrium occurred via the fast pathway and the AP caused eccentric AVNRT. This case highlights the importance of the retrograde atrial activation sequence in the localization of APs and suggests that differential pacing maneuvers are not feasible in transitional tachycardia with atrial fusion.