Discussion
The differential diagnoses of WCT with LBBB morphology similar to that during SR, an HH interval preceding a VV interval, and presenting with 1:1 VA conduction include bundle-branch reentry ventricular tachycardia (BBR-VT), any supraventricular tachycardia (SVT) with LBBB such as AVN reentrant tachycardia (AVNRT), orthodromic reciprocating tachycardia (ORT) with AV accessory pathway (AP), ORT with node-fascicular/node-ventricular (NF/NV) AP, junctional tachycardia (JT), and atrial tachycardia (AT). Antidromic reciprocating tachycardia, pathway-to-pathway tachycardia, and any SVT with a bystander AP were unlikely because His deflection preceded the QRS onset with the HV interval identical to that during SR (Figure 1). During WCT, AH block and the subsequent VA perturbation followed without termination (Figure 2). This finding could exclude ORT with AV AP, ORT with NF/NV AP, JT, and BBR-VT because these tachycardias include the ventricle or His as a part of the circuit. AVNRT with infra-Hisian block also was an unlikely diagnosis because there was VA perturbation without termination of the tachycardia (Figure 2). Although we did not confirm the dual pathway physiology of the AVN by an extra pacing maneuver, gradual prolongation of AH leading to the fixed AH interval after AH block during WCT (Figure 2) was not inconsistent with the transition of the antegrade pathway from the fast to the slow pathway. Based on these findings, AT was diagnosed.
VA relationships were interrupted by atrial pacing from the two different sites of the right atrium without termination of the tachycardia.1, 2 Mapping was performed during WCT, and the earliest atrial activation site was near the His region. RFCA was performed and the WCT was terminated. This was complicated by complete AV block. Before ablation, the patient was informed of the potential risk of AV block with the necessity for a pacemaker and of being the potential candidate for cardiac resynchronization therapy (CRT) because of class 2 congestive heart failure with an impaired EF and LBBB; therefore, CRT was implanted.
We described a case of AT presenting with a wide QRS-complex, 1:1 VA conduction, HH variability preceding VV variability and short VA. The differential diagnoses of WCT with 1:1 AV conduction are challenging, and some pacing maneuvers should be encouraged. However, in our case, only one ECG tracing, a transient VA perturbation following AH block during WCT, immediately ruled out other SVT and VT, which is the most important differential diagnosis.
An observation of the spontaneous phenomenon during EP study is occasionally sufficient to make a diagnosis.