Answer:
PACs initiate an incomplete RBBB morphology tachycardia with an ”A on V” morphology and a normal HV interval. The differential diagnoses include typical AVNRT, atrial tachycardia and orthodromic tachycardia via a concealed nodofascicular or nodoventricular pathway. The tachycardia initiated with A-H-H-A sequence suggests that it can be typical AVNRT initiating with a double fire (2 for 1 phenomenon) and rules out the other differential diagnoses enumerated above. Interestingly, in our case, the HV interval during the tachycardia is shorter than in sinus rhythm, suggesting that ventricular tachycardia (VT) involving the conduction system could be a differential diagnosis. In supraventricular tachycardia, the HV interval during the tachycardia is equal to or longer than in sinus rhythm, while the HV interval is generally negative in myocardial VTs and preexcited tachycardia.
The diagnosis of fascicular VT was confirmed by performing atrial overdrive pacing during the tachycardia, which revealed entrainment of the tachycardia as seen by fusion beat and at a faster atrial rate, the QRS narrowed (ruling out rate related RBBB). The HV interval during the tachycardia (HV 34 ms) was less than during sinus rhythm (54 ms). After 6 mg of adenosine, the tachycardia continued with ventriculoatrial block. During fascicular VT, the His is typically activated retrogradely, resulting in a pseudo HV interval with a negative HV usually.1
Mapping was performed both during tachycardia and during sinus rhythm, with the earliest fascicular signals detected in the upper septum (upper septal fascicular VT) just below the His region. After discussing the patient and his relatives, RF energy was delivered during tachycardia, which terminated the tachycardia within 2 seconds. Even with isoprenaline, no tachycardia was induced after RF energy application. During a 6-month follow-up, the patient was symptom-free.
The interesting points in our case were: 1) For moments during the tachycardia, it resembles typical AVNRT due to the ‘A on V’ pattern. Hence the patient was treated as AVNRT with slow pathway ablation attempted twice in other hospitals. 2) The shorter HA was due to robust VA conduction through the AV node alongwith a relatively slow conduction time down the septal fascicle. This case illustrates the importance of measuring the HV interval during the tachycardia and comparing it with the HV interval during sinus rhythm. 3) A pseudo-normal HV interval during fascicular VT has not been reported before.
Among the idiopathic fascicular VTs, the upper septal variant is the least common type (<1%). This variant can mimic supraventricular arrhythmias like AVNRT, and an incorrect diagnosis can result in RF energy delivery in the region of the AV node, potentially causing damage to the conduction system.2.3