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