Commentary
Careful examination of the Holter monitoring showing frequent
extrasystoles revealed sinus rhythm marching though the tracing without
perturbation (Figure 2A). Differential diagnoses for the extrasystoles
would include ventricular ectopy, His-Purkinje ectopy, or
supraventricular ectopy conducting with aberrancy, with the latter
including atrial ectopy, atrio-ventricular nodal echo beats and more
rarely dual AV nodal physiology with ‘double fire’ response. Similarly,
examination of irregular narrow complex tachycardia initially reported
as “atrial fibrillation” revealed putative P waves consistent with
sinus rhythm, with supraventricular extrasystolic beats in a grouped
beating pattern.
The 12-lead rhythm strip during tachycardia is helpful (Figure 2B).
Again, sinus rhythm is observed without perturbation in rate. The
extrasystoles exhibited variable degrees of aberrancy. The 12-lead
analysis also allows one to be reasonably confident that there are no P
waves preceding the extrasystolic beats making atrial bigeminy much less
likely. Single AV nodal echo beats would also be unlikely in the absence
of significant PR perturbation when one compares the first 2 sinus beats
on the tracing. The remaining differential diagnoses would include: 1)
ectopic beats arising from the His-Purkinje system; 2) dual AV nodal
pathways with ‘double fire’ response, conducting with varying aberrancy.
An electrophysiological study (EPS) was then performed under conscious
sedation. Quadripolar catheters were positioned at the high right
atrium, and right ventricular apex. Decapolar catheters were positioned
in the coronary sinus, and His bundle and right bundle region to assess
conduction through the His-Purkinje conduction system.
The clinical arrhythmia was reproducible with slow atrial pacing in the
range of 700 – 900 ms. Again, the extrasystolic beats exhibited either
normal QRS morphology, right bundle branch block (RBBB), or RBBB and
left anterior fascicular block, all with the same coupling interval
(Figures 3A). Examination of the intracardiac electrograms revealed
abrupt changes in AH interval between short AH and very long AH
intervals (680ms), suggestive of dual AV nodal physiology, as well as
potential ‘two for one’ AV response (Figure 3A).
Although it was difficult to definitively exclude junctional
extrasystoles as an alternative explanation for the extrasystoles,
retrograde His activation with shortening of the HV interval is often
described with junctional extrasystoles.1 In our case,
the His activation was always proximal to distal and the HV interval was
unchanged compared to that during sinus rhythm. Also, the coupling
interval of the extrasystolic beat appeared consistent with both
decremental atrial pacing and atrial extra-stimuli (Figure 3B). The
extrasystoles were only observed during a specific range of atrial
pacing intervals and they were suppressed with faster atrial pacing and
during isoprenaline. Taken together, these features are most compatible
with a dual AV nodal non-re-entrant tachycardia or ‘double fire’
response, where sinus beats are simultaneously conducted anterogradely
over both a slow and fast pathway. This produces a long-short coupling
of the His-Purkinje bundle, resulting in aberrancy pattern (RBBB and/or
LAFB) on the second beat. Notably, AV nodal re-entrant tachycardia was
not inducible.
Ablation of the slow pathway is the treatment of choice for double fire
tachycardia with reports of 100% acute success and 94% long term
success2. One patient was treated by increasing the
lower rate pacing of his implanted device from 40 bpm to 70 bpm to
supress slow pathway conduction3. In our case,
radiofrequency energy was applied in the slow pathway region. Occasional
junctional extrasystole was observed during application of RF energy.
Following ablation, there was no further evidence of extrasystolic
beats.
Dual AV nodal response conduction is a rare phenomenon. For it to occur,
the effective refractory period of the His-Purkinje system must be
shorter than the difference in AH intervals between fast and slow
pathway conduction. Usually retrograde ventriculo-atrial conduction is
poor or absent, as in the present case.4
Dual AV node physiology with “double fire response” can mimic a
variety of other arrhythmias, including PVCs and atrial fibrillation as
was the case in our patient.5 It can cause symptomatic
palpitations and also tachycardia-induced cardiomyopathy. These findings
emphasize the importance of carefully examining the ECG of an irregular
arrhythmia, with close attention to the presence of grouped beating, and
include double response in the differential diagnosis.