ABSTRACT
N/A
1 │INTRODUCTION
Electrical cardioversion of atrial fibrillation resulting in
simultaneous initiation of atrio-ventricular nodal reentrant tachycardia
is a rarely reported electrophysiological phenomenon whose mechanisms
are poorly understood. This case report aims to speculate the plausible
underlying mechanisms.
2 │CASE PRESENTATION
A 70-year-old male with a history of regular complex short RP
supraventricular tachycardia (SVT) underwent an invasive
electrophysiology study (EPS). The SVT was repeatedly terminated with
adenosine in the past. During EPS, dual atrioventricular node (AVN)
physiology was demonstrated based on AH jump. Single atrial extra
stimulus reproducibly induced an ‘A on V’ tachycardia with a cycle
length (CL) of 360 milliseconds (ms). Its mechanism was ascertained to
be typical slow-fast AVN reentrant tachycardia (AVNRT) after performing
standard EP maneuvers.1,2 Atrial burst pacing induced
sustained atrial fibrillation (AF) at one instance. Synchronized
electrical direct current (DC) cardioversion (150 joules) resulted in
termination of AF and initiation of a regular narrow complex
tachycardia, both at the same instance (Figure 1). Such an occurrence is
rarely reported in literature. What are the plausible explanations
behind this phenomenon?
3 │DISCUSSION
The left side of the tracing (Figure 1) shows variable RR intervals on
the surface electrocardiogram leads with concurrent chaotic atrial
activity in the coronary sinus (CS) electrograms. This is consistent
with AF. Pursuant to the DC shock artifact, there is termination of AF
as evidenced by a brief pause of 390 ms. This is followed by resumption
of the same AVNRT (similar CL of 360 ms and ‘A on V’ sequence). The
underlying mechanism is not precisely clear but can be speculated as
enunciated below.
The prerequisite for initiation of a reentrant tachycardia such as AVNRT
is the difference in the refractory periods of the dual limbs of the
circuit with unidirectional conduction block in one limb and slow
propagation in the other. The degree of perturbation of such a circuit
is directly proportional to the duration of the excitable gap. The
latter is a measure of the relative refractory period between the head
and tail end of the electrical wave front propagating within the
circuit. With this predisposing construct already in existence in this
patient, a superimposed DC shock can be postulated to convert AF to
AVNRT in one of the following ways.
In the initial scenario, termination of AF may be immediately followed
by the first beat of typical slow-fast AVNRT. The implication is that
the critically-timed DC shock stimulus propagated through the limbs of
the circuit but created selective and conducive differential
refractoriness within them. Thus, there is antegrade conduction in the
slow pathway but block in the fast pathway. The AVNRT ensues as the
conduction from the slow pathway retrogradely enters the fast pathway
following its recovery from refractory period.
Alternatively, it is plausible that a shock-induced accelerated
junctional beat follows the termination of AF and initiates AVNRT.
Initiation of typical AVNRT is a well understood electrophysiological
phenomenon predicated on the same construct as detailed above. Given the
fact that there was irregular ventricular conduction during AF, it is
unlikely that there was dual tachycardia (AF and AVNRT) and that the
shock simply unmasked the AVNRT.
This seemingly rare phenomenon is reported only once before but without
any mechanistic explanation.3 The authors anticipate
that this report would engender a nuanced deliberation regarding this
phenomenon.
References
- Veenhuyzen GD, Quinn FR, Wilton SB, Clegg R, Mitchell LB. Diagnostic
pacing maneuvers for supraventricular tachycardia: part
1. Pacing Clin Electrophysiol . 2011;34(6):767-782.
doi:10.1111/j.1540-8159.2011.03076.x
- Veenhuyzen GD, Quinn FR, Wilton SB, Clegg R, Mitchell LB. Diagnostic
pacing maneuvers for supraventricular tachycardias: part
2. Pacing Clin Electrophysiol . 2012;35(6):757-769.
doi:10.1111/j.1540-8159.2012.03352.x
- Nair KK, Namboodiri N, Priyadarshini A, et al. Electrical
Cardioversion of an Irregular Narrow QRS Tachycardia to a Regular
Narrow QRS Tachycardia. J Cardiovasc Electrophysiol .
2017;28(3):353-354. doi:10.1111/jce.13124
Figure Legends
Figure 1- Three- channel electrocardiogram (ECG) and intracardiac
electrograms from a decapolar catheter placed in the coronary sinus
showing electrical cardioversion (notice the shocking artifact) of an
irregular narrow complex tachycardia to a regular narrow complex ‘A on
V’ tachycardia