An Isoproterenol Dependent Broad QRS Tachycardia: What is the
Mechanism?
Weizhuo Liu MD1, Nanqing Xiong MD2*,
Wentao Gu MD2
1. Department of Critical Care Medicine, Shanghai Chest Hospital,
Shanghai Jiaotong University, 241 West Huaihai Road, Shanghai 200030,
China
2. Department of Cardiology, Huashan Hospital Fudan University, 12
Wulumuqizhong Road, Shanghai 200040, China
*Corresponding author: Nanqing Xiong MD, Department of Cardiology,
Huashan Hospital Fudan University, 12 Wulumuqizhong Road, Shanghai
200040, China. E-mail:
huashancardio@163.com
Key words: wide QRS complex tachycardia, intermittent preexcitation,
Wolff-Parkinson-White syndrome, antidromic AV reentrant tachycardia,
AV-nodal-refractory atrial extrastimulus
This research did not receive any specific grant from funding agencies
in the public, commercial, or not-for-profit sectors.
A 27-year-old female presenting with recurrent palpitation during
exercise was referred for electrophysiology study. Electrocardiogram
(ECG) during symptom was not documented. Her baseline ECG was sinus
rhythm with normal PR interval. Ventricular pacing and programmed
extrastimulus showed retrograde conduction via AV node. A-H interval was
progressively prolonged during atrial extrastimuli without preexcitation
(Figure 1). No tachycardia was inducible until isoproterenol infusion,
after which a wide QRS tachycardia was initiated by ventricular burst
pacing. Figure 2 showed 2 intracardiac recordings during tachycardia.
What can be learned from the tracings?
Figure 2A showed a wide QRS complex tachycardia with left bundle branch
block morphology, left-inferior frontal axis and 1:1 VA relationship,
the cycle length of which was about 365ms. There was no His electrogram
preceding each QRS, instead, a putative retrograde His could be observed
after local ventricular electrogram on His catheter with an H-A interval
around 100ms. Differential diagnosis included ventricular tachycardia
(VT) from right ventricular outflow tract and different types of
preexcited tachycardia. A sensed atrial extrastimulus was delivered when
AV node was refractory, indicated by the unchanged timing of atrial
signal on His catheter, which advanced the next QRS with identical
morphology to tachycardia. This phenomenon suggested the presence of an
anterograde conducting accessory pathway. Measurement of A-A interval
indicated the tachycardia was reset, providing the evidence that the AP
was participating in the reentry [1]. VT can be
ruled out by the unchanged QRS morphology during sensed atrial
extrastimulus and entrainment from atrium, while preexcited AV nodal
reentry and nodoventricular tachycardia was precluded by the resetting
of tachycardia with AV-nodal-refractory atrial extrastimulus. The fact
that atrial extrastimulus with earlier prematurity terminated the
tachycardia without conduction to the ventricle (Supplemental Figure)
also argued against AVNRT with innocent bypass tract, which in this
setting would require the timing of the extrastimulus to exactly
encounter the refractory period of the pathway when terminating AV nodal
reentry. Furthermore, the tachycardia could also terminate spontaneously
with an atrial signal, making preexcited atrial tachycardia highly
unlikely (Figure 2B). Therefore, the diagnosis of antidromic
atrioventricular reentrant tachycardia (AVRT) was made based on all the
evidence above.
But how could the patient have antidromic AVRT whose preexcitation was
absent at baseline, during atrial extrastimuli, and pacing from both
atria (not shown)? If we look into the sinus QRS configuration after
tachycardia cessation in Figure 2B, prominent delta waves could be
observed, which emerged only during isoproterenol infusion in this case.
In Wolff-Parkinson-White syndrome, intermittent anterograde pathway
conduction dependent on isoproterenol, although not common, has been
reported [2]. The pathway conduction was
unsustainable when isoproterenol was discontinued, proving the
unidirectional conducting accessory pathway was highly
isoproterenol-sensitive, which served as the anterograde limb of the
antidromic AVRT. On the other hand, isoproterenol also facilitated the
retrograde conduction of AV node, which played the role of the
retrograde limb. The pathway was then ablated near 12 o’clock site of
tricuspid annulus, after which no delta wave was present with
isoproterenol infusion, and the tachycardia was therefore not inducible.