Responses to Ventricular Overdrive Pacing during Wide QRS
Tachycardia: What is the Mechanism?
Wentao Gu MD1, Xinping Luo MD1, Jian
Li MD1, Jinjin Zhang MD1, Nanqing
Xiong MD1*
1. Department of Cardiology, Huashan Hospital Fudan University, 12
Wulumuqizhong Road, Jing’an District, Shanghai 200040, China
*Corresponding author: Nanqing Xiong MD, Department of Cardiology,
Huashan Hospital Fudan University, 12 Wulumuqizhong Road, Jing’an
District, Shanghai 200040, China. E-mail:
huashancardio@163.com
Key words: wide QRS complex tachycardia, ventricular overdrive pacing,
supraventricular tachycardia, AV reentrant tachycardia, orthodromic His
activation
Funding: (None)
A 35-year-old female presenting with recurrent palpitation was referred
for electrophysiology study. Short episodes of narrow QRS tachycardia
could be recorded on ECG (Figure 1). During electrophysiology study, the
tachycardia could be readily induced with atrial pacing or programmed
atrial extrastimuli, but usually terminated in seconds, making it
difficult to perform diagnostic maneuvers. When it spontaneously changed
to a wide QRS tachycardia, a burst of ventricular overdrive pacing (VOD)
was delivered which turned it back to the narrow complex one (Figure 2).
What can be learned from the response?
The first 4 beats in this tracing showed wide complex tachycardia (WCT)
with right bundle branch block (RBBB) morphology and 1:1
ventriculo-atrial ratio. Candidates for diagnosis could be ventricular
tachycardia, supraventricular tachycardia (SVT) with RBBB, bundle branch
reentrant tachycardia and preexcited tachycardia. An H-V interval of
53ms approximating that during sinus rhythm excluded ventricular
tachycardia from myocardium and preexcited tachycardia. The last 4 beats
were clinical narrow complex tachycardia with the same cycle length, H-V
interval and atrial activation sequence (earliest A at CS7-8) as the
WCT, highly indicating that the WCT was SVT with functional RBBB caused
by continuous concealed activation from left bundle branch before VOD
peeled back the refractoriness of the right bundle branch in the
following beats. In addition, A right-sided accessory pathway (AP) was
unlikely given the same H-A interval (127ms, measured to CS7-8) with and
without RBBB [1].
The middle 4 beats demonstrated progressive change in QRS, from fusion
to probable fully-paced morphology during VOD. The 3rdbeat advanced the subsequent A without atrial activation change,
indicating the presence of a septal AP. However, post-pacing interval
(PPI) was 125ms over tachycardia cycle length (TCL) with V-A-H-V
response, which argued against AV reentrant tachycardia (AVRT) utilizing
a septal AP. Note that His signal was found after the
3rd and the 4th stimulating
artifact. Considering the extremely short interval between the
3rd artifact and the subsequent His (17ms), it could
hardly be a retrograde His, but was activated in an orthodromic
direction instead. A progressively increased A-H interval after the
2nd, 3rd and 4thpacing suggested decremental conduction in AV node before orthodromic
His capture. When atrium was entrained, anterograde conduction to the
His bundle during VOD supported the diagnosis of AVRT and excluded AV
nodal reentry [2]. The long PPI was associated
with the delay of the first return His-V following VOD-induced
decremental AV nodal conduction. The corrected PPI-TCL was 78ms after
subtracting the difference between the first return A-H and tachycardia
A-H from PPI [3]. The AP was successfully ablated
at left posterior septum, which rendered the tachycardia non-inducible.