Contact: ojas.mehta@gmail.com
Background
A 78 year old lady sought medical attention for palpitations with
documented regular wide complex tachycardia with a left bundle branch
block (figure 1) as well as atrial fibrillation. She presented for a
catheter ablation for atrial fibrillation and presumed CTI-dependent
atrial flutter. As a part of the procedure, we performed an
electrophysiology study.
We performed pacing manoeuvres to evaluate for the presence of a
concealed accessory pathway. She had an underlying left bundle branch
block observed in sinus rhythm. Results of para-Hisian pacing maneuver
are displayed in figure 2 with intracardiac electrograms and
corresponding 12-lead ECG. Para-Hisian pacing is performed at a constant
pacing interval through the distal electrode of the His bundle catheter.
There is a decapolar catheter in the coronary sinus with the proximal
electrode positioned at the Os.
Here are some questions to consider:
There are 4 different paced morphologies observed. What does each one
of these represent?
How can we interpret the findings of this maneuver? What role does
pre-existing left bundle branch block play in our interpretation?
Considerations
In this example, para-Hisian pacing is performed through the distal
electrode of the ablation catheter. The retrograde atrial activation
sequence is eccentric and identical in all beats. The shortest V-A
interval, to the earliest atrial signal is constant in all beats
displayed. However, the paced QRS morphologies are different. There are
four differing QRS morphologies with ventricular pacing via the distal
His electrode. Paced beats 1, 2, 3 and 5 have different morphologies
while paced beats 3 and 4 are identical. The Stim-QRS interval and the
initial component of the QRS complex varies between the beats.
Depolarisation of ventricular myocardium via the conduction system
results in rapid initial deflections of the QRS due to the rapid
conduction velocity, direction of activation and mass of myocardium
activated concurrently. Direct capture of ventricular myocardium results
in shorter Stim-QRS intervals with a slurred onset representing slower
cell to cell conduction. Typically, the para-Hisian pacing maneuver
involves capture of the His bundle and the ventricular myocardium,
comparing this to a situation when there is only ventricular myocardial
capture (without His bundle capture). When this occurs, the pattern of
atrial activation is evaluated and the V-A interval is compared. Changes
in atrial activation pattern suggest more than one pathway of electrical
conduction from ventricle to atrium. Changes in V-A interval suggests
conduction from ventricle to atrium is reliant on the AV node and
conduction system (1).
A typical nodal response, with retrograde conduction exclusively through
the AV node, is one where the retrograde atrial activation sequence is
preserved and the VA interval is longer in the absence of His bundle or
conduction system capture, than it is with His bundle or conduction
system capture.
When performing this manoeuvre it is important to confirm what is being
captured by the pacing electrode. Below is our explanation of the four
paced QRS morphologies:
- QRS 1: Capture of His bundle only
- QRS 2: Capture of His bundle only and correction of left bundle branch
block
- QRS 3 and 4: Capture of His bundle and local ventricular myocardium
- QRS 5: Capture of His bundle with correction of left bundle branch
block and capture of local ventricular myocardium
Our His catheter shows evidence of local ventricular myocardial signals
which are seen in the initial 2 beats, as distinct from the pacing
stimulus. This occurs as the local ventricular myocardium has not been
captured by the pacing stimulus.
Interpretation
Given the above explanations, we can then interpret the findings. In
this instance we have conduction system capture in all beats. Therefore,
this response is in fact nodal as there is no change in the VA interval.
The presence of a septal accessory pathway would allow for quicker
activation of the atrium, or shorter VA interval, when there was direct
ventricular myocardial capture alongside His bundle capture. We would
also expect a shift in retrograde atrial activation sequence.
Additionally, overcoming typical left bundle branch block with His
bundle capture is a phenomenon that has been observed with His bundle
pacing (2). There are multiple mechanisms proposed to explain this with
the seminal concept that fibers within the His bundle itself are
predestined for the left or right bundle branch (3). Therefore, bundle
branch block can be overcome by pacing distal to the block within the
His bundle, or by using pacing outputs to improve the source-sink
mismatch that would otherwise render the left bundle fibres inactive
(4).
Conclusion
This case highlights an example of attempted para-Hisian pacing with
responses contrary to what is typically described due to the capture of
tissue in an unusual manner. Therefore, it is in fact exactly what is to
be observed with conduction through the atrioventricular node, despite
the apparent eccentric retrograde atrial activation sequence.
Disclosure
The authors of this manuscript have no conflicts of interest to
disclose.
References
1. Ali H, Foresti S, Lupo P, De Ambroggi G, Mantovani R, De Lucia C, et
al. Para-Hisian Pacing: New Insights of an Old Pacing Maneuver. JACC
Clin Electrophysiol. 2019;5(11):1233-52.
2. Huang W, Su L, Wu S, Xu L, Xiao F, Zhou X, et al. Long-term outcomes
of His bundle pacing in patients with heart failure with left bundle
branch block. Heart. 2019;105(2):137-43.
3. James TN, Sherf L. Fine structure of the His bundle. Circulation.
1971;44(1):9-28.
4. Ali N, Keene D, Arnold A, Shun-Shin M, Whinnett ZI, Afzal Sohaib SM.
His Bundle Pacing: A New Frontier in the Treatment of Heart Failure.
Arrhythm Electrophysiol Rev. 2018;7(2):103-10.
Figures