This week’s electrogram interpretation:
What is observed throughout the following electrogram?
Two quadripolar catheters are being utilized. One is locate near the His bundle (although no H is observed), and the second is in the RV.
*Bonus – Are we pacing from the RVOT or RV apex?
Answer
Fusion Beat – NOT VERP
- Extrastimulus testing is being performed from the RVOT. Notice the negative QRS in V1 (LBBB Morphology) which indicates that pacing is originating in the right ventricle. Next, is to look at the inferior leads. The inferior leads are II, III, and aVF with the positive pole at the foot. Here, aVF is displayed and has a positive QRS indicated the wavefront is traveling towards the positive pole at the foot… high to low. However, in the following illustrations, RV apex pacing is displayed for ease.
Let’s break this down one impulse at a time…
- The first displayed impulse demonstrates ventricular pacing and conduction to the atrium. There is a wide QRS morphology with V pacing with the A to follow.
- The last displayed impulse shows a pacing artifact that did not capture the ventricular tissue as it was still refractory from the prior impulse.
- The sinus node fired, and the impulse conducted to the ventricles a relatively narrow QRS complex.
- The 3rd displayed beat is a fusion beat. This is a fusion of ventricular-paced morphology and sinus rhythm morphology. Notice how the QRS complex is a blend between the two morphologies.
- This happened because the sinus node fired (A on His channel) and began conducting to the ventricle. As this conduction is occurring, the ventricle is paced and begins depolarizing the ventricle.
- This impulse is a blend of the two.
- Now putting it together again, let’s walk through the electrogram.
- Impulse 1 – captured the V & conducted to the A.
- Impulse 2 – captured the V & was conducting to the A, but the SA node fired which is why the A appears to be earlier than the prior impulse. However, the SA node did not fire early enough for conduction the ventricle which would alter the QRS morphology.
- Impulse 3 – the SA node now fired before the RV was paced, depolarization of the ventricle is a fusion of the antegrade and retrograde conduction.
- Impulse 4 – the SA node fired, and the pacing impulse did not capture the ventricle; therefore, the antegrade conduction (from A to V) caused the depolarization of the ventricle creating a relatively narrow QRS complex.
- NOT VERP – since the ventricle did not capture on the extrastimulus (S2) and the SA node was competing with the ventricular pacing, we are unable to interpret this as VERP. The pacing run must be repeated, most likely at a faster drive train rate (55o or 500 ms).
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If you like this, find hundreds of electrograms to interpret in our Understanding EP workbook – included in the Understanding EP book set.
For more information: EP Essentials – Understanding EP: A Comprehensive Approach
