Electrogram Interpretation – 9

Accessory Pathway Block

• This example demonstrates decremental pacing from CS 9,10… pacing faster and faster by small increments to test antegrade conduction from the atrium to the ventricle. 
• First, notice that each pacing artifact does correspond with local capture as seen by an A on the egm. 
• Next, each impulse throughout the electrogram did conduct to the ventricle. 

So, now what??

• Don’t forget to use the surface ECG and observe the activation pattern from each paced A to the V.
• The first two displayed impulses travel from the A to the V via the accessory pathway. Notice the preexcitation (delta wave) on the surface ECG. 
• On the third impulse, the accessory pathway blocks, but the impulse is still able to travel to the ventricle through the AV node. 
• Also of note is the longer AV interval on the last three impulses. Once the accessory pathway “shortcut” blocks, the impulse had to travel solely via the AV node. The conduction time through the AV node is longer as it delays the impulses to the ventricle. Recall, it is the gatekeeper of the heart and slows the impulses or decrements to protect the ventricles from rapid depolarization such as in AF. 

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For more information:  EP Essentials – Understanding EP: A Comprehensive Approach – sections on pacing and AVRT.