This week’s electrogram interpretation:
What is observed with the third impulse in the following electrogram?
The PV catheter is placed in the LSPV.
Answer
Exit Block
- PVI or pulmonary vein isolation is performed to treat the most common trigger of atrial fibrillation…firing from within the pulmonary vein.
- When ectopy or tachycardia occurs in the pulmonary vein, it may exit into the left atrium. This may trigger atrial fibrillation. (more detail explaination found in Understanding EP: A Comprehensive Guide)
- In this example, the pacing output started off at 15 mV and was then decreased.
- The first 2 impulses capture the pulmonary vein as noted by the pacing artifact and immediate pulmonary vein depolarization is clearly observed on PV 19,20 through PV 7,8.
- These impulses also appear that they have conducted to the atrium, notice the corresponding A on the other catheters beginning with CS 1,2.
- However, on impulse #3 with 5 mV, there is clear PV capture but no corresponding A on the CS…?
- This is exit block. The impulse from the pulmonary vein was unable to travel to the left atrium. PVI of the LSPV was successful and they may proceed with the next vein.
- So what was happening on impulse #1 and #2?
- At the higher amplitude pacing (15 mV and 10 mV), the physician was capturing both the pulmonary vein AND the left atrium. Since the LSPV is nearest CS 1,2, the earliest A in the atrium was on distal CS and ablation (LAA) catheters.
What’s the lesson??
Slow down and look at each impulse and be mindful of the pacing output. In this example, if the mV had not been reduced it would appear that there was no exit block. It may fool the physician into thinking that the vein is not isolated and they may continue with unneeded ablations.
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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
