Let’s combine looking at 3D images along with the corresponding electrograms to determine entrance block.
This 3D geometry was created for a pulmonary vein isolation procedure for atrial fibrillation. We are viewing the posterior wall of the left atrium. The four protusions are the pulmonary veins (blue – LSPV, brown – LIPV, gray – RSPV, green – RIPV) and the pink appendage slightly showing (pink) which is an anterior structure.
As you recall, the pulmonary veins are often a trigger for atrial fibrillation. If a line of block is created outside the veins, then the impulse is unable to propagate from the PV to the LA and cause AF.
Let’s take a closer look at a few different examples.
Here is a pre and post electrogram acquired from a circular mapping catheter placed in the LSPV (left superior pulmonary vein). The patient is in sinus rhythm and the physician is pacing the distal CS (close to the vein) to observe entrance block.
Notice in the pre electrogram, signal can travel from the CS into the pulmonary vein (labeled A). After RF ablation, the signal may no longer enter the pulmonary vein. We will observe the same thing on the next 3 images of the LIPV, RSPV, and RIPV.
On the next two examples, pacing is occuring closer to the veins via the proximal CS.
On this patient, entrance block was observed on all four pulmonary veins. If the AF trigger is from one of the veins, then it will not be able to travel to the atrium and cause atrial fibrillation.
Ideally, exit block would also like to be seen by either pacing the pulmonary veins or observing spontaneous PV firing that does not escape into the atrium due to the ablation line created.
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