This week’s electrogram interpretation:
After ablation of the CTI, the following maneuver was performed. What is observed?
Answer
Lateral-to-Medial Block
The cavotricuspid isthmus (CTI) if ablated for typical right atrial flutter. After ablation, pacing on either side of the line is performed to look for bidirectional block. If the wavefront is able to traverse the line, further ablation is necessary.
In the first example, pacing is occurring on the lateral aspect of the ablation line via RA 1,2. A measurement is made from the pacing artifact to the A on CS 9,10, which is on the medial aspect. The measurement of 160ms correlates with lateral-to-medial block. Here, there was not a pre-ablation measurement to compare.
In the second example, pacing is now occurring from RA 5,6. The measurement to CS 9,10 is now shorter at 140ms. This differential site pacing helps to determine if there is indeed lateral-to-medial block or just delayed activation. Pacing further up the catheter (ie. RA 9,10), would elicit an even shorter measurement.
Pacing will then also be performed from CS 9,10 and measured to the closet channel on the lateral aspect of the line (RA 1,2) to observe medial-to-lateral block as well.
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For more information: EP Essentials – Understanding EP: A Comprehensive Approach – section on atrial flutter ablaiton.
