Are we done? AKA How to verify the block?

• how to be sure the CTI is blocked
• where to look for a gap
• tips and tricks for catheter movement

Confirming complete block across the CTI is usually straightforward and can be documented in various ways. It is advisable to pick one method that works best for you and your procedural setting, but you should also be familiar with others to find confirmation if something is off.

• The differential pacing or A1/A2 method is usually used in a setting with a CS-catheter in place. This technique is done by pacing from the proximal CS pole at a steady rate (e.g., CL of 600ms) and measuring the distance from the stimulus artifact to the local atrial potential on both sides of the ablation line. The meaning of this is quite straightforward: the impulse (propagation) travels from the CS to the medial ablation site without any obstacle and gives a very short A1 of approximately 5-30ms. In the case of CTI block, it propagates through the roof of the RA (in some cases through the posterior wall) and then reaches the lateral side with a long delay of at least 110ms. It is crucial to bear in mind that only the difference (A2 minus A1) is important and needs to be longer than 110ms. Using this method, a bidirectional block can also be documented. To do that, simply reverse the pacing and measurement side: pace from the lateral ablation catheter and measure A2 on the CS catheter. To make your measurement more “bulletproof,” you can also make a second A2 measurement more superior to the first one. If you notice a shortening of the time interval, then you have confirmed the block as by going higher you are moving towards the propagation wavefront.
• Wide double potentials are also a good sign of the block. As described in a previous lesson, double potentials along the ablation line with spacing of more than 110ms are a good indicator of complete block. The best method to do this is by using CS pacing to provide stable propagation from one side of the ablation line. In an optimal setting, you should be able to see a reverse polarity of those two signals.
• Mapping systems also provide a possibility to verify the block by color-coding the activation around (or through CTI in the case of incomplete block). However, this method requires a high-resolution map to be effective.
• We cannot forget about verification with a multipolar Halo-catheter; that’s why the whole next lesson covers this topic.