Double Potentials & Fractionation in Flutter – Why Care?

Answer: This EGM indicates that counterclockwise block has likely been achieved.

It initially demonstrates a narrowly spaced double potential on the ABL 1,2 EGM, followed by a suddenly widely spaced double potential (3rd beat). The fluro image indicates that the ablation catheter is on the CTI region.

Widely spaced double potentials where the combined Stim-Potential measurements exceed 90% of the AFL cycle length has a 100% specificity for denoting CTI block. How to measure this will be explained below.

 

Specifically in the context of CTI ablation (although the principal holds in other areas), double potentials observed during HALO or CS pacing indicate a line of partial or complete block. The more widely spaced the double potentials, the more likely they denote a true line of block, rather than just slow conduction.

Widely spaced double potentials generally have >90ms isoelectric period between each Potential/EGM. The above diagram describes why this is the case. If the ablation catheter is located on a truly blocked CTI line, then it will detect lateral CTI depolarization shortly after distal Halo pacing stimulation (D1).

After a prolonged conduction time, the ablation electrode will eventually detect medial CTI depolarization resulting in another EGM (D2) occurring >90ms later than D1.

Double Potentials indicating failed CTI Block

• Partial Split with <90ms of isoelectric period between double potentials often indicate an absence of CTI block, indicating that there is a “leak” of CTI conduction.

• When walking the catheter along the CTI line, you may notice a convergence of the double potentials into a prolonged continuous fractionated EGM. This usually indicates a “Gap” in the ablation line. This may represent a target site for ablation.

• Keep in mind that electrode size and spacing may not offer sufficient resolution to dectect fractionated EGM’s in all cases, and sometimes fractionation can be present due to diseased tissue, rather than due to a “Gap” in CTI ablation line

“Must Know” Double Potential measurments indicating CTI Block

Rhee et al. validated an objective measurement of double potentials for determining CTI Block:

Essentially SD1 + SD2 >90% of AFL CL indicates CTI block.

• SD1 is the measurement from Prox CS pacing stimulus to the first double potential.
• SD2 is the measurement from Prox CS pacing stimulus to the second double potential.

The summation of these times should = 90% of the AFL Cycle length. This has a 94% sensitivity & 100% specificity for CTI block.

My Two cents:

There is far more to flutter ablation than simply measuring trans-isthmus time and differential pacing. The morphology & spacing between double potentials can indicate CTI block, conduction & potential target sites for ablation.

• Double potentials offer objective indications of CTI block. During CS or Halo Pacing, the summation of Stim-D1 + Stim-D2 measurements should be > 90% of AFL cycle length. This indicates CTI block.
• Partially split potentials indicate an absence of CTI block, with continuous fractionation potentially indicating CTI “gap” & potential target sites for ablation.

Active observation & assessment of these potentials throughout the case can add value & insight during the procedure.

Cheers
Mitch & CPP Team

Structured & in-depth learning on double potentials & measurements relevant in Flutter ablations can be found in our newly launched Program 3 of EP in Practice: Atrial Flutter!

• Rhee KS, Kwon KS, Lee SH, Lee KH, Lee SR, Chae JK, Kim WH, Ko JK, Nam GB, Choi KJ, Kim YH. Simple method of counterclockwise isthmus conduction block by comparing double potentials and flutter cycle length. Korean Circ J. 2009 Dec;39(12):525-31.
• Christopoulos G, Siontis KC, Kucuk U, Asirvatham SJ. Cavotricuspid isthmus ablation for atrial flutter: Anatomic challenges and troubleshooting. HeartRhythm Case Rep. 2020 Mar 16;6(3):115-120.
• https://www.semanticscholar.org/paper/Septal-Accessory-Pathway%3A-Anatomy%2C-Causes-for-and-Macedo-Patel/e27c794bd0fb33c45e259f352cdcbf3a028c9f6f/figure/1