Activation Mapping in 2D & 3D for Focal Arrhythmias

Answer: This rhythm is a long RP tachcyardia. This makes it likely Atypical AVJRT, or Focal Atrial Tachycardia arising from the CS os region given the proximal CS EGM is >20ms pre P wave onset.

Both of these arrhythmias would demonstrate early activation in the posteroseptal region of the RA. This rhythm was actually a CS os Focal AT which was successfully mapped with 3D mapping, & targeting for ablation.

 

Understanding Activation Mapping

• Activation mapping identifies the “earliest” atrial EGM preceding P wave onset. It involves moving the ablation catheter to different anatomical sites during tachycardia to find the earliest atrial signal relative to P wave onset.

EGM characteristics of the Focal site of origin:

• EGM occurs 15-45 ms before P wave onset. (You should reference every EGM of interest during activation mapping to your P wave onset).

•  The target EGM should precede all other atrial EGMs, indicating the focal source of tachycardia.

• Unipolar QS wave

• Bipolar fractionation (not always present)

Examples Successful & Unsuccessful Sites of Ablation for Focal AT

Why should the EGM at the Focal Origin of Tachycardia precede P wave onset?

ECGs are insensitive for detecting cardiac voltage. The P wave onset occurs only after enough myocardial mass has depolarised to generate sufficient voltage. Thus, P wave onset appears AFTER the cells at the focal AT origin have spontaneously depolarised.

EP catheters are more sensitive, with closely spaced electrodes detecting small amounts of electrical activity. When placed at the tachycardia origin, the catheter can capture atrial depolarisation before P wave onset, even when minimal atrial muscle mass is recruited.

A key point to note, is that if the atrial EGM on the mapping catheter occurs after P wave onset, that site is not the focal origin.

• 3D mapping systems add value to these procedures by colour coding locations where early and late EGM’s were found.
  • White” or “Red” areas indicate the earliest mapped EGM’s so far.
  • Orange areas contain EGM’s that were not quite as early as red, but earlier than areas colour coded yellow, and Purple areas contain the latest mapped EGM’s thus far.

• This colour coding system produces a visually digestible map, where “early” anatomical regions are clearly demarcated as regions of interest by their red/white colouring and allowing you to visually track how the chamber is depolarising.

3D Activation Mapping of Focal AT for our example patient

There are no good colours, only good signals

• One thing to note, is that red areas ONLY represent the earliest EGM’s mapped “SO FAR” & may not represent the region of tachycardia origin.

• The only truth is the morphology and timing of the individual EGMs mapped. Just because a 3D mapping system has colour coded it as red or white, doesn’t mean it is the origin of tachycardia.

My Two Cents:

• Activation mapping Focal AT involves comparing EGM’s at different anatomical sites to the P wave onset during Focal AT.
• 3D mapping is simply a visual representation of 2D mapping principles, producing a visual representation of how the chamber is depolarising.
• Successful sites for ablation should ideally contain the earliest atrial EGM, precede P wave onset by 15-45ms & display a unipolar QS morpgology.
• The site of the proximal CS electrode with an EGM occurring >20ms pre P wave onset during tachycardia in the example above represents a good site of interest to investigate for the origin of tachycardia.

Thanks for tuning in :)
Cheers
Mitch & CPP Team

Activation mapping lectures for Focal AT can be found in our EP in Practice education course