Where should I look?

• where are EATs typically located
• how to distinguish the origin of EAT
• what are anatomy related pitfalls during ablation

Distinguishing of ectopy from other arrhythmias might be challenging but using a step by step approach will help. Start by looking at surface ECG:

• isoelectric line between P waves suggesting an ectopic character of the arrhythmia (as opposed to continues “flutter” wave of atrial reentry)
• typical findings are acceleration and deceleration of EAT at the beginning and the end of episode (as opposed to atrial reentry with rather stable CL)
• adenosine testing showing ongoing tachycardia despite AV block excludes reentry involving AV node and helps assess the isoelectric line between P waves
   
  The origins of EAT are distributed quite equally with 1/3 coming from RA, 1/3 from midline region and 1/3 from LA.  Using CS catheter for defining EAT origin is quite straight forward as activation from proximal to distal indicates RA or midline origin. LA origin is characterized by distal to proximal activation. The most dominate sites are:
• RA: Crista terminalis (located in posterior-lateral wall) and regions of the tricuspid anulus. You can distinguish those two origins by looking at inferior leads (II, III, aVF) with Crista giving positive P-waves and anulus negative ones.
• Midline: common EAT originate from CS ostium and parahisian region.
• LA: most frequent location are pulmonary veins as well as appendage and mitral anulus. The left sided origin can be suspected when P wave is strongly positive.
   
  In order to avoid complication during ablation a special consideration regarding adjacent structures must be given. Region of crista terminalis might be in close vicinity of right phrenic nerve. Moreover, catheter instability in this region might cause ablation close to sinus node.  Septal EAT are frequently located dangerously to conduction system and special attention must be put into allocation of his potentials.