How to be sure its ectopic and not reentry?

• what is the mechanism of ectopic tachycardia
• how to distinguished it from reentry mechanism
• what pacing maneuvers are most reliable and easy to conduct

Ectopy atrial tachycardia can be caused by two different mechanism: triggered and automaticity. Automaticity results from spontaneous depolarization at the end of the action potential. This is a physiological feature of automatic cells such as the SN, AV node, His bundle and Purkinje cells, however in other cells it is considered to be abnormal.  Triggered activity has some features of both automaticity. Simular to automaticity, it involves increase of action potential in late phase 3 or early phase 4 (so called afterdepolarization) that can initiate action potential. However, unlike automaticity, triggered activity is not always spontaneous and it can be provoked by premature beats. Thus, like reentry, it can be induced with pacing.
You can distinguish ectopy from the reentry already before conducting EP study by looking into ECG:

• isoelectric line between P waves is characteristic for ectopy while reentry produce continues “flutter” waves
• the CL of ectopy vary frequently (worming up at the beginning and cooling down at the end) as the reentry CL remains more or less stable
• prolonged and sustained episodes of tachycardia are highly suggestive of reentry because ectopy is frequently restricted to individual premature beats or short minutes lasting arrhythmias
• termination of the tachycardia after administration of adenosine indicates an ectopic character (or an AV node involving reentry)
    
  Discrimination between two mechanism is even easier during EP procedure:
• using mapping system, you can visualize typical ectopy pattern of one punctual central activation spreading all directions in opposite to reentry early-meets-late loop pattern
• in case of ectopy total activation time is much shorter than CL (due to relation between P-wave and isoelectric line) while in case of well mapped reentry those numbers should be identical
• during entrainment (or overdrive suppression) the PPI is dependent on pacing CL in case of the ectopy while pacing CL has no influence on PPI in reentry
   
  CAVE: you should try the scenarios multiple times as it will randomly switch from triggered to automatic EAT. With more training you will easily recognize the difference.