This could be scar tissue (e.g. from a ischemic insult, surgery etc) or normal but “different” tissue separated by normal anatomical boundaries – the classic examples being the slow pathway (SP) in AV Nodal re-entry and Cavo Tricuspid Isthmus (CIT) In right atrial flutter.
The critical distinction between the two groups of myocardium should be different conduction properties. Usually this means one group conducts slows and recovers fast whereas the other group conducts fast and recovers slowly. In a normal heart, majority of the tissues are fast conducting and slow recovering. That’s why when there is myocardial injury, slowly conducting tissues form this type of arrhythmia zones- i.e. the scar forms the obstruction (anatomical separation) and the still alive tissue in the scar forms slowly conducting segments. Collectively this facilitates re-entry – especially with premature impluses as described below.
Slowly conducting tissue recovers fast and fast conducting tissue takes time to recover