Re-entry Circuits

Junctional tachycardias are those arising from the junction between the atria and ventricles, including the AV node, and are due to re-entry circuits.

A ‘re-entry circuit’ occurs when they are two separate conducting pathways – the normal ‘fast’ conducting pathway, plus another area of conducting tissue allowing impulses to pass only at a slower rate – the ‘slow’ pathway.

Typically, the slow pathway has a shorter refractory period meaning impulses from the atria travel down both pathways but those from the slower pathway meet refractory tissue within the fast pathway and stop.

Animation of ‘slow’ & ‘fast’ pathways, without development of a re-entry circuit

In some circumstances, such as a well-timed atrial premature beat, antegrade transmission travels down the slow pathway, which has already recovered first, and then can pass back up the fast pathway as it is no longer refractory. It will then pass again down the slow pathway to form a loop, which will now continue in the same direction. This circular ‘re-entry’ pathway causes increased depolarisation of the ventricles, resulting in a tachycardia.

Animation of the development of a re-entry circuit