Re-entry circuit is the major cause of most types of the tachyarrhythmias.
There should be two adjacent pathways, which have different electrophysiologic properties, and they are connected proximally and distally. One of the pathways has slow conduction and short refractory period; another one has fast conduction and long refractory period. Refractory period is a period when cells cannot be excited.
Impulse is conducted by the fast pathway and it reaches distal connection more quickly than impulse conducted by the slow pathway. As there is some delay in the impulse conduction by the slow pathway, impulse conducted by the fast pathway spreads to the distal part of the slow pathway. When impulses encounter each other, tissues on both sides are in the refractory period. So, impulse cannot be conducted in any side. There is no circulating loop.
But if the impulse arrives to these pathways prematurely, when tissues of the fast pathway are in the refractory period, the fast pathway is not excited. The impulse is spread by the slow pathway, it comes to the distal connection, and also, it spread on the fast pathway. The impulse spreads by fast pathway in two directions: to the proximal connection and to the slow pathway again. In this case it begins to circulate by this re-entry loop, spreading very frequently in both directions. This leads to a number of arrhythmias: atrioventricular re-entrant tachycardia, atrial flutter, ventricular tachycardia occurring due to myocardial scarring. In these cases there is a fixed anatomical pathway, and the mechanism is called macro-re-entry. Also, micro-re-entry should be mentioned. It occurs in cases of atrioventricular nodal re-entrant tachycardia, atrial fibrillation, atrial tachycardia, ventricular fibrillation, ventricular tachycardia. Micro-re-entry doesn’t require the obvious circuit accessory pathways; there may be one or multiple micro-re-entry loops.