Arrhythmias can be mechanistically separated into those due to abnormal impulse generation and those due to abnormal impulse conduction. The mechanisms of arrhythmias may be broken down into three categories: reentry, automaticity, and triggered arrhythmias.
The reentry mechanism of arrhythmias has been well defined by George Ralph Mines, who hypothesized that bidirectional conduction with unidirectional block leads to reentry. This is very common in patients who have ventricular tachycardia with a scar in which there is slow conduction in one area, allowing for recovery with the ability for conduction to travel up another area, thus creating a circus movement tachycardia. Reentry is likely the mechanism responsible for the majority of clinically significant tachycardias. For a reentrant tachycardia to exist, several conditions need to be met: 1) there must be a zone of tissue that conducts slowly; 2) unidirectional block must occur in part of the tissue involved in the reentrant circuit; and 3) the conduction velocity must be slow enough to allow the blocked tissue to recover.