Cardiac arrhythmias are the abnormalities or perturbations in the normal activation or beating of heart myocardium. The sinus node sends a depolarization wave over the atrium and depolarizing atrioventricular (AV) node propagating over His-Purkinje system and depolarizes ventricle in systematic way. There are many types of cardiac arrhythmias or abnormal heart beating. The normal rhythm of heart is called as sinus rhythm which can be disturbed through failure of automaticity as a sick sinus syndrome (SSS) or as an inappropriate sinus tachycardia.
The mechanisms responsible for cardiac arrhythmias are generally divided into two major categories – enhanced or abnormal impulse generation (i.e., focal activity), and conduction disturbances (i.e., reentry). Focal activity includes enhanced automaticity and triggered activity. Automaticity causes spontaneous generation of APs that do not require induction by previous beats. Healthy myocardium is not normally automatic, but disease conditions (e.g.heart failure, HF) can lead to resting membrane potential depolarization to more positive values causing abnormal automaticity. The most common causes of focal arrhythmias are early afterdepolarizations (EADs) that precede full repolarization (typically corresponding to phase-2 and phase-3 repolarization of the human AP) and delayed afterdepolarizations (DADs) that occur after full repolarization.

References

1.Fu DG. Cell Biochem Biophys. 2015;73(2):291–296.