Cardiac arrhythmia is the disturbance of the regular rhythm of the heart (1). The heart's electrical system performs two functions, the initiation and conduction of impulses. Specialized cardiac tissue possesses the ability to initiate impulses (spontaneous depolarization), which is the pillar of automaticity. This tissue, such as the sinoatrial node (SA Node) determines when the heart beats, as well as its frequency.
Arrhythmia occurs when tissues become automatic that do not demonstrate automaticity under normal conditions, and can result in tachycardia or bradycardia. If this affects the atria, then it is referred to as atrial arrhythmia, whereas a pathology in the ventricles results in ventricular arrhythmia. Both of these are usually manifested as tachycardia.
One common cause of arrhythmia is reentry, in which there is a restimulation of cardiac muscle due to conduction abnormalities that are usually circular and result in reexcitation. This is a common cause of tachycardia, though it never causes bradycardia. Reentry requires several conditions to exist. There must be a zone of tissue that conducts slowly, which allows previously activated tissue to recover and become reexcited by the time the slowly-conducted signal reaches. In order for this to occur, there must also be a unidirectional block in the part of the tissue involved in the reentrant circuit.
Another potential cause of arrhythmia is triggered activity. Triggered activity is the abnormality of impulse initiation due to ionic currents that produce abnormal action potentials. This causes (triggers) depolarizations after depolarization, also known as afterdepolarization. One way to achieve this is with a long QT syndrome, in which the repolarization happens over a longer period of time. If the repolarization is over a certain threshold, it may cause a second action potential, which ultimately results in arrhythmia and tachycardia.