Most common cause of arrhythmia is -

Ischemic injury is the most common cause of rhythm disorders, because of direct damage or due to the dilation of hea chambers with consequent alteration in conduction system firing. Far less common are inherited causes of arrhythmias. These are caused by mutations in genes that regulate various ion channels that regulate depolarization and repolarization of myocardial cells. Such channelopathies are impoant (but founately uncommon) substrates for fatal arrhythmias. They underlie some cases of sudden cardiac death. Ref : Harrisons 20th edition pg 1443