A single beat generated from a focus outside the SA node but within the conduction system is known as an ectopic beat. Ectopic beats can either be atrial or ventricular in origin. An ectopic beat arising from the atria is known as an atrial ectopic beat. 

Ectopic beats arise from irritability in the conduction system that is strong enough to override the SA node. They can also be caused by an impulse from an escape mechanism. The two types of ectopic beats can be distinguished from one another. A prolonged R-R cycle followed by an ectopic beat is usually due to an escape beat. An early premature beat is usually caused by an irritable focus.