Others suggest that the main cause of arrhythmias is a difference in the electrical properties between the inside (endocardium) and outside (epicardium) of the heart (known as the repolarisation hypothesis).[1] The shape of the action potential differs between the epicardium and the endocardium. The action potential in cells from the epicardium shows a prominent notch after the initial spike due to a transient inward current. This notch is far less evident in cells from the endocardium, and the difference between the endocardium and epicardium are most clearly seen in the right ventricle.