Endomyocardial fibroelastosis is a cause of unexplained childhood cardiac insufficiency. It results from diffuse thickening of the endocardium leading to dilated myocardiopathy in the majority of cases and restrictive myocardiopathy in rare cases. It may occur as a primary disorder or may be secondary to another cardiac malformation, notably aortic stenosis or atresia.

An atrial arrhythmia characterized by disorganized atrial activity without discrete P waves on the surface EKG, but instead by an undulating baseline or more sharply circumscribed atrial deflections of varying amplitude an frequency ranging from 350 to 600 per minute.