Arrhythmogenic right ventricular cardiomyopathy, or ARVC, is a type of cardiomyopathy that is characterised by the replacement of heart muscle with fatty and fibrous tissue. It is a type of cardiomyopathy that affects the ventricles (the lower pumping chambers) of the heart and causes arrhythmias (abnormal heart rhythms). It affects the right ventricle, and often also affects the left ventricle. For this reason it is sometimes called arrhythmic cardiomyopathy (as the main symptoms are arrhythmias). It doesn't affect the atria (upper chambers) of the heart.
It is caused by a defect in the proteins that join the cells of the heart muscle together. This means that the proteins do not develop properly and cannot keep the muscle cells together. When this happens the muscle cells detach and die and are replaced with fibrous scar tissue and fat deposits. This affects the structure of the heart muscle, and it becomes thin and stretched, causing arrhythmias and preventing the heart from pumping blood properly.