The following is from theheartbeatclinic.com/tria... - though it is stated elsewhere on the web in less detail.
A second problem that atrial fibrillation causes is a weakening of the heart muscle, called cardiomyopathy. Specifically, if atrial fibrillation causes the ventricles to beat rapidly (tachycardia) for a long time, the muscle of the ventricles may fatigue and weaken. The problem becomes more serious if the patient has other heart diseases. The time it takes to develop tachycardia-induced cardiomyopathy may be several weeks to months. However, it doesn't happen in hours, or usually even days. Even without other heart disease, rapid heart rates may lead to heart failure with these symptoms: shortness of breath, weakness, fatigue, and exercise intolerance (tiring easily with activity). Cardiomyopathy occurs in fewer than one in every five people who have long-term atrial fibrillation. The risk of developing cardiomyopathy, however, stresses the importance of using medication to slow the electrical impulses traveling from the atria to the ventricles.