Dilated cardiomyopathy (DCM) is a disease affecting the heart muscle. It is the second most common heart disease in dogs (after mitral valve disease). In DCM the heart is unable to contract normally and as the heart muscle stretches the heart gets larger. DCM affects mainly middle-aged large and giant breed dogs and some spaniels. Cats and small breeds of dog are occasionally affected. Male dogs are more likely to be affected than females. Animals with DCM usually develop signs of heart failure and abnormal heart rhythms.