Hypertrophic cardiomyopathy, often called HCM, is a form of heart disease characterized by enlargement and thickening of the walls of one or more heart chambers. As the walls thicken, they become stiff and less flexible, interfering with the flow of blood entering and being pumped out of the heart. As a result, fluid builds up in and around the heart and lungs. In severe cases, the blood becomes sluggish and stagnant; sluggish blood can’t transport oxygen and other nutrients throughout the body efficiently. What causes HCM isn’t known, but experts suspect a genetic component. Some dogs with HCM don’t act sick. Others have difficulty breathing, pant, cough, vomit, lose their appetite, lose weight, become lame, listless, tired, weak and depressed, collapse or even unexpectedly drop dead. Owners should familiarize themselves with these signs so they can get their dog to a veterinarian as quickly as possible. Fortunately, HCM is fairly uncommon in dogs.