Here is a concise overview of HCM taken from a talk by Dr. Katheryn Meurs:

"Hypertrophic cardiomyopathy (HCM) is a primary heart muscle disease and is the most common heart disease in cats. It usually results in a thickening of the heart muscle, generally in the left ventricle and often leads to an increase in blood pressure in the upper chambers of the heart and causes a backward flow of blood into the lungs. The symptoms cat owners notice include difficulty in breathing (open-mouth breathing) or heart failure. Sometimes blood clots result, often moving to the back legs and causing paralysis (saddle thrombosis)....

HCM is often inherited or susceptibility to the disease is “genetically programmed” to develop in a specific individual. It is, however, not always inherited. At this time, we believe there are inherited forms of HCM in Maine Coon, Devon Rex, Ragdoll, American Shorthair, and British Shorthair cats...."