Restrictive cardiomyopathy (RCM) is a common cause of severe congestive heart failure and death in older cats. RCM is a poorly understood form of feline heart disease that affects the heart muscle. It leads to the replacement of the cardiac muscle with scar tissue and the progressive inability of the heart to effectively pump blood. The prognosis is poor for cats with this type of heart disease and treatment drugs to improve the pumping performance of the heart muscle have not been identified. The hypothesis of this study is that a cytokine, TNFa, causes inflammation and plays a role in the development and progression of feline restrictive cardiomyopathy. Further, that pentoxifylline, which has been successfully used to treat humans with similar conditions, may be a useful method of treating cats. Additionally, it is believed that this study will generate data that will improve general understanding of this type of feline heart disease.