(NYHA) functional classification. The NYHA functional classes (I–IV) begin with class I, which is defined as a person who experiences no limitation in any activities and has no symptoms from ordinary activities. People with NYHA class II heart failure have slight, mild limitations with everyday activities; the person is comfortable at rest or with mild exertion. With NYHA class III heart failure, a marked limitation occurs with any activity; the person is comfortable only at rest. A person with NYHA class IV heart failure is symptomatic at rest and becomes quite uncomfortable with any physical activity. This score documents the severity of symptoms and can be used to assess response to treatment. While its use is widespread, the NYHA score is not very reproducible and does not reliably predict the walking distance or exercise tolerance on formal testing. In its 2001 guidelines, the American College of Cardiology/American Heart Association working group introduced four stages of heart failure: Stage A: People at high risk for developing HF in the future, but no functional or structural heart disorder Stage B: A structural heart disorder, but no symptoms at any stage Stage C: Previous or current symptoms of heart failure in the context of an underlying structural heart problem, but managed with medical treatment Stage D: Advanced disease requiring hospital-based support, a heart transplant, or palliative care The ACC staging system is useful since stage A encompasses "pre-heart failure" – a stage where intervention with treatment can presumably prevent progression to overt symptoms. ACC stage A does not have a corresponding NYHA class. ACC stage B would correspond to NYHA class I. ACC stage C corresponds to NYHA class II and III, while ACC stage D overlaps with NYHA class IV. The degree of coexisting illness: i.e. heart failure/systemic hypertension, heart failure/pulmonary hypertension, heart failure/diabetes, heart failure/kidney failure, etc. Whether the problem is primarily increased venous back pressure (preload), or failure to supply adequate arterial perfusion (afterload) Whether the abnormality is due to low cardiac output with high systemic vascular resistance or high cardiac output with low vascular resistance (low-output heart failure vs. high-output heart failure) Histopathology Histopathology can diagnose heart failure in autopsies. The presence of siderophages indicates chronic left-sided heart failure, but is not specific for it. It is also indicated by congestion of the pulmonary circulation. Prevention A person's risk of developing heart failure is inversely related to level of