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Heart failure can be described by the side of the heart affected, its functional pattern, cardiac output state, and whether it is acute or chronic. Systolic dysfunction reflects impaired contractility and reduced ejection fraction; diastolic dysfunction reflects impaired relaxation or compliance, with preserved ejection fraction and elevated filling pressure. Left-sided failure raises pulmonary venous pressure, while right-sided failure raises systemic venous pressure; right-sided failure is often secondary to left-sided disease, though pulmonary and valvular causes also occur. Increased preload and cardiac remodeling may initially help maintain output. Over time, sympathetic and renin–angiotensin–aldosterone activation, fluid retention, vasoconstriction, and fibrosis can worsen the burden on the heart. Natriuretic peptides provide a counter-regulatory response.