What Is Heart Failure?

Heart failure is a chronic, progressive condition in which the heart cannot pump enough blood to meet the body's needs — either because it cannot fill properly, pump forcefully enough, or both. Despite its name, "heart failure" does not mean the heart has stopped or is about to stop — it means the heart is not working as efficiently as it should, leading to a cascade of compensatory mechanisms and, eventually, symptoms that significantly affect quality of life and longevity.

Heart failure affects approximately 6.7 million Americans and is the leading cause of hospitalization in adults over age 65. It is responsible for more than 80,000 deaths annually as the primary cause, and is a contributing cause in many more deaths.

The Heart As A Pump

To understand heart failure, it helps to understand the heart's pumping cycle:

The left ventricle — the heart's main pumping chamber — fills with blood from the lungs during diastole (relaxation). During systole (contraction), it ejects blood into the aorta and systemic circulation. The proportion of blood ejected with each heartbeat is the ejection fraction (EF) — normally 55–70%.

Heart failure can arise from dysfunction of either phase: systole (pump failure — the heart contracts weakly) or diastole (filling failure — the heart is stiff and cannot fill normally).

Causes Of Heart Failure

Heart failure is not a disease but a syndrome — the end result of many conditions that damage or overload the heart:

Coronary artery disease / prior heart attack: The most common cause in the United States. Scar tissue from myocardial infarction replaces functioning muscle, reducing contractile mass.

Hypertension (high blood pressure): Chronically elevated pressure forces the left ventricle to work harder to eject blood. The ventricle hypertrophies (thickens) — initially a compensation, but ultimately becomes stiff and dysfunctional.

Cardiomyopathies: Diseases of the heart muscle itself:

• Dilated cardiomyopathy: The ventricle enlarges and weakens (can be genetic, from alcohol, chemotherapy, viral myocarditis, or idiopathic)
• Hypertrophic cardiomyopathy (HCM): Abnormal thickening of the heart muscle, often genetic
• Restrictive cardiomyopathy: The heart muscle becomes stiff and cannot fill normally (from amyloidosis, sarcoidosis, and other infiltrative diseases)

Heart valve disease: Stenotic (narrowed) or regurgitant (leaking) valves increase the work load on the heart chambers.

Arrhythmias: Uncontrolled rapid heart rhythms (especially atrial fibrillation) can cause or worsen heart failure.

Other causes: Thyroid disease, severe anemia, chemotherapy cardiotoxicity, alcohol, cocaine, viral infections, and congenital heart disease.

Types Of Heart Failure

Heart failure is classified by ejection fraction — because different types have different pathophysiology, respond differently to specific treatments, and have different prognoses:

HEART FAILURE WITH REDUCED EJECTION FRACTION (HFrEF) — EF < 40%

Historically called "systolic heart failure." The left ventricle is enlarged, dilated, and contracts weakly — pumping a smaller fraction of its volume with each beat. The most common type; the type for which the most evidence-based treatments exist.

HEART FAILURE WITH PRESERVED EJECTION FRACTION (HFpEF) — EF ≥ 50%

Historically called "diastolic heart failure." The heart contracts with normal or near-normal force, but the ventricle is stiff and cannot relax and fill properly. Blood backs up because the heart isn't receiving it efficiently. This type is increasingly recognized and is now the most common type overall — driven by the epidemic of hypertension, obesity, and diabetes. It is more common in women and older adults.

HEART FAILURE WITH MILDLY REDUCED EJECTION FRACTION (HFmrEF) — EF 40–49%

An intermediate zone recognized as a distinct category in recent guidelines. May represent transitional states between HFrEF and HFpEF.

How Heart Failure Progresses: Compensatory Mechanisms And Decompensation

When the heart begins to fail, the body activates compensatory mechanisms — initially helpful, ultimately harmful over time:

Neurohormonal activation: The renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system activate — raising blood pressure, retaining sodium and water, and increasing heart rate and contractility. Short-term benefits: maintaining blood pressure. Long-term: excess fluid retention, additional cardiac stress, and worsening cardiac remodeling.

Ventricular remodeling: The heart enlarges and changes shape — initially accommodating the increased volume, eventually becoming less efficient.

These compensations can maintain near-normal function for years, during which the patient may be asymptomatic. Eventually, the heart can no longer maintain adequate output despite these mechanisms — the patient decompensates, and symptoms emerge.

Symptoms Of Heart Failure

Heart failure symptoms reflect inadequate cardiac output and/or fluid buildup:

Shortness of breath (dyspnea):

• With activity (exertional dyspnea): Earliest symptom — the patient notices breathlessness at increasingly lower levels of activity
• At rest: As heart failure worsens
• Lying flat (orthopnea): Fluid redistributes when lying down; patients sleep with multiple pillows
• Paroxysmal nocturnal dyspnea (PND): Waking suddenly from sleep with severe shortness of breath

Fluid retention:

• Leg swelling (peripheral edema): Typically worst in the ankles and lower legs; sock marks are a classic sign
• Abdominal swelling (ascites): In advanced heart failure
• Rapid weight gain: From fluid accumulation — daily weight monitoring detects worsening before symptoms become severe

Fatigue and weakness: Reduced cardiac output means less blood delivered to muscles and organs.

Reduced exercise tolerance: Progressively declining ability to perform activities of daily living.

Cough or wheezing: From pulmonary congestion.

Cognitive changes: Reduced cerebral perfusion in advanced heart failure.

The New York Heart Association (Nyha) Classification

Heart failure severity is commonly classified by functional status:

• Class I: No limitation of physical activity
• Class II: Slight limitation; comfortable at rest; ordinary activity causes symptoms
• Class III: Marked limitation; comfortable at rest; less than ordinary activity causes symptoms
• Class IV: Unable to carry on any activity without symptoms; symptoms at rest