An ejection fraction is the percentage of blood pumped out of a heart chamber during the contraction phase of each heartbeat (systole). The term typically refers to a measurement of the left ventricle (the lower left chamber of the heart), which pumps oxygen-rich blood out to the body through the aorta. An ejection fraction can also be measured in the right ventricle (the lower right chamber), which pumps blood returning from the body to the lungs for oxygen. However, this measurement is nearly always referred to as a right ventricular ejection fraction.

Even in a healthy heart, some blood always remains within the heart chambers after each heartbeat. Therefore an ejection fraction is a percentage of the blood within the chamber that is pumped out with every heartbeat. Normally, the left ventricle pumps 55 to 75 percent of the blood within that chamber out to the body with each heartbeat. A higher than normal ejection fraction could indicate the presence of certain heart conditions, such as hypertrophic cardiomyopathy.

An ejection fraction of less than 40 may indicate heart failure, a chronic condition in which at least one heart chamber is not pumping well enough to meet the body’s needs. Heart failure leads to congestion of blood vessels and fluid backup and swelling in the lungs, legs and ankles, shortness of breath, and fatigue. An ejection fraction between 40 and 50 percent may indicate damage to the heart muscle (e.g., from a prior heart attack). Typically, this EF level alone is not low enough to lead to heart failure.

A low ejection fraction may result from some kind of cardiomyopathy, a condition in which the heart is abnormally enlarged, thickened or stiffened. Low EF is an important risk factor for sudden cardiac death (SCD), a condition that occurs when the heart stops abruptly (cardiac arrest), usually as a result of ventricular fibrillation. Patients with a low EF are significantly more likely to suffer sudden cardiac death within two years than patients with a normal ejection fraction.