The ejection fraction is a measurement of the heart's efficiency and can be used to estimate the function of the left ventricle, which pumps blood to the rest of the body.

The left ventricle pumps only a fraction of the blood it contains. The ejection fraction is the amount of blood pumped divided by the amount of blood the ventricle contains. A normal ejection fraction is more than 55% of the blood volume. If the heart becomes enlarged, even if the amount of blood being pumped by the left ventricle remains the same, the relative fraction of blood being ejected decreases. For example:

• A healthy heart with a total blood volume of 100 mL that pumps 60 mL to the aorta has an ejection fraction of 60%.
• A heart with an enlarged left ventricle that has a total blood volume of 140 mL and pumps the same amount (60 mL) to the aorta has an ejection fraction of 43%.