Heart failure often develops after other conditions have damaged or weakened the heart. However, heart failure can also occur if the heart becomes too stiff. In heart failure, the main pumping chambers of the heart (the ventricles) may become stiff and not fill properly between beats. In some people, the heart muscle may become damaged and weakened. The ventricles may stretch to the point that the heart can't pump enough blood through the body. Over time, the heart can no longer keep up with the typical demands placed on it to pump blood to the rest of the body. Your doctor can determine how well your heart is pumping by measuring how much blood is pumped out with each beat (ejection fraction). Ejection fraction is used to help classify heart failure and guide treatment. In a healthy heart, the ejection fraction is 50% or higher — meaning that more than half of the blood that fills the ventricle is pumped out with each beat. But heart failure can occur even with a normal ejection fraction. This happens if the heart muscle becomes stiff from conditions such as high blood pressure. Heart failure can involve the left side (left ventricle), right side (right ventricle) or both sides of your heart. Generally, heart failure begins with the left side, specifically the left ventricle — your heart's main pumping chamber.