There are several ways to classify heart failure. The first approach describes the phase of the heart cycle resulting in heart failure. This method classifies heart failure as either systolic or diastolic. Systolic heart failure is caused by the decreased pumping ability of the heart due to weakness of the heart muscle, such as that caused by a heart attack. As a result, fluid builds up in the lungs and leads to difficulty breathing.

Diastolic heart failure is caused by the inability of the heart muscle to relax. This limits the flow of blood into the left ventricle, which is the main pumping chamber of the heart. An example of this is when the left ventricle is stiff as a result of high blood pressure. Blood flow from the left atrium to the left ventricle is limited and results in blood backing up into the lungs.

Both diastolic and systolic heart failure can result in blood backing up into the lungs. Diastolic heart failure may account for up to half of all cases of heart failure, and is more common in women and the elderly.

Another way to describe heart failure is in its timing. Chronic heart failure describes a condition that develops gradually. The symptoms are often subtle at first, becoming more obvious over time. Chronic heart failure is not a specific diagnosis, but is a result of another underlying condition. Finding the underlying condition is important because the treatment often depends on what that condition is. Unlike chronic heart failure, acute heart failure occurs more suddenly. An example of this is a large heart attack where enough muscle is seriously injured to prevent adequate pumping of blood. Another example is when a heart valve suddenly stops functioning properly.

Heart failure can also be classified based on which part of the heart is having difficulty. When the left side of the heart is affected, the left ventricle often cannot pump enough oxygen-rich blood to the organs, including the heart itself. This is known as left-sided failure, and can lead to the build up of fluid in the lungs. Left-sided heart failure can be either acute or chronic.

Blood may also back up in the right ventricle, causing a condition known as right-sided heart failure. During right-sided failure, blood becomes backed up in the bloodstream, causing fluid to accumulate in the body. This condition is referred to as edema. The legs and feet are likely to swell as a result of the fluid build-up. Right-sided heart failure can also be either acute or chronic.

Both types of heart failure are often present at the same time. In fact, right-sided heart failure is often the result of left-sided heart failure. Right-sided heart failure can also be caused by severe lung disease, in which the right side of the heart can't generate enough force to pump blood through the damaged lungs.