How Edema Works

In years gone by, when someone's lower legs, ankles and feet were swollen, people called it "dropsy." Today, however, we call it edema. It's important to note that edema isn't a disease, but the symptom of an underlying condition, like heart failure (in which the heart is not pumping well enough to meet the body's demand for oxygen).

Edema is swelling caused by fluid (primarily water) that gets trapped in the tissues of your body. It usually occurs in the feet, ankles and legs (where it's called peripheral edema), but it also can occur nearly anywhere in the body, including the lungs (pulmonary edema) and the abdomen (ascites).


In most cases, edema begins when fluid leaks from the body's smallest blood vessels into the surrounding tissues. When the body senses that fluid is being lost from these vessels, it signals the kidneys to hold on to more fluid, increasing the volume of fluid in the vessels and leading to additional leakage.

Edema can occur if you sit or stand in one place for too long, as gravity pulls water down into your legs and feet. It can also result from a weakening in the valves of the veins in your legs, making it harder for them to push blood back up to the heart. Pregnant women sometimes get edema, and people who eat a diet too high in sodium (salt) can also develop it.

Certain diseases, such as kidney or liver disease, can cause edema or make it worse. One of the most common -- and most serious -- causes of edema is heart failure. In heart failure, the heart is weakened and its pumping action is impaired. There are two types of heart failure, based on which side of the heart is most affected. Each type is associated with a different type of edema:

  • Right-sided heart failure happens when the right chamber of the heart is not pumping adequately. As a result, the oxygen-poor blood that normally travels from the rest of the body to the heart begins to get backed up, causing fluid buildup in the veins and edema in the legs, ankles, and feet. Sometimes, right-sided heart failure can cause swelling in the abdomen (ascites).
  • Left-sided heart failure occurs when the left chamber of the heart cannot adequately pump oxygen-rich blood from the heart to the rest of the body. As a result, the oxygen-rich blood that normally travels from the lungs to the heart begins to get backed up, leading to the buildup of fluid in the lungs (pulmonary edema). Additional symptoms of left-sided heart failure include shortness of breath, fatigue and coughing, especially at night or while lying down.

­Signs that you might have edema include:

  • Stretched and shiny-looking skin over a swollen area
  • Increased abdomen size (ascites)
  • Shortness of breath or difficulty breathing (­pulmonary edema)
  • Tightness of jewelry, clothing or accessories
  • Low output of urine, even when you are drinking as much fluid as normal
  • A dimple in the skin covering the swollen area that remains for a few seconds after a pressing finger has been released

People who have a more serious form of edema (for example, pulmonary edema) may also experience the following:­

  • Rapid and labored breathing
  • Shortness of breath (especially at night and/or while lying down)
  • Coughing, sometimes with frothy blood
  • ­A sense of feeling suffocated



Edema Diagnosis and Treatment

Make an appointment with your doctor if you think you have edema. He or she will obtain a detailed medical history and conduct a full exam -- an examination of the legs and veins in the neck can provide valuable clues. The doctor will also listen to your lungs through a stethoscope, which can detect the presence of fluid, which indicates pulmonary edema.

Your doctor might order tests to help make a diagnosis. Blood and urine tests are useful in diagnosing certain forms of edema related to kidney or liver disease. If heart failure is suspected, your doctor may order tests to detect abnormalities in your heart's electrical activity, size, shape and function during rest and exercise.


How Is Edema Treated?

When edema isn't too severe, it can be treated with a program of diet and lifestyle changes.

When water builds up in your tissues, one of the most effective ways of getting rid of it is to reduce the amount of salt or sodium in your diet. The typical American diet is very high in sodium. Even if you don't add salt while cooking or avoid using the salt shaker at the table, you're probably eating too much sodium. That's because we eat processed foods, like frozen dinners, boxed noodle and rice dishes and canned soups and vegetables. Most processed foods are high in sodium.

Leg, ankle and foot edema can be improved if you elevate your legs above the level of your heart for 30 minutes three or four times each day. Leg edema can also be prevented and treated with the use of support stockings (also called compression hose).

Your doctor may prescribe oral medicines (diuretics or water pills) to reduce your edema. Diuretics cause the kidneys to pass more water and sodium, reducing fluid volume throughout the body. Unfortunately, these oral medicines typically become less effective over time, and many people with severe edema eventually require hospitalization.

Typically, an in-hospital treatment for a serious case of edema includes intravenous medications (given with a needle into the bloodstream) that remove excess fluid from the body. In addition to the diuretics, some commonly used medications include:

  • Vasodilator drugs, which cause blood vessels to expand, thereby increasing blood flow
  • Inotropic drugs, which stimulate the heart to expel more blood with each beat

Intravenous drug therapy requires an average hospital stay of six days.

If the intravenous drug therapy doesn't relieve the edema, patients receive a procedure known as ultrafiltration or aquapheresis (an advanced and highly automated form of ultrafiltration). In this procedure, blood containing excess fluid is withdrawn, passed through a special filter and returned to the body. Aquapheresis therapy usually requires a three- to four-day hospital stay.

Follow the links on the next page to learn more about edema.­­


Lots More Information

Related HowStuffWorks Articles

  • How Heart Failure Works
  • How Fluid Overload Works
  • How Diuretics Work
  • How Low-Sodium Diets Work
  • How Ultrafiltration Works
  • How Aquapheresis Works

More Great Links

  • ­Costanzo MR, Guglin ME, Saltzberg MT, et al. Ultrafiltration versus ­intravenous diuretics for patients hospitalized for acute decompensated heart failure. J Am Coll Cardiol. 2007; 49:675-683.
  • Costanzo MR, O'Sullivan J, et al. Early ultrafiltration in patients with decompensated heart failure and diuretic resistance. J Am Coll Cardiol. 2005; 46:2047-2051.
  • ­Elkayam U, Hatamizadeh P, Janmohamed M. The challenge of correcting volume overload in hospitalized patients with decompensated heart failure. J Am Coll Cardiol. 2007; 49:684-686.