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Sunday, October 23, 2005

Pulmonary Edema - Page Three

Pulmonary Edema - Causes

Your lungs are two spongy, elastic organs inside your rib cage that stretch and contract as you breathe. Although your lungs can hold up to 4 quarts of air, you generally inhale only a fraction of that with each breath.

Two major airways (bronchi) carry air into your lungs. These airways subdivide into smaller airways (bronchioles) that finally end in clusters of tiny air sacs. Each lung has about 300 million air sacs, which inflate like miniature balloons every time you inhale.

Wrapped around each air sac are capillaries that connect the arteries and veins in your lungs. The capillaries are so narrow that red blood cells have to pass through them in single file. Each red blood cell absorbs oxygen, while the plasma — the fluid containing the red blood cells — releases carbon dioxide.

But in certain circumstances the alveoli fill with fluid instead of air, preventing oxygen from being absorbed into your bloodstream. A number of factors can cause fluid to accumulate in your lungs, but most have to do with your heart (cardiac pulmonary edema). Understanding the relationship between your heart and lungs can help explain why.

How your heart works

Your heart is composed of two upper and two lower chambers. The upper chambers (the right and left atria) receive incoming blood. The lower chambers, the more muscular right and left ventricles, pump blood out of your heart. The heart valves — which keep blood flowing in the correct direction — are gates at the chamber openings.

Normally, deoxygenated blood from your body enters the right atrium and flows into the right ventricle, where it's pumped through large blood vessels
(pulmonary arteries) to your lungs. There, the blood releases carbon dioxide and picks up oxygen. The oxygen-rich blood then returns to the left atrium through the pulmonary veins, flows through the mitral valve into the left ventricle, and finally leaves your heart through another large artery, the aorta. The aortic valve at the base of the aorta keeps the blood from flowing backward into your heart. From the aorta, the blood travels to the rest of your body.

What goes wrong

Cardiac pulmonary edema — also known as congestive heart failure — occurs when the left ventricle isn't able to pump out enough of the blood it receives from your lungs. As a result, pressure increases inside the left atrium and then in the pulmonary veins and capillaries, causing fluid to be pushed through the capillary walls into the air sacs.

Congestive heart failure can also occur when the right ventricle is unable to overcome increased pressure in the pulmonary artery, which usually results from left heart failure, chronic lung disease, or high blood pressure in the pulmonary artery (pulmonary hypertension).

Medical conditions that can cause the left ventricle to become weak and eventually fail include:

Coronary artery disease. Over time, the arteries that supply blood to your heart can become narrow from fatty deposits (plaques). A heart attack occurs when a blood clot forms in one of these narrowed arteries, blocking blood flow and damaging the portion of your heart muscle supplied by that artery. The result is that the damaged heart muscle can no longer pump as well as it should. Although the rest of the heart tries to compensate for this loss, it's either unable to do so effectively or it's weakened by the extra workload. Normally, the ventricles pump about 50 percent to 60 percent of the blood they contain with each contraction. But when the pumping action of the heart is weakened, this figure may fall as low as 15 percent. In that case, blood backs up into the lungs, forcing fluid in the blood to pass through the capillary walls into the air sacs.

Cardiomyopathy. When your heart muscle is damaged by causes other than blood flow problems, the condition is called cardiomyopathy. Often, cardiomyopathy has no known cause, although it sometimes runs in families. Less common causes include infections (myocarditis), alcohol abuse and the toxic effects of drugs such as cocaine and some types of chemotherapy. Because cardiomyopathy weakens the left ventricle — the heart's main pump — it may not be able to respond to conditions that require it to work harder, such as a surge in blood pressure or infections. When the left ventricle can't keep up with the demands placed on it, fluid backs up into the lungs.

Heart valve problems. In mitral valve disease or aortic valve disease, the valves that regulate blood flow either don't open wide enough (stenosis) or don't close completely (insufficiency). This allows blood to flow backward through the valve. When the valves are narrowed, blood can't flow freely into your heart and pressure in the left ventricle builds up, causing the left ventricle to work harder and harder with each contraction. The increased pressure extends into the left atrium and then to the pulmonary veins, causing fluid to accumulate in your lungs. On the other hand, if the mitral valve leaks, some blood is backwashed toward your lung each time your heart pumps. If the leakage develops suddenly, you may develop sudden and severe pulmonary edema.

High blood pressure (hypertension). Untreated or uncontrolled high blood pressure causes a thickening of the left ventricular muscle, and accelerates coronary artery disease.

If pulmonary edema persists, it can raise pressure in the pulmonary artery and eventually the right ventricle begins to fail. The right ventricle has a much thinner wall of muscle than does the left side. The increased pressure backs up into the right atrium and then into various parts of the body, where it can cause leg swelling (edema), abdominal swelling (ascites) or a buildup of fluid in the pleural space (pleural effusion).

Noncardiac pulmonary edemaNot all pulmonary edema is the result of heart disease. Fluid may also leak from the capillaries in the lungs' air sacs because the capillaries themselves become more permeable or leaky, even without the buildup of back pressure from the heart. In that case, the condition is known as noncardiac pulmonary edema because the heart isn't the cause of the problem.

Some factors that can cause increased capillary permeability leading to noncardiac pulmonary edema are:

Lung infections. When pulmonary edema results from lung infections, such as pneumonia, the edema occurs only in the part of the lung that's inflamed.

Exposure to certain toxins. These include toxins you inhale — such as chlorine, ammonia or nitrogen dioxide — as well as those that may circulate within your own body. For example, women giving birth may develop pulmonary edema when amniotic fluid reaches the lungs through the veins of the uterus (amniotic fluid embolism).

Severe allergic reactions (anaphylaxis). You can have serious allergic reactions to some medications as well as to certain foods and insect venom.

Smoke inhalation. Children and older adults are especially vulnerable to lung damage caused by breathing smoke from structural fires. Smoke contains chemicals that damage the membrane between the air sacs and the capillaries, allowing fluid to enter the lungs.

Drug overdose. More than 20 drugs — ranging from narcotics such as heroin to diabetes medications and aspirin — are known to cause noncardiac pulmonary edema. Aspirin-induced pulmonary edema can occur in people who take increasingly large doses of aspirin to relieve pain or other symptoms. For reasons that aren't clear, smokers who use aspirin are at greater risk.

Acute respiratory distress syndrome (ARDS). This serious disorder occurs when your lungs suddenly fill with fluid and inflammatory blood cells. Many conditions can cause ARDS, including severe injuries (trauma), systemic infection (sepsis), pneumonia or shock. ARDS sometimes develops after extensive surgery. Symptoms usually appear within 24 to 72 hours after the original illness or trauma.

High altitudes. Mountain climbers and people who live in or travel to high-altitude locations run the risk of developing high altitude pulmonary edema (HAPE). This condition — which typically occurs at elevations above 8,000 feet — can also affect hikers or skiers who start exercising at higher altitudes without first becoming acclimated. But even people who have hiked or skied at high altitudes in the past aren't immune. Although the exact mechanism isn't completely understood, HAPE seems to develop as a result of increased pressure from constriction of the pulmonary capillaries. Symptoms include headaches, insomnia, fluid retention, cough and shortness of breath. Without appropriate care, HAPE can be fatal.

Mayo Clinic

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