Your lungs contain numerous small, elastic air sacs called alveoli. With each breath, these air sacs take in oxygen and release carbon dioxide. Normally, the exchange of gases takes place without problems.
But in certain circumstances, the alveoli fill with fluid instead of air, preventing oxygen from being absorbed into your bloodstream. A number of things can cause fluid to accumulate in your lungs, but most have to do with your heart (cardiogenic 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 and pump it into the lower chambers. 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 all over 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.
Heart-related (cardiogenic) pulmonary edema
Cardiogenic pulmonary edema is a type of pulmonary edema caused by increased pressures in the heart.
This condition usually occurs when the diseased or overworked left ventricle isn't able to pump out enough of the blood it receives from your lungs (congestive heart failure). As a result, pressure increases inside the left atrium and then in the veins and capillaries in your lungs, causing fluid to be pushed through the capillary walls into the air sacs.
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 muscle 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. Or it isn't a clot that brings on the problems, but rather it is a gradual narrowing of the coronary arteries resulting in a weakness of the left ventricular muscle.
Although the rest of your heart tries to compensate for this loss, either it's unable to do so effectively or it's weakened by the extra workload. When the pumping action of your heart is weakened, blood gradually backs up into your lungs, forcing fluid in your blood to pass through the capillary walls into the air sacs. This is chronic congestive heart failure.
Cardiomyopathy. When your heart muscle is damaged by causes other than blood flow problems, the condition is called cardiomyopathy.
Because cardiomyopathy weakens the ventricles — your heart's main pump — your heart may not be able to respond to conditions that require it to work harder, such as a surge in blood pressure, a faster heartbeat with exertion, or using too much salt that causes water retention or infections. When the left ventricle can't keep up with the demands that are placed on it, fluid backs up into your lungs.
Heart valve problems. In mitral valve disease or aortic valve disease, the valves that regulate blood flow in the left side of your heart either don't open wide enough (stenosis) or don't close completely (insufficiency). This allows blood to flow backward through the valve (regurgitation).
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 left ventricle also dilates to allow more blood flow, but this makes the left ventricle's pumping action less efficient. Because it's working so much harder, the left ventricular muscle eventually thickens, which puts greater stress on the coronary arteries, further weakening the left ventricular muscle.
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 can lead to damage to the heart muscle and worsening of coronary artery disease.
Other conditions may lead to cardiogenic pulmonary edema, such as high blood pressure due to narrowed kidney arteries (renal artery stenosis) and fluid buildup due to kidney disease or heart problems.
Non-heart-related (noncardiogenic) pulmonary edema
Pulmonary edema that isn't caused by increased pressures in your heart is called noncardiogenic pulmonary edema.
In this condition, fluid may leak from the capillaries in your lungs' air sacs because the capillaries themselves become more permeable or leaky, even without the buildup of back pressure from your heart. Some factors that can cause noncardiogenic pulmonary edema include:
- Acute respiratory distress syndrome (ARDS). This serious disorder occurs when your lungs suddenly fill with fluid and inflammatory white blood cells. Many conditions can cause ARDS, including severe injuries (trauma), systemic infection (sepsis), pneumonia and severe bleeding.
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 generally occurs at elevations above 8,000 feet (about 2,400 meters) — can also affect hikers or skiers who start exercising at higher altitudes without first becoming acclimated, which can take from a few days to a few weeks. But even people who have hiked or skied at high altitudes in the past aren't immune.
Although the exact cause isn't completely understood, HAPE seems to develop as a result of increased pressure from constriction of the pulmonary capillaries. Without appropriate care, HAPE can be fatal, but this risk can be minimized.
- Nervous system conditions. A type of pulmonary edema called neurogenic pulmonary edema can occur after some nervous system conditions or procedures — such as after a head injury, seizure or subarachnoid hemorrhage — or after brain surgery.
- Adverse drug reaction. Many drugs — ranging from illegal drugs such as heroin and cocaine to aspirin — are known to cause noncardiogenic pulmonary edema.
- Pulmonary embolism. Pulmonary embolism, a condition that occurs when blood clots travel from blood vessels in your legs to your lungs, can lead to pulmonary edema.
- Viral infections. Pulmonary edema can be caused by viral infections such as the hantavirus and dengue virus.
Lung injury. Pulmonary edema can occur after surgery to remove blood clots from your lungs, but it occurs only in the part of the lung from which the clots were removed. It can also occur after a collapsed lung has been re-expanded or rarely after volumes of fluid have been removed from the lung.
Noncardiogenic pulmonary edema also occurs in the lung directly below blunt trauma to the chest wall with the most common cause being auto accidents.
- Exposure to certain toxins. These include toxins you inhale as well as those that may circulate within your own body, for example, if you inhale (aspirate) some of your stomach contents when you vomit. Inhaling toxins such as ammonia and chlorine, which can occur with train accidents, causes intense irritation of the small airways and alveoli, resulting in fluid accumulation.
- Smoke inhalation. Smoke from a fire contains chemicals that damage the membrane between the air sacs and the capillaries, allowing fluid to enter your lungs.
- Near drowning. Inhaling water causes noncardiogenic pulmonary edema that is reversible with immediate attention.