What condition is when fluid accumulates in the lungs, preventing them from breathing adequately?

Acute respiratory distress syndrome (ARDS) occurs when the millions of tiny air sacs in the lungs, called alveoli, fill with excess fluid. This can be the result of any kind of injury to or illness in the lung.

Key Info

• ARDS occurs when the air sacs in the lungs, the alveoli, fill with fluid. This prevents adequate oxygen transfer into the blood
• Medical Treatment for ARDS includes supplemental oxygen, mechanical ventilation, or extracorporeal membrane oxygen (ECMO)
• ECMO functions as an artificial lung, providing blood with oxygen and returning it to the body

In healthy lungs, the alveoli fill with inhaled air, transferring oxygen into the blood carried by small neighboring vessels. The oxygen-rich blood can then travel throughout the body to deliver its cargo to the kidneys, brain, liver, and other organs. 

Certain lung conditions or injuries can cause the body to initiate an inflammatory response, sending excess fluid to the lungs. When fluid accumulates in the alveoli, they can no longer fill with air, and oxygen cannot pass as easily into the blood. This is called acute respiratory distress syndrome, or ARDS.

Soon after the initial injury or illness, blood oxygen levels decline, and breathing becomes fast and difficult as the body tries to compensate. There may also be signs such as confusion or low blood pressure that result from the vital organs not getting enough oxygen. In some cases, the lung may try to heal itself, creating scar tissue that decreases the lung's elasticity and makes it still harder to breathe.

Conditions that can lead to ARDS include:

• Pneumonia
• Trauma
• Sepsis
• Inhalation of stomach contents
• Inhalation of smoke

Medical Treatment for ARDS

The first goal in treating ARDS is to increase oxygen levels in the blood so that organs function better. Three main ways to provide more oxygen are:

• Supplemental oxygen. For milder symptoms or as a temporary measure, oxygen may be delivered through a mask that fits tightly over the nose and mouth.
• Mechanical ventilation. Most people with ARDS need the help of a machine to breathe. A mechanical ventilator pushes air into the lungs and forces some of the fluid out of the air sacs.
• Extracorporeal membrane oxygenation. In this process, the blood is removed from the body and oxygenated externally before it is returned.

ECMO for ARDS

Extracorporeal membrane oxygenation (ECMO) is a term used to describe a circuit outside the body ("extracorporeal") that directly oxygenates and removes carbon dioxide from the blood. ECMO functions as an artificial lung working in addition to the failing lungs.

When ECMO is used for respiratory failure, a catheter is placed in a central vein, usually near the heart. A mechanical pump draws blood from the vein into the circuit, where the blood passes along a membrane (referred to as an "oxygenator" or "gas exchanger"), providing an interface between the blood and freshly delivered oxygen.

The blood may be warmed or cooled as needed, and is returned either to a central vein ("veno-venous ECMO") or to an artery ("veno-arterial ECMO"). Veno-venous ECMO provides respiratory support alone, while veno-arterial ECMO provides both respiratory and hemodynamic (blood pressure) support. Examples of scenarios where ECMO may benefit include the following:

• In cases of life-threatening acute respiratory failure with profound gas exchange abnormalities, ECMO may be used as salvage therapy.
• ECMO may also be used in those with ARDS who would benefit from lung-protective ventilation strategies but who are unable to tolerate such strategies.

In addition to receiving oxygen therapy, people with ARDS are given medication to:

• Prevent and treat infections
• Relieve pain and discomfort
• Prevent clots in the legs and lungs
• Minimize gastric reflux

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Pulmonary edema is a serious medical condition that happens when excess fluid begins to fill the lungs' air sacs (the alveoli). When the alveoli are filled with fluid, they cannot adequately add oxygen to, or remove carbon dioxide from, the blood. So pulmonary edema produces significant breathing difficulties, and may often become a life-threatening problem.

The alveoli are where the real work of the lungs takes place. In the alveolar air sacs, the fresh air we breath comes in close proximity to the capillaries carrying oxygen-poor blood from the body’s tissues. (This oxygen-poor blood has just been pumped from the right side of the heart out to the lungs, via the pulmonary artery.)

Through the thin walls of the alveoli, critical gas exchanges occur between the air within the alveolar sac and the “spent” blood within the capillaries. Oxygen from the alveoli is taken up by the capillary blood, and carbon dioxide from the blood diffuses into the alveoli. The blood, now oxygen-rich once again, is carried to the left side of the heart, which pumps it out to the tissues. The “used” alveolar air is exhaled out to the atmosphere, as we breathe.

Life itself is dependent on the efficient exchange of gasses within the alveoli.

With pulmonary edema, some of the alveolar sacs become filled with fluid. The critical exchange of gasses between inhaled air and capillary blood can no longer occur in the fluid-filled alveoli. If sufficient numbers of alveoli are affected, severe symptoms occur. And if the pulmonary edema becomes extensive, death can ensue.

Pulmonary edema may occur acutely, in which case it commonly causes severe dyspnea (shortness of breath), along with coughing (which often produces pink, frothy sputum), and wheezing. Sudden pulmonary edema also may be accompanied by extreme anxiety and palpitations. Sudden-onset pulmonary edema is often called “flash pulmonary edema,” and it most often indicates a sudden worsening of an underlying cardiac problem. For instance, acute coronary syndrome can produce flash pulmonary edema, as can acute stress cardiomyopathy.

Acute pulmonary edema is always a medical emergency and can be fatal.

Chronic pulmonary edema, which is often seen with heart failure, tends to cause symptoms that wax and wane over time, as more or fewer alveoli are affected. Common symptoms are dyspnea with exertion, orthopnea (difficulty breathing while lying flat), paroxysmal nocturnal dyspnea (waking up at night severely short of breath), fatigue, leg edema (swelling), and weight gain (due to fluid accumulation).

Doctors usually divide pulmonary edema into one of two types: cardiac pulmonary edema, and non-cardiac pulmonary edema.

Heart disease is the most common cause of pulmonary edema. Cardiac pulmonary edema happens when an underlying heart problem causes pressures on the left side of the heart to become elevated. This high pressure is transmitted backward, through the pulmonary veins, to the alveolar capillaries. Because of the elevated pulmonary capillary pressure, fluid leaks out of the capillaries into the alveolar air space, and pulmonary edema occurs.

Almost any kind of heart disease can eventually lead to elevated left-sided cardiac pressure, and thus, to pulmonary edema. The most common types of heart disease causing pulmonary edema are:

With chronic cardiac pulmonary edema, elevated pressures within the capillaries can eventually cause changes to occur in the pulmonary arteries. As a result, high pulmonary artery pressure may occur, a condition called pulmonary hypertension. If the right side of the heart has to pump blood against this elevated pulmonary artery pressure, right-sided heart failure can eventually develop.

With some medical conditions, the alveoli can fill up with fluid for reasons unrelated to elevated cardiac pressure. This can occur when the capillaries in the lungs become damaged, and as a consequence, they become “leaky” and allow fluid to enter the alveoli.

The most common cause of this sort of non-cardiac pulmonary edema is acute respiratory distress syndrome (ARDS), which is caused by a diffuse inflammation within the lungs. The inflammation damages the alveolar walls and allows fluid to accumulate. ARDS is typically seen in critically ill patients and may be caused by infection, shock, trauma, and several other conditions.

In addition to ARDS, non-cardiac pulmonary edema may also be produced by:

• Pulmonary embolism
• High altitude sickness
• Drugs (especially heroin and cocaine)
• Viral infections
• Toxins (for instance, inhaling chlorine or ammonia)
• Neurologic problems (such as brain trauma or subarachnoid hemorrhage)
• Smoke inhalation
• Near drowning

Rapidly making the correct diagnosis of pulmonary edema is critical, and especially critical is correctly diagnosing the underlying cause.

Diagnosing pulmonary edema is usually accomplished relatively quickly by performing a physical examination, measuring the blood oxygen levels, and doing a chest X-ray.

Once pulmonary edema has been found, steps must be taken immediately to identify the underlying cause. The medical history is very important in this effort, especially if there is a history of heart disease (or increased cardiovascular risk), drug use, exposure to toxins or infections, or risk factors for pulmonary embolus.

An electrocardiogram and an echocardiogram are often quite helpful in detecting underlying heart disease. If heart disease is suspected but cannot be demonstrated by noninvasive testing, a cardiac catheterization may be necessary. A range of other tests may be needed if a non-cardiac cause is suspected.

Non-cardiac pulmonary edema is diagnosed when pulmonary edema is present in the absence of elevated left heart pressures.

The immediate goals in treating pulmonary edema are to reduce the fluid buildup in the lungs and restore blood oxygen levels toward normal. Oxygen therapy is virtually always given right away. If signs of heart failure are present, diuretics are also given acutely. Medicines that dilate blood the vessels, such as nitrates, are often used to reduce pressures within the heart.

If blood oxygen levels remain critically low despite such measures, mechanical ventilation may be required. Mechanical ventilation can be used to increase the pressure within the alveoli, and drive some of the accumulated fluid back into the capillaries.

However, the ultimate treatment of pulmonary edema—whether it is due to heart disease or to a non-cardiac cause—requires identifying and treating the underlying medical problem. 

Pulmonary edema is a serious medical condition caused by excess fluid in the alveoli of the lungs. It is most often due to cardiac disease, but can also be produced by a range of non-cardiac medical problems. It is treated by rapidly addressing the underlying cause, using diuretics, and sometimes with mechanical ventilation.