Though it has become part of a vocabulary around COVID-19, Acute Respiratory Distress Syndrome, or ARDS for short, refers to a type of lung damage that can result from a variety of causes, including illness, trauma, or even as a complication that occurs following certain medical procedures. ARDS is a dangerous, potentially fatal respiratory condition in which the lungs sustain a serious, widespread injury that diminishes their ability to provide the body’s organs with enough oxygen. The condition causes fluid to accumulate in the lungs, which in turn reduces blood oxygen to dangerously low levels. ARDS is a medical emergency.

ARDS is always caused by an injury to the lungs, whether from illness or injury, but it can affect people of any age. In fact, in the United States, about 190,000 Americans are diagnosed with ARDS each year. All ARDS patients must be given supplemental oxygen therapy and most will be placed on a mechanical ventilator to help them breathe. Though there is no cure for ARDS, it’s not uniformly fatal. With treatment, an estimated 60% to 75% of those who have ARDS will survive the disease.

“We know how to support people through ARDS very well,” says Lauren Ferrante, MD, MHS, a Yale Medicine pulmonary and critical care specialist. “But it’s still a serious problem. Even though we can support the body to give it time to recover, the lungs still have to start the recovery process themselves. There is no magic bullet treatment for ARDS.” 

The term “acute” appears in the name of ARDS, because the condition arises from a recent injury to the lungs.  It is characterized by the accumulation of fluid in the lungs and below-normal levels of oxygen in the blood (the medical term for this is hypoxemia).

While a variety of medical conditions may lead to ARDS, at a microscopic level they all result in damage to air sacs in the lungs (called alveoli) and the tiny neighboring blood vessels (called capillaries).  

The average person has close to 500 million alveoli in their lungs. Each of these is responsible for performing two critical tasks—transporting oxygen into the blood in the capillaries and removing carbon dioxide from the blood. (All of our tissues and organs need a constant supply of oxygen-rich blood to stay healthy.)

Damage to the alveoli and neighboring capillaries reduces the ability of the lungs to send oxygen into the blood. This happens because the lung injury causes fluid to leak into the spaces between the capillaries and the alveoli. Pressure on the alveoli increases, and eventually fluid gets in there, too. 

This is what gives ARDS its characteristic trait—accumulation of fluid in the lungs, causing the alveoli to collapse. This leads to a series of cascading problems, each further decreasing the lungs’ capacity to move oxygen into the blood and directly impacting the body’s tissues and organs.

What’s more, ARDS also triggers an immune response. The injury causes a release of cytokines—a type of inflammatory protein—which then bring neutrophils, a type of white blood cell, to the lung. But problems arise when some of these proteins and cells leak into nearby blood vessels and, via the circulatory system, are sent throughout the body, causing inflammation in other organs. This inflammation, in combination with low levels of blood oxygen, can lead to such problems as organ failure and sometimes multiple organ failure. 

Doctors divide the causes of lung injury that lead to ARDS into two broad categories: direct lung injury and indirect lung injury. Direct lung injuries are those that occur in or directly affect the lungs. Indirect injuries are those that occur elsewhere in the body, yet ultimately end up harming the lungs.

Conditions that cause direct lung injury include:

• Pneumonia
• Aspiration (inhalation of stomach contents into the lungs)
• Inhalation of toxic substances
• Bruising of the lungs caused by chest trauma
• Near-drowning
• Fat embolism (when a clot of fat enters the pulmonary circulation)
• Lung transplantation
• Viral infection of the lungs, including by SARS-CoV-2, the coronavirus that causes COVID-19 infection.

Conditions that cause indirect lung injury include:

• Sepsis (the body’s overreaction to an infection that can damage multiple organ systems, including the lungs)
• Severe trauma to the body (head trauma, burns, multiple bone fractures)
• Massive blood transfusion
• Pancreatitis (inflammation of the pancreas)
• Cardiopulmonary bypass (heart-lung bypass surgery)
• Drug overdose

Out of these possible causes of lung injury, two of them—sepsis and pneumonia—make up 40% to 60% of all ARDS cases.

When the virus that causes COVID-19 enters the body, it frequently attaches to cells in the upper airway. This sets off an immune response that causes inflammation, leading to symptoms such as cough, sore throat, and fever.

In some cases, the virus travels beyond the upper airway, moves through the lungs, and ends up in the alveoli. When this occurs, COVID-19 can lead to ARDS, typically setting in about eight days after the onset of initial symptoms. Certain risk factors increase the likelihood of the development of ARDS in people with COVID-19, including advanced age, diabetes, and high blood pressure.

The first symptoms of ARDS usually develop within a few hours to a week after lung injury. 

People typically experience extreme difficulty breathing and shortness of breath. This is often accompanied by rapid, shallow breathing. Low oxygen levels in the blood can also produce a range of other symptoms, including confusion, dizziness, excessive sweating, low blood pressure, and rapid heart rate. Some people may notice that their fingertips, lips, or skin take on a bluish hue, a sign of insufficient blood oxygen level.

In most cases, ARDS develops in people who are already hospitalized for another illness or injury. In these cases, health care providers will take note of these symptoms and provide immediate treatment. Anyone who experiences these symptoms outside of a hospital, however, should seek urgent medical assistance. 

People with certain medical conditions and lifestyle habits are at increased risk of developing ARDS. In general, older people, as well as those who smoke cigarettes or misuse alcohol are at greater risk. In rare cases, some medical procedures such as cardiovascular surgery may lead to ARDS, and people with chronic liver disease, cirrhosis, or chronically suppressed immune systems have increased likelihood of developing ARDS. 

No single test can diagnose ARDS. Instead, doctors will try to assess possible underlying problems that may cause it. In general, a doctor will evaluate a patient’s medical history, perform a physical exam, and order diagnostic tests.

Because ARDS is a disease of the lungs, the physical exam centers on evaluating lung function. A doctor will listen to the lungs, using a stethoscope to check for characteristic crackling sounds produced by fluid-filled lungs. He or she will also measure the patient’s heart rate and blood pressure, and will check for signs of what’s called cyanosis, a bluish discoloration of the skin.

The doctor may also measure the patient’s blood oxygen level with a pulse oximeter, a small, electronic device that painlessly clips onto a fingertip or ear lobe. In most cases, patients will need to have their blood drawn so that doctors can check for signs of infection and evaluate organ function.

In addition to bloodwork, doctors may order X-rays and a CT scan of the patient’s chest. These images allow doctors to see signs of fluid accumulation in the lungs.

In some cases, a doctor may recommend additional tests, including an electrocardiogram (a test to check the heart's rhythm and electrical activity) to rule out heart conditions that might mimic ARDS symptoms. They may also order a bronchoscopy, a test in which a small tube equipped with a camera is inserted into the lungs allowing for a clear view of lung tissues.

Treatment for ARDS typically aims to increase blood oxygen levels, provide breathing support, and treat the underlying cause of the disease.

• Oxygen and Ventilation. Most ARDS patients are placed on a mechanical ventilator, usually in the intensive care unit of a hospital. A ventilator takes over a person’s breathing when they are unable to breathe on their own.
• Medications to manage symptoms. Often patients are given diuretics to help clear away excess fluid buildup in the lungs, and pain medication to relieve discomfort.
• Treating underlying cause. In addition to increasing blood oxygen levels, doctors must treat the underlying medical condition that led to ARDS.
• Prone positioning. For ARDS in general and especially with COVID-19, having a patient lie on their belly (on or off a ventilator) can help patients use parts of their lungs that aren’t used when lying on their back. 

In some cases, if ventilator treatment fails to adequately improve blood oxygen levels or if the patient is unsuited to mechanical ventilation, doctors will place ARDS patients on Extracorporeal Membrane Oxygenation, or ECMO, therapy. In ECMO therapy, a machine takes over the functions usually performed by the lungs, and if necessary, the heart.

ARDS is a serious condition. Even with treatment, about 25% to 40% of people with ARDS do not survive.

In general, people with ARDS caused by direct lung injury have worse outcomes than those with indirect causes of lung injury. Other issues that can have a negative effect on outcome include advanced age and certain chronic medical conditions, including liver disease, cirrhosis, alcohol abuse, and long-term immunosuppression.

While the mortality rate for ARDS is significant, recent advances in treatment have significantly increased the chances of survival and recovery. Patients who survive ARDS typically require some form of physical therapy to rebuild muscle tone. Most people who survive ARDS go on to recover their normal or close to normal lung function within six months to a year.

Others may not do as well, particularly if their illness was caused by severe lung damage or their treatment entailed long-term use of a ventilator. Their reduced lung function may affect daily routine and activities, or it may only occur during strenuous activity, for instance, while exercising.

At Yale Medicine, the ICU team is very experienced in the use of evidence-based strategies to provide the highest-quality care to ARDS patients. These strategies include lung-protective ventilation, prone positioning, close attention to volume status, and other strategies to give the patient the best chance of recovery. The ICU at Yale also has an established mobility program, called the STEPS-ICU program, in which physical and occupational therapists work with ICU patients to help them start the recovery process as soon as they start to improve.