Epidemiology

Acute respiratory distress syndrome affects approximately 200,000 people in the United States every year, with around 3 million cases occurring worldwide annually. This condition is responsible for roughly 10% of all admissions to intensive care units and is the reason at least 25% of people need mechanical ventilation during their hospital stay^[1]. In fact, about 190,000 Americans are diagnosed with ARDS each year, though the actual number may be higher because the condition can be difficult to diagnose and may not always be recognized^[8].

ARDS can affect people of any age, from infants to elderly adults. Unlike some conditions that primarily target specific groups, ARDS does not discriminate based on age or whether someone was previously healthy. However, certain demographic factors do influence risk. Advanced age is associated with a higher likelihood of developing the condition, as is female gender. People who smoke or consume alcohol regularly also face increased risk^[16]. Most people who develop ARDS are already critically ill or have sustained major injuries, often finding themselves hospitalized for another reason when the syndrome emerges.

The mortality rate for ARDS remains concerning despite advances in medical care. The condition carries a high death rate, with mortality increasing as age advances and as the illness becomes more severe. Close to 50% of people with severe ARDS do not survive^[13]. However, with treatment, an estimated 60% to 75% of those who develop ARDS will survive the disease^[8]. Even among survivors, many face long-term health challenges and a prolonged recovery period.

Causes

Acute respiratory distress syndrome is always triggered by an injury to the lungs, but that injury can come from many different sources. Medical professionals divide these causes into two main categories: direct lung injuries and indirect lung injuries. Direct injuries happen in or directly affect the lungs themselves, while indirect injuries occur elsewhere in the body but ultimately harm the lungs through a chain reaction of inflammation and other processes^[8].

Sepsis stands out as the most common cause of ARDS. Sepsis happens when you have a serious infection in your lungs, such as pneumonia, or in other organs, and the infection triggers widespread inflammation throughout your body^[1]. When the body’s response to infection becomes overwhelming, it can damage the delicate structures in the lungs even if the original infection started somewhere else entirely.

Other direct lung injuries that can cause ARDS include aspiration pneumonia, which occurs when stomach contents are inhaled into the lungs. This might happen if someone vomits and accidentally breathes in the vomit, or if food or liquid goes down the wrong way. The stomach’s acidic contents can cause severe damage to lung tissue^[1]. Breathing in toxic substances also directly injures the lungs. This includes inhaling high concentrations of chemical fumes or smoke from fires, which can burn and damage the airways and air sacs^[1].

Major trauma or burns represent another significant cause. Accidents, falls, or severe injuries may directly damage the lungs or trigger severe inflammation that spreads to the lungs. Near-drowning incidents cause water to enter the lungs, leading to fluid buildup and potential ARDS^[1]. Drug overdoses, particularly involving cocaine and opioids, can also trigger the condition^[1].

Some medical treatments carry risks as well. Receiving more than 15 units of blood in a short period of time increases the risk of developing ARDS^[1]. Additionally, severe inflammation in the pancreas, known as pancreatitis, can lead to ARDS even though the pancreas is not part of the respiratory system^[1]. The COVID-19 virus has also been recognized as a cause that can develop into severe ARDS^[1].

Risk Factors

While ARDS can develop in anyone, certain groups of people and certain behaviors increase the likelihood of developing this serious condition. ARDS usually affects people who are already critically ill or dealing with major injuries. Most often, the syndrome develops while someone is hospitalized for treatment of another infection, illness, or trauma^[1].

People with existing medical problems face higher risk. Those with lung disease are more vulnerable to developing ARDS when faced with additional health challenges^[7]. Advanced age is a significant risk factor, as older adults are more likely to develop the condition and less likely to survive it^[16]. Women face slightly higher risk compared to men^[16].

Lifestyle factors play an important role as well. Smoking damages lung tissue over time, making the lungs more vulnerable to acute injuries that could trigger ARDS. Regular alcohol use also increases risk, though the exact mechanisms are not fully understood^[16]. These habits weaken the body’s defenses and make it harder for the lungs to withstand stress from illness or injury.

Certain medical events dramatically increase ARDS risk. Having a blood infection, sustaining severe injuries from accidents or violence, experiencing major burns, undergoing multiple blood transfusions, or developing severe pancreatitis all place someone in a high-risk category^[6]. People who inhale vomit, smoke from fires, or toxic fumes are also at elevated risk^[6]. Even some medical procedures can increase risk, particularly lung transplantation^[6].

Symptoms

The symptoms of acute respiratory distress syndrome depend on what caused the condition, how severe the case is, and whether someone has preexisting lung or heart problems. However, certain core symptoms appear consistently across most cases. The primary and most distressing symptom is severe shortness of breath. People with ARDS feel as though they cannot get enough air, no matter how hard they try to breathe^[1].

Breathing becomes fast and labored. Instead of the normal, effortless breathing pattern, each breath requires significant effort and happens more rapidly than usual. This rapid breathing reflects the body’s desperate attempt to bring in more oxygen and remove carbon dioxide^[1]. The heart also responds to the lack of oxygen by beating faster, creating a rapid heart rate that patients may notice as heart pounding or racing^[1].

A particularly concerning sign is when the fingernails and lips turn bluish in color. This cyanosis happens because the blood does not contain enough oxygen. When oxygen levels drop too low, the blood takes on a darker, bluish tone that becomes visible through the skin and nail beds^[1]. This is a medical emergency requiring immediate attention.

Additional symptoms may include coughing, which sometimes produces white or pink frothy material from the lungs. Some people experience chest discomfort or pain. Confusion and extreme tiredness can develop as the brain and other organs fail to receive adequate oxygen^[2]. In cases related to infection, fever may be present as well^[8].

What makes ARDS particularly dangerous is how quickly it develops and worsens. The syndrome tends to develop within a few hours to a few days of the event that caused it. Once it begins, ARDS can worsen rapidly, sometimes progressing from mild symptoms to life-threatening respiratory failure in a very short time^[1]. This rapid progression means that early recognition and immediate medical intervention are crucial for the best possible outcomes.

Prevention

Because acute respiratory distress syndrome develops as a complication of other serious illnesses or injuries, preventing ARDS often means preventing or promptly treating the conditions that cause it. There is no vaccine or specific preventive medication for ARDS itself. However, several strategies can reduce your risk or catch potential triggers early.

Prompt treatment of infections represents one of the most important preventive measures. Since sepsis and pneumonia are leading causes of ARDS, seeking medical attention when you have signs of infection and taking prescribed antibiotics as directed can prevent these infections from becoming severe enough to trigger lung injury. Getting vaccinated against influenza, pneumonia, and COVID-19 can prevent the serious infections that might lead to ARDS^[6].

Lifestyle modifications provide another layer of protection. Quitting smoking is crucial, as smoking damages lung tissue and makes it more vulnerable to injury. Reducing alcohol consumption also lowers risk. Both of these habits weaken the lungs’ natural defenses and healing capacity^[16]. Maintaining overall good health through regular exercise, adequate nutrition, and management of chronic conditions helps keep your body strong and more resistant to severe complications when illness or injury does occur.

For people hospitalized with conditions that increase ARDS risk, healthcare teams employ various preventive strategies. These include careful monitoring of oxygen levels, judicious use of blood transfusions, elevation of the head of the bed to reduce the risk of aspiration, and protocols to prevent hospital-acquired pneumonia. Early recognition and treatment of sepsis, along with lung-protective ventilation strategies when mechanical breathing support becomes necessary, can prevent ARDS from developing or reduce its severity^[17].

Pathophysiology

Understanding what happens inside the body during ARDS helps explain why the condition is so dangerous and why certain treatments are used. At a microscopic level, ARDS involves damage to the thin barrier between air and blood in the lungs. Your lungs contain approximately 500 million tiny air sacs called alveoli. Each alveolus is surrounded by equally tiny blood vessels called capillaries. In healthy lungs, oxygen from the air you breathe passes through the thin walls of the alveoli into the blood in the capillaries, while carbon dioxide moves from the blood into the alveoli to be breathed out^[8].

When ARDS develops, the barrier between alveoli and capillaries becomes damaged. This damage is associated with capillary endothelial injury and diffuse alveolar damage, medical terms that describe widespread harm to the blood vessel linings and air sac structures^[5]. The injury causes inflammation and swelling throughout the lungs. This swelling breaks down the protective membranes that normally keep fluid in the blood vessels and out of the air spaces^[2].

As these membranes fail, fluid begins leaking from the capillaries into the spaces around the alveoli. At first, fluid fills the spaces between blood vessels and air sacs. As pressure builds, fluid eventually floods into the alveoli themselves. When fluid fills the air sacs, they cannot fill with air. The alveoli collapse under the weight of the fluid. This is the hallmark problem in ARDS: instead of being filled with air, the lungs become filled with fluid^[8].

Additionally, a foamy substance called surfactant that normally keeps the alveoli open breaks down during ARDS. Surfactant acts like a coating that prevents the air sacs from collapsing. When it stops working properly, the alveoli become stiff and prone to collapse, making it even harder for the lungs to expand and take in air^[3].

The lungs also become stiff and non-compliant, meaning they resist expansion. Breathing requires much more effort than normal. More importantly, with fluid-filled and collapsed alveoli, oxygen cannot pass from air into blood effectively. This causes critically low oxygen levels in the bloodstream, a condition called hypoxia. Without enough oxygen, all the organs in the body suffer. The brain, heart, kidneys, liver, and other organs need a constant supply of oxygen-rich blood to function^[1].

The injury also triggers an immune response. The body releases cytokines, which are inflammatory proteins, and these attract white blood cells called neutrophils to the lungs. While this immune response is meant to heal injury, it can backfire. Some of these inflammatory proteins and cells leak into nearby blood vessels and travel throughout the body, causing inflammation in organs far from the lungs. This widespread inflammation, combined with low oxygen levels, can lead to organ failure or even multiple organ failure^[8].

Once ARDS develops, patients usually experience varying degrees of constriction in the arteries that supply the lungs. This pulmonary artery vasoconstriction may lead to pulmonary hypertension, which is high blood pressure in the lung circulation. This puts additional strain on the right side of the heart, which must work harder to pump blood through the constricted lung vessels^[5]. This can sometimes lead to acute cor pulmonale, a condition where the right side of the heart becomes overwhelmed and struggles to function properly.

The combination of fluid-filled lungs, collapsed air sacs, stiff lung tissue, breakdown of surfactant, widespread inflammation, and strain on the heart creates a cascade of problems that make ARDS so life-threatening. The body cannot get the oxygen it needs, and without oxygen, every system begins to fail. This explains why people with ARDS require intensive medical support, often including machines to breathe for them while their lungs attempt to heal.