ARDS stands for acute respiratory distress syndrome, a life-threatening condition in which the lungs suddenly fill with fluid, making it extremely difficult to breathe. It develops rapidly, typically within one week of a triggering illness or injury, and requires immediate treatment in an intensive care unit. Hospital mortality ranges from roughly 35% in mild cases to 46% in severe cases, making it one of the most dangerous conditions treated in critical care.
What Happens in the Lungs
In healthy lungs, tiny air sacs called alveoli inflate with each breath and pass oxygen into the bloodstream through thin, delicate membranes. In ARDS, those membranes become damaged and leak. Protein-rich fluid floods the air sacs, collapsing them and blocking the normal exchange of oxygen and carbon dioxide. The lungs essentially become waterlogged, but not from heart failure. The fluid comes from inflammation and tissue damage in the lungs themselves or from a massive inflammatory response elsewhere in the body.
This flooding also disrupts the production of surfactant, a slippery coating that normally keeps the air sacs open. Without enough surfactant, more air sacs collapse, and the lungs become progressively stiffer and harder to inflate. The result is severe, rapidly worsening oxygen deprivation that the body cannot correct on its own.
Common Causes
ARDS is never the primary illness. It’s always triggered by something else, and those triggers fall into two categories.
Direct lung injuries damage the air sacs from the inside. The most common are pneumonia and aspiration, which is when stomach contents, food, or liquid are accidentally inhaled into the lungs. Inhaling smoke, toxic fumes, or near-drowning can also cause it.
Indirect injuries reach the lungs through the bloodstream. Sepsis (a body-wide infection that triggers overwhelming inflammation) is the leading indirect cause. Severe pancreatitis, major traumatic injuries with shock, and receiving multiple blood transfusions are also significant risk factors. In these cases, the lungs aren’t the original problem, but they bear the brunt of the body’s extreme inflammatory response.
How ARDS Is Diagnosed
Doctors use a standardized set of criteria known as the Berlin definition to diagnose ARDS. Four conditions must be met:
- Timing: Respiratory failure develops within one week of a known illness or injury.
- Chest imaging: X-rays or CT scans show fluid (called bilateral infiltrates) in both lungs, not just one side.
- Ruling out the heart: The fluid in the lungs cannot be fully explained by heart failure. If there’s any doubt, an echocardiogram is used to check heart function.
- Oxygen levels: Blood oxygen is dangerously low relative to how much supplemental oxygen is being delivered. This is measured as a ratio comparing arterial oxygen pressure to the concentration of oxygen being given.
Mild, Moderate, and Severe ARDS
Once diagnosed, ARDS is classified into three severity levels based on how badly oxygen exchange is impaired. The worse the ratio between blood oxygen and delivered oxygen, the more severe the condition:
- Mild ARDS: Oxygen exchange is reduced but still partially functional. Hospital mortality is approximately 35%.
- Moderate ARDS: Oxygen exchange is significantly impaired. Hospital mortality rises to about 40%.
- Severe ARDS: The lungs are barely able to transfer oxygen into the blood. Hospital mortality reaches roughly 46%.
These aren’t fixed categories. A patient diagnosed with mild ARDS can worsen into moderate or severe over hours or days, which is why continuous monitoring in an ICU is essential.
How ARDS Is Treated
There is no drug that cures ARDS. Treatment focuses on keeping oxygen levels high enough to prevent organ damage while giving the lungs time to heal. Nearly all patients with moderate or severe ARDS need a mechanical ventilator, a machine that pushes air into the lungs through a tube placed in the airway.
The key challenge with ventilation is that the lungs are already injured, and forcing air into them can cause additional damage. To prevent this, doctors use a strategy called lung-protective ventilation, delivering smaller breaths than a normal person would take. The recommended target is 4 to 6 milliliters of air per kilogram of the patient’s ideal body weight, roughly half the size of a normal breath. This gentler approach has been one of the most important advances in ARDS survival.
For patients with severe ARDS who aren’t responding to standard ventilation, turning the patient face-down (prone positioning) can dramatically improve oxygen levels. Lying on the stomach redistributes blood flow and opens collapsed areas in the back of the lungs. Clinical trials have shown a clear survival benefit when patients are kept in this position for at least 16 hours at a time over several consecutive days. Sessions shorter than 12 hours showed no benefit.
Recovery and Long-Term Effects
Surviving ARDS is only the beginning. ICU stays are often measured in weeks, and the recovery process extends far beyond hospital discharge. Some survivors make a full recovery, but many face lasting consequences that affect daily life for months or years.
Breathing problems are the most common lingering issue. Many survivors gradually regain most of their lung function over several months to two years, but the process is slow. Even people who recover well typically deal with shortness of breath and fatigue in the meantime, and some need supplemental oxygen at home for months after leaving the hospital. A smaller number have permanent breathing impairment.
The effects extend well beyond the lungs. Prolonged time on a ventilator and in the ICU causes significant muscle wasting and weakness, sometimes so severe that patients need to relearn basic movements like standing and walking. Cognitive problems, including difficulty with memory, concentration, and processing speed, are reported by many survivors. Depression is also common, both as a direct effect of critical illness on the brain and as a response to the long, difficult recovery process. Taken together, these physical, cognitive, and psychological aftereffects are sometimes referred to as post-intensive care syndrome, and they can persist long after the lungs themselves have healed.