Acute respiratory distress syndrome is a type of respiratory (lung) failure resulting from many different disorders that cause fluid to accumulate in the lungs and oxygen levels in the blood to be too low.
The acute respiratory distress syndrome (ARDS) is a medical emergency. It may occur in people who already have lung disease or in those with previously normal lungs. This syndrome used to be called the adult respiratory distress syndrome, although it can occur in children. The less severe form of this syndrome is called acute lung injury (ALI).
Any disease or condition that injures the lungs can cause ARDS. More than half of the people with ARDS develop it as a consequence of a severe, widespread infection (sepsis) or pneumonia.
When the small air sacs (alveoli) and tiny blood vessels (capillaries) of the lungs are injured, blood and fluid leak into the spaces between the air sacs and eventually into the sacs themselves. Collapse of many alveoli (a condition called atelectasis—see Bronchiectasis and Atelectasis: Atelectasis) may also result because of a reduction in surfactant, a liquid that coats the inside surface of the alveoli and helps to keep them open. Fluid in the alveoli and the collapse of many alveoli interfere with the movement of oxygen from inhaled air into the blood, causing oxygen levels in the blood to decrease sharply. Movement of carbon dioxide from the blood to air that is exhaled is affected less, and levels of carbon dioxide in the blood change very little.
The decreased blood oxygen levels caused by ARDS and the leakage into the bloodstream of certain proteins (cytokines) produced by injured lung cells and white blood cells can lead to inflammation and complications in other organs. Failure of several organs (a condition called multiple organ system failure) may also result. Organ failure can begin soon after the onset of ARDS or days or weeks later. Additionally, people with ARDS are less able to fight lung infections, and they tend to develop bacterial pneumonia.
ARDS usually develops within 24 to 48 hours of the original injury or disease but may take as long as 4 or 5 days to occur. The person first experiences shortness of breath, usually with rapid, shallow breathing. Using a stethoscope, a doctor may hear crackling or wheezing sounds in the lungs. The skin may become mottled or blue (cyanosis) because of low oxygen levels in the blood, and other organs such as the heart and brain may malfunction, resulting in a rapid heart rate, abnormal heart rhythms (arrhythmias), confusion, and lethargy.
Analysis of a blood sample taken from an artery indicates low levels of oxygen in the blood, and chest x-rays show fluid in spaces that should be filled with air. Further tests may be needed to ensure that heart failure is not the cause of the problem (see Heart Failure).
Without prompt treatment, the severe oxygen deprivation causes death in 90% of people with ARDS. However, with appropriate treatment, about three fourths of people with ARDS survive.
People who respond promptly to treatment usually recover completely with few or no long-term lung abnormalities. Those whose treatment involves long periods on a ventilator are more likely to develop lung scarring. Such scarring may decrease over a few months after the person is taken off the ventilator. Lung scarring, if extensive, can impair lung function in ways that are noticeable during certain day-to-day activities. Less extensive scarring may impair lung function only when the lungs are stressed, such as during exercise or an illness.
Many people lose large amounts of weight and muscle during the illness. Rehabilitation in the hospital can help them regain their strength and independence.
People with ARDS are treated in an intensive care unit. Successful treatment usually depends on treating the underlying disorder (for example, pneumonia). Oxygen therapy, which is vital to correcting low oxygen levels, also is given.
If oxygen delivered by a face mask or nasal prongs does not correct the low blood oxygen levels, or if very high doses of inhaled oxygen are required, mechanical ventilation must be used. Usually a ventilator delivers oxygen-rich air under pressure using a tube inserted through the mouth into the windpipe (trachea). For people who have ARDS, the ventilator pressure is delivered during the inhaled breath and at a lower pressure during exhalation (called positive end-expiratory pressure), which helps keep the alveoli open at the end of exhalation.
Last full review/revision January 2008 by Brian K. Gehlbach, MD; Jesse B. Hall, MD