Acute Respiratory Distress Syndrome
Acute respiratory distress syndrome is a type of 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.
Acute respiratory distress syndrome (ARDS) is a medical emergency that often occurs in people who have severe lung disease. However, even people who previously had normal lungs can develop ARDS. This syndrome is sometimes called adult respiratory distress syndrome, although it can occur in children. The less severe form of this syndrome is called acute lung injury.
 See the sidebar Causes of Acute Respiratory Distress Syndrome.
Causes
Any disease or condition that injures the lungs can cause ARDS. About one third of the people with the syndrome develop it as a consequence of a severe, widespread infection (sepsis). Other people develop ARDS because of significant damage at first to another organ, such as the pancreas. Damage to the pancreas can release proteins such as enzymes and cytokines, which are capable of injuring other organs and tissues in the body, including the lungs.
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 Section 4, Chapter 48)) may also result because of a reduction in surfactant activity. Surfactant is the liquid that coats the inside surface of the alveoli, helping to hold 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.
Symptoms and Diagnosis
ARDS usually develops within 24 to 48 hours of the original injury or disease. The person first experiences shortness of breath, usually with rapid, shallow breathing. Through a stethoscope, a doctor may hear crackling or wheezing sounds in the lungs, or the doctor may hear nothing abnormal. Because of low oxygen levels in the blood, the skin may become mottled or blue (cyanosis), and other organs such as the heart and brain may malfunction, resulting in a rapid heart rate, confusion, and lethargy.
The oxygen deprivation caused by ARDS and the leakage into the bloodstream of certain proteins (cytokines) produced by 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.
Analysis of a blood sample taken from an artery indicates low levels of oxygen in the blood (see Section 4, Chapter 39), 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 Section 3, Chapter 25).
Treatment and Prognosis
People with ARDS are treated in an intensive care unit. Successful treatment depends on correcting the underlying cause; oxygen therapy, which is vital to correcting low oxygen levels, is combined with treatment of the underlying cause.
If oxygen delivered by a face mask or nasal prongs does not correct the low blood oxygen levels, or if excessively high doses of inhaled oxygen are required, a ventilator must be used; this treatment is called mechanical ventilation (see Section 4, Chapter 55). A ventilator delivers oxygen-rich air under pressure using a tube inserted through the mouth into the trachea. For people who have ARDS, the ventilator pressure is delivered both during the inhaled breath and at a lower pressure during exhalation (called positive end-expiratory pressure). The pressure supplied by the ventilator during and after a breath opens collapsed (atelectatic) regions of the lung and allows oxygen to move through the walls of the injured lungs into the blood.
The pressure and volume of air that the ventilator delivers to the lungs with each breath must be adjusted to help keep the small airways and alveoli open while avoiding rupturing the fragile air sacs, which can lead to air accumulating around the lung and collapsing it (called pneumothorax (see Section 4, Chapter 52)). Monitoring and adjusting the pressure also ensures that the lungs do not receive an excessive concentration of oxygen, which can damage the lungs and worsen ARDS. Limiting the volume of air with each breath also ensures that the lungs are not further damaged from overstretching. Sedating drugs such as midazolam are often given to calm the person and reduce their sense of being short of breath.
In some cases, diuretic drugs may be needed to help remove fluid from the lungs. Antibiotics are usually needed for people who develop bacterial pneumonia. Some people may benefit from the use of intravenous corticosteroids in the later stages of ARDS. Other supportive treatment, such as providing liquid nutritional supplements through a small feeding tube placed in the stomach or small intestine (see Section 12, Chapter 153), is also important because dehydration or malnutrition may increase the likelihood of multiple organ system failure. If a person cannot be adequately fed in this way, food may need to be delivered intravenously.
Without prompt treatment, the severe oxygen deprivation resulting from ARDS causes death in 90% of people with the disorder. However, with appropriate treatment, about half of all people with severe 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 improve over a few months after the person is taken off the ventilator.
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