Acute Respiratory Distress Syndrome

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Acute respiratory distress syndrome (ARDS) is an acute, life-threatening disorder of the lungs. Patients with ARDS experience severe shortness of breath and often require mechanical ventilation (life support) because of respiratory failure. ARDS is not a specific disease; instead, it is a type of lung dysfunction that is associated with a variety of diseases, such as pneumonia, shock, sepsis (a severe infection in the body), and trauma. ARDS can be confused with congestive heart failure, another common condition that can also cause respiratory distress.

ARDS leads to low oxygen levels in the blood even with high amounts of inhaled oxygen and mechanical support. Because adequate oxygen is needed for the rest of the body to function, ARDS is very serious and often fatal.


Other Names

Other names for ARDS include:

  • Acute lung injury
  • Increased permeability pulmonary edema
  • Noncardiac pulmonary edema
  • Adult respiratory distress syndrome

ARDS used to be called stiff lung, shock lung, and wet lung.

The term acute lung injury (ALI) is sometimes used in the same setting as ARDS, but also includes less severe lung injury.

Signs and Symptoms

The major signs and symptoms of ARDS are:

  • Shortness of breath, including the inability to speak in full sentences and retractions (using the rib muscles to help inhale).
  • Fast, labored breathing
  • A bluish skin color (due to a low level of oxygen in the blood)
  • A lower amount of oxygen in the blood

Many other conditions can cause similar symptoms of pulmonary distress, including pneumonia, asthma, heart failure, pulmonary embolus, pulmonary edema, and many more. For this reason, the diagnosis of ARDS must meet certain criteria. The definition and diagnosis of ARDS is based on results of tests such as x-rays and blood tests. Other signs and symptoms include low blood pressure, lightheadedness, confusion, and fatigue.

Clinicians watch for these signs and symptoms in patients who have conditions that might lead to ARDS. People who develop ARDS may be too sick to complain about having trouble breathing or other related symptoms. If a patient shows signs of developing ARDS, doctors will do tests to confirm that ARDS is the problem.


ARDS usually occurs within the first 24 hours of a critical illness. A few examples of predisposing conditions that may lead to ARDS:

  • Severe infections (sepsis)
  • Acute pancreatitis
  • Shock—events leading to low blood pressure that does not bring adequate blood to the body
  • Inhaling gastric contents, leading to pneumonia
  • Lung bruising (contusion) from a direct blow
  • Severe injury
  • Inhaling toxic gases
  • Near-drowning
  • Repeated blood transfusion (transfusion-related acute lung injury [TRALI] ["Shock Lung"])

To understand exactly how ARDS develops, it is necessary to understand lung function.

Physiology of the lung

With each breath, air passes through the nose and mouth into the trachea (windpipe). The air then travels to the lungs via the bronchus and bronchioles to the actual lung tissue. Finally, the last portion of the lung reached are the air sacs, called alveoli. This is where oxygen is transferred to the bloodstream, and, wastes like carbon dioxide are removed in the opposite manner through exhalation.

Small blood vessels called capillaries run through the walls of the alveoli. Capillaries are the body's smallest blood vessels, and their wall lining is so thin that the body's nutrients can cross them. In this case, the nutrient is oxygen. The red blood cells in the capillaries run through in single file. As they are squeezed against the vessel wall on the surface of the alveoli, the red blood cells pick up oxygen. The job of the red blood cells is mainly to transfer oxygen, and it is done by the portion of the red blood cell called hemoglobin. Oxygen binds to the hemoglobin within the red blood cell, and it is then carried into the bloodstream. Blood carries the oxygen to all parts of the body, including the body's organs, where it is released by the hemoglobin and used to make energy.

Mechanism of ARDS

When the body is challenged with very severe infection or injury, it uses all of its available resources to fight these conditions. One way it does this is with inflammation, which involves the release of molecules called cytokines that stimulate the immune system to fight infection and to start the repair process. One way that cytokines do this is by making capillaries leakier than normal to allow white blood cells and nutrients in the bloodstream to reach these affected areas of the body. This works well in small or moderate injuries or infection, but when the damage to the body is too severe, the number of cytokines released by the body becomes too much for the organs to handle. ARDS is the result of this process in the lung.

In ARDS, massive cytokine release causes the lung's capillaries to leak more fluid than normal into the alveoli. This fluid blocks adequate exchange of oxygen and other nutrients from the capillaries to the alveoli. Thus the patient's oxygen levels are decreased because of the inability to transfer it from the lungs out to the bloodstream. The lungs become stiff, and the patient must work much harder to move air in and out of the lungs. When this occurs, the body's organs do not receive enough oxygen. Without oxygen, the organs may not work properly or may stop working. Since oxygen is required to repair the damage caused by overwhelming infections, the disease process that caused the ARDS is then worsened by lack of oxygen.

Underlying conditions in ARDS

Many conditions or factors can directly or indirectly lead to injury of the lungs or ARDS.

Direct lung injury

Conditions that can directly injure the lungs include:

  • Pneumonia
  • Aspiration (inhalation of food, beverages, or vomit)
  • Breathing of harmful fumes or smoke
  • Use of a ventilator, a machine that helps people breathe, but can injure the lungs
  • Near-drowning

Indirect lung injury

Conditions that can indirectly injure the lungs include:

  • Sepsis, a condition in which bacteria infect the bloodstream.
  • Severe bleeding due to an injury to the body or having many blood transfusions
  • An injury to the chest or head
  • Pancreatitis, a condition in which the pancreas becomes irritated or infected
  • Fat embolus, a condition in which fat released from a broken bone blocks an artery within the lung. This is similar to a pulmonary embolus.
  • Drug overdose

It is not clear why some very sick or injured people develop ARDS and others do not. Researchers are trying to find out why ARDS develops and how to prevent it.


Clinical diagnosis

ARDS is a syndrome that must meet certain criteria to be diagnosed.

  • It must occur acutely (suddenly).
  • X-ray of the chest must show disease ("infiltrates") in both lungs
  • Blood tests must show that the lung surface is not working efficiently to carry and transfer oxygen. This requires the use of arterial blood gas tests. To diagnose ARDS, the amount of oxygen in the air being breathed by the patient must be much higher than the amount of blood that enters the arteries.
  • It must be determined that the fluid build-up in the alveoli is not due to to fluid overload caused by other systems in the body. For example, in congestive heart failure, the heart is unable to effectively pump, and that leads to a backup of fluid into the lungs. ARDS is injury to the lung tissue itself causing pulmonary edema (fluid in the lung).

Initial tests

The first tests done are:

  • An arterial blood gas test (ABG). This blood test shows the oxygen level in the blood. A low level of oxygen in the blood may be a sign of any pulmonary disease including ARDS.
  • Chest x-ray. This test is used to take a picture of the lungs. In ARDS, it should reveal extra fluid (infiltrates) in both lungs.
  • Blood tests, such as a complete blood count, blood chemistries, and blood cultures. These tests help to identify the cause of ARDS, such as an infection.
  • Sputum cultures. Again, this would be used to identify a lung infection causing ARDS. This test studies whether there are any bacteria in the airways or the lungs.
  • pulmonary function test may be of help.
  • peak expiratory flow rate(PEFR).

Other tests

Other tests used to diagnose ARDS include:

  • Computed tomography, or a CT scan. This test uses a computer to take more detailed x-rays of the lungs. It may show fluid in the lungs, signs of pneumonia, or a lung tumor.
  • Cardiac tests that look for signs of heart failure. Heart failure is a condition in which the heart is not functioning properly. This condition can cause fluid to build up in the lungs.


Treating ARDS is difficult. The main aim of treatment is to increase the oxygen levels in the blood and to prevent organ failure from lack of oxygen. Oxygen is the first treatment. The easiest method to administer oxygen is by inhalation, either through a face mask or through a special tubing that fits in the nose (a nasal cannula). These methods, though, are not effective for the treatment of ARDS because the hypoxemia (decreased blood levels of oxygen) is so severe. In ARDS, a breathing machine, or mechanical ventilator, is required. A ventilator consists of a tube placed directly into the patient's trachea while a machine regulates the amount of oxygen administered and the rate at which the patient breathes. The patient is sedated and paralyzed because of the discomfort the tube causes. The other treatments in ARDS aim at aggressive treatment of the condition causing the ARDS.[1]


The air from the atmosphere that we breathe is 20% (one-fifth) oxygen. This means that oxygen can be added to the air that people with ARDS breathe in to increase the oxygen content. Pure (100%) oxygen can be inhaled for a short period of time; however, over long periods, it can cause damage to the lung surface. For this reason, doctors will use increased levels of oxygen but maintain levels below 100% in ARDS.

Intravenous fluid

Having enough fluid in the body, particularly the circulation, is important for producing adequate blood pressure. When there is too little fluid in the circulation, the blood pressure decreases and this can cause organs such as the kidneys to lose function. On the other hand, if there is too much fluid, the heart is stressed by the fluid overload. The body deals with this situation by leaking fluid from the circulation into the tissues (edema) and the lungs (pulmonary edema).

In ARDS, the lungs are already full of fluid from the cytokines, so it is important not to use too much fluid. On the other hand, enough fluid must be used to make sure that blood reaches the organs and they continue to function. For this reason, balancing the amount of fluid in the body is very important in ARDS patients, and doctors pay a lot of attention to this issue.

Mechanical ventilation

The mechanical ventilator is another name for a breathing machine, or life-support machine. This device is often used in ARDS. It forces air in and out of the lungs of patients who are too sick, weak, or unconscious to be able to breathe for themselves. In ARDS, one of the main problems is that the lungs fill with fluid and become very stiff. This takes a lot of effort for a patient who may already be very ill with another illness. Patients quickly become tired with this effort. The ventilator is used to allow breathing to continue without requiring this huge effort from the patient.

Unfortunately, mechanical ventilation provides its own problems. During ARDS, the alveoli again are under duress because of the fluid collection. There is decreased surface area along the alveoli for effective oxygen transfer, and they are likely to collapse (remember they are like sacs or balloons). To treat ARDS with mechanical ventilation, an extra mode on the ventilator is chosen to increase the pressure in the alveoli to hold them open like a balloon to aid in oxygenation of the bloodstream. This feature, though helpful, may cause damage and rupture the alveoli if the pressure is too high. This is monitored closely. It can cause lungs to collapse (pneumothorax) from excessive pressure (barotrauma). This added feature in mechanical ventilation to treat ARDS is very effective and is now standard protocol, but it is also carefully monitored.

Other treatments

One other therapy that may be employed is the use of blood transfusions if the blood count is too low. Having enough red blood cells improves the delivery of oxygen to the tissues.

There are a number of experimental treatments for ARDS. None has been shown to increase the chances of survival.

Living With ARDS

Some people fully recover from ARDS. Others continue to have health problems. After leaving the hospital, the following problems are common:

  • Shortness of breath: After treatment, lung function in most people who have ARDS improves within 3 months and is back to normal within 6 months. For others, it may take up to a year or more. Some people have breathing problems for the rest of their lives.
  • Tiredness and muscle weakness: Being in the hospital and on a ventilator can cause the muscles to become weak as they are not being used.
  • Depression: Many people who have had ARDS become depressed for a while after treatment. This is common in all people who survive intensive care unit treatment.
  • Problems with memory and thinking clearly: Certain medicines and a low oxygen level in the blood can cause these problems. This problem can be lifelong.

Chances of Developing ARDS

ARDS treatment has improved in recent years. As a result, more people are surviving ARDS. About 190,000 people in the United States each year are diagnosed with ARDS or its less severe counterpart, acute lung injury (ALI).[2] About 35%–40% of people with ARDS or ALI die.[2] The prognosis depends in part on the severity of the cause of the ARDS.

Over the long term, ARDS can cause chronic lung problems. This can be due to damage from the mechanical ventilator if high pressures were needed. Some people who survive recover completely. Others may have lasting damage to their lungs and other health problems.

Related Problems


ARDS often leads to other medical problems while in the hospital. The most common are infections, pleurisy, pneumothorax (collapsed lung), scarring of the lungs, and blood clots.

  • Infections. Being in the hospital and being bedridden for a long length of time can make patients prone to infections, such as pneumonia. Being on a ventilator also increases chances for infection because the machine does not allow coughing to clear the airways. Infections are treated with antibiotics.
  • Pneumothorax. This is a condition in which one or both lungs collapse due to a puncture of the lung tissue. Air collects in the space around the lungs. The pressure of the air from a ventilator can cause this condition. Pneumothorax can be treated with a chest tube, which is a tube placed through the chest wall into the space around the lung to remove the air.
  • Lung scarring. ARDS causes the lungs to become stiff (scarred) and makes it hard for them to expand and fill with air. Being on a ventilator also can cause lung scarring.
  • Blood Clots. Lying down for long periods can cause blood clots to form in the legs. A blood clot that forms in a vein deep in the body is called a deep vein thrombosis (DVT). This type of blood clot can break off, travel through the bloodstream to the lungs, and block blood flow. This is called a pulmonary embolus.

Clinical Trials

For a list of government-sponsored clinical trials on ARDS, visit here.

The pre-eminent US-based ARDS Research Network is funded by the NIH. It is called the ARDSNet. They have performed several important clinical trials related to ARDS that have helped to change modern therapeutic strategy.


  1. ↑ ARDS Support Center, Inc. Understanding ARDS.
  2. ↑ 2.0 2.1 Rubenfeld GD, Herridge MS. Epidemiology and outcomes of acute lung injury. Chest. 2007 Feb;131(2):554-62. Abstract | Full Text

External Links

  • American Lung Association: Adult (Acute) Respiratory Distress Syndrome (ARDS) Fact Sheet

Source: Medpedia content in its entirety

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