The Pulmonary and Critical Care Division of Harborview Medical Center, University of Washington School of Medicine, Seattle, Washington has granted permission to include this article in the Northwest ARDS Support Network website.

"WHAT IS ARDS, ANYWAY?"

"Four of the most frightening letters I have ever had to deal with."

That was how a mother, whose son eventually recovered from ARDS, once described this disease. Acute Respiratory Distress Syndrome, or ARDS (also referred to as Adult Respiratory Distress Syndrome), is a form of sudden and often severe lung failure. Lung failure means the lungs can no longer carry out their normal function of getting oxygen into the blood and removing carbon dioxide from the body. In order to understand how ARDS can cause this, it is important first to review how the lung works.

Air, which contains oxygen, is inhaled through the nose and mouth. It then passes into the windpipe (trachea). From the trachea the air flows through tubes called bronchi. These bronchial tubes go to the microscopic air sacs called alveoli. Very small blood vessels (capillaries) sit next to these air sacs. Oxygen passes out of the air sacs into the bloodstream and carbon dioxide passes from the bloodstream into the air sacs. The carbon dioxide is then exhaled. Unfortunately, ARDS prevents this normal process from taking place.

ARDS causes the lungs to become inflamed. Sometimes the inflammation can be mild but it is often very severe. ARDS usually involves both lungs; in the early stages of the illness the inflammation can start in one lung, but often spreads to involve the other lung as well. When this inflammation occurs, it causes a great deal of lung damage, specifically to the alveoli and the capillaries.

When alveoli are damaged, they can collapse and lose the ability to receive oxygen. When capillaries are damaged, they leak fluid (edema) into the lungs and alveoli. With some alveoli collapsed and other alveoli filled with fluid, it becomes very difficult for the lungs to absorb oxygen and get rid of carbon dioxide. If this inflammation continues, the lung, like any other part of the body, can become scarred as it tries to heal itself. When that happens, much of the fluid in the lungs becomes replaced by scar tissue (fibrosis). If too much fibrosis occurs, it will also interfere with the exchange of oxygen and carbon dioxide.

ARDS often comes on very quickly, but it is not always easy to diagnose. It can be easily confused with pneumonia (or "double pneumonia" if both lungs are involved) and with congestive heart failure, two other conditions that can lead to inflammation and edema collection in the lungs. However, pneumonia results from an infection in the lung whereas ARDS is inflammation without direct infection. Congestive heart failure causes fluid to back up into the lungs because the heart is weak and loses its ability to pump normally, but there is no actual damage to the lungs. A chest X-ray would be abnormal in all three of these situations and sometimes it is very difficult to tell them apart. When that happens, other tests and procedures are sometimes necessary to properly diagnose ARDS.

To summarize, ARDS occurs when there is severe inflammation in both lungs resulting in an inability of the lungs to function properly.  The following questions and answers discuss the causes, prognosis and treatment of ARDS.

WHAT CAUSES ARDS?
HOW DO YOU GET ARDS?

"He was sick enough already and now this had to happen!"

What causes this widespread lung inflammation called ARDS? Although this sounds like a simple question, the answer is not well known. There is a large amount of scientific information that supports many different theories about how ARDS develops, but the truth is that no one knows the precise reason why ARDS occurs.

What is known, however, is that ARDS can be caused by two basic mechanisms. The first is a direct physical or toxic injury to the lungs. Examples of this include the inhalation of vomited stomach contents (aspiration), smoke or other toxic fumes, and a severe "bruising" of the lungs that usually occurs after a severe blow to the chest.

The other mechanism that causes ARDS is more common but is more difficult to understand. When a person is very sick or the body severely injured, some chemical signals are released into the bloodstream. These signals reach the lung, and the lung reacts to these messages by becoming inflamed, thus causing lung failure. Examples of this type of indirect lung injury include the presence of a severe infection somewhere in the body (sepsis), a severe injury (trauma) to some part of the body, severe bleeding that requires many units of blood (massive transfusion), and some types of drug overdose. There are several other rare causes of ARDS, but the two most common causes are sepsis and severe trauma.  

Not everyone who has these problems, however, develops ARDS.  This is fortunate since all of the above problems are relatively common. So why should some patients with sepsis or trauma develop ARDS and not others? There are not any good answers to this question. It does not appear that cigarette smoking, or the presence of other lung diseases such as asthma, emphysema, chronic bronchitis or lung cancer, makes one more susceptible to ARDS. It seems that only a small percentage of the patients who are at risk for ARDS because of their other illnesses or injuries actually develop ARDS. We cannot predict with any certainty who will get ARDS and who will escape it. This unpredictable nature makes ARDS a very frustrating complication of other illnesses that may be serious enough by themselves.

HOW COMMON IS ARDS?

"I had never even heard of ARDS until my son John was in a car crash."

This is an all too common statement from people affected by ARDS. Almost everyone knows about cancer, strokes, and heart attacks, but most people have never even heard of ARDS until someone they know develops the disease. Yet the statistics are surprising. It is estimated that there are approximately 150,000 cases of ARDS each year in the U.S. alone.

HOW SERIOUS IS ARDS?

ARDS is not only more common than most people think, it is also a very serious problem. Since ARDS was first described in 1967, the prognosis has improved only slightly despite rapid advancements in medical science and technology. Statistics reveal that approximately one half of the 150,000 people who develop ARDS each year will survive.

Younger people and those who have fewer chronic health problems are more likely to recover. It is known that people with a milder form of ARDS tend to have a better chance of recovering than those with a more severe form of the illness.

It is also known that the cause of a patient's ARDS helps predict that patient's chances for survival. For example, patients who develop ARDS due to sepsis usually do not do as well as patients whose ARDS is related to trauma. Finally, those patients who do survive after developing ARDS usually improve over several months with a return to normal or near normal lung function.

As suggested above, very few cases of ARDS are alike. Some people get better quickly within a matter of several days and others take weeks or months to improve. Some people have no complications and others seem to develop every possible complication of ARDS. Finally, some people will die quickly while others die after a long and trying illness.

Dealing with the seriousness and the unpredictability of ARDS is extremely frustrating and can be emotionally devastating for patients, family, friends, and for the patient's doctors and nurses as well. Hopefully, current and future research will make ARDS a more treatable and hence much less serious and more predictable illness than it is now.

HOW IS ARDS TREATED?

At this time there is no specific treatment for ARDS although several therapies are now being tested. Patients with ARDS are supported on a breathing machine (ventilator) to maintain enough oxygen in the bloodstream while they recover from ARDS and their other injuries or illness. When a person is on a ventilator there is an artificial airway or endotracheal tube, a tube that goes into the windpipe or trachea through the mouth or nose or through a surgical incision in the neck. This tube is connected to the ventilator. While in place, the tube temporarily interferes with the patient's ability to speak since it passes between the vocal cords. Positive end-expiratory pressure (PEEP), is a special setting on the ventilator that keeps the lungs expanded to help get oxygen from the lungs into the bloodstream. Patients may be placed on a special bed, such as an "air bed" or a rotating bed to position them properly to help prevent complications such a bed sores and pneumonia.

Medications are used to treat the original injury or illness as well as complications that may occur. Some general categories of medications are listed below:

ANTIBIOTICS - drugs that fight infection
ANALGESICS - pain-relieving drugs
SEDATIVES - anti-anxiety drugs
CARDIOVASCULAR DRUGS - raise the blood pressure or stimulate the heart
MUSCLE RELAXANTS - drugs that prevent voluntary muscle movement and reduce the body's demand for oxygen.

WHAT IS A VENTILATOR AND HOW 
IS IT USED TO SUPPORT ARDS PATIENTS?
WHAT DOES IT FEEL LIKE TO HAVE IT BREATHE FOR YOU?

A ventilator is a breathing machine that is connected to a tube in the patient's windpipe, an endotracheal tube or tracheostomy tube. It can breathe completely for a patient or assist a patient's own breathing. There are a number of controls or settings on the ventilator that are ordered by the doctor. The amount of oxygen that the patient receives can be adjusted. The air we normally breathe contains 21% oxygen. It is possible to give a patient as much as 100% oxygen through a ventilator but this, too, can cause damage to the lung so an effort is made to give the lowest amount of oxygen necessary. PEEP is one way to avoid giving the patient high levels of oxygen.

PEEP, which is positive end expiratory pressure, is adjusted through the ventilator. It keeps some pressure in the lungs at the end of each breath.  This pressure helps keep the alveoli, the tiny air sacs where oxygen passes into the bloodstream, from collapsing. The pressure is measured and carefully adjusted because there can be complications with high levels of PEEP. The amount of PEEP is often increased and decreased gradually but occasionally it is important to change the level of PEEP more quickly.

Other adjustments on the ventilator include the size of each breath (tidal volume) the patient receives and the number of breaths (respiratory rate) the patient receives each minute. The ventilator can be adjusted so that it does all of the breathing for the patient or so that the patient breathes partially on his or her own. These settings are adjusted, depending on the amount of oxygen and carbon dioxide in the blood as well as other tests of lung function.

The ventilator can sense when the patient takes a breath of his or her own, timing the set number of breaths to the patient's own breathing rhythm. Often the amount of breathing needed by the body is much more than the patient is able to do on his or her own. The patient may require sedatives or relaxing drugs to help them breathe with the ventilator. PEEP is an odd sensation because it feels like the lungs do not empty at the end of each breath. However, the patient may also feel better because it can make breathing easier and gets more oxygen into the bloodstream.

NOW THAT YOU HAVE ARDS, WHAT HAPPENS NEXT?

The course of events after ARDS has developed is determined, in part, by the degree of abnormality in lung function and in part by the illness or injury that led to the development of ARDS. Most patients need to be on the ventilator for several days. If the underlying medical condition has stabilized and no new complications develop, then it is likely that the lungs will begin to heal, allowing the patient to do more and more of the breathing on his or her own, and the ventilator may be removed within a week. This happens in about a third of the patients with ARDS.

In another third of the patients, the underlying condition is so severe that even intensive therapy is not able to reverse the abnormalities. Such patients may have, or develop, progressive or irreversible damage to other vital organs. Sometimes the healing process is further compromised by chronic illnesses or advanced age. Although intensive medical care is sometimes able to prolong survival by a few days, such patients often die within the first week.

Those who survive the first week but whose ARDS has not yet improved enter what might be termed the "chronic phase" of ARDS. These patients need to be on the ventilator for up to three or four weeks and sometimes longer. Sometimes this happens because of the original injury or illness, but often it occurs because of other complications. Even when there is a satisfactory response to treatment of the underlying condition, a small number of patients have persistent inflammation in the lung and seem unable to begin the healing process. The outcome in patients who enter this chronic phase is dependent on reversing the inflammation and preventing or treating complications, especially infection. Sometimes unusual or experimental treatments may be considered. About half of the patients with "chronic" ARDS will get better and leave the hospital, but recovery is slow and may be incomplete.

WHAT ARE THE COMMON COMPLICATIONS THAT OCCUR WITH ARDS?

Barotrauma is one complication that may occur with ARDS. The word means injury caused by pressure. In ARDS, the lung is weakened and, combined with the high pressures of the ventilator, there is a risk of lung rupture. This is called a pneumothorax and leads to an accumulation of air in the pleural cavity. The pleura form a smooth, moist lining around the lungs. Normally, there is no air in the small space between the pleura and the lungs. When a pneumothorax develops, a chest tube is inserted by a physician through the patient's chest wall, into the pleural cavity outside the partially collapsed lung to remove the air. The tube is connected to a suction machine to help the lung reinflate. The suction machine or wall suction is used until the patient's lung is healed enough to stay inflated on its own. A pneumothorax may also be related to trauma or to other procedures used in treating the patient.

Bacterial infections are a common complication of ARDS and contribute to continued lung injury. The lung is the most common site of infection. Lung infection or pneumonia may be difficult to diagnose in a patient with ARDS because, as we've said, the patient's chest X-ray is already very abnormal. The nurse or respiratory therapist will obtain a specimen of phlegm or sputum from the lungs by suctioning through the patient's endotracheal tube when the patient has a fever. The specimen is sent to the laboratory for a culture, a method that allows any bacteria that are present to grow. In this way, bacteria that may be causing an infection can be identified and the sensitivity of the bacteria to antibiotics can be determined. Sometimes the doctor may want to obtain a sputum specimen from deeper in the lung. In that case a bronchoscopy may be performed by a physician. The bronchoscope is a flexible tube like instrument that contains a light and an eyepiece. It is inserted through the patient's endotracheal tube and the doctor can see inside the patient's airways. A special small brush is passed through the bronchoscope and into an area of the lung that appears infected. This brush is then sent to the laboratory for culture and sensitivity tests. Sedative medications are used to keep the patient comfortable and the ventilator is adjusted during this procedure so the patient's breathing continues without a problem.

Bacterial infections may also occur in other parts of the body such as the bloodstream, the urinary tract, sinuses, skin or muscle, the abdomen or the spinal fluid.

All of these areas are tested for infection in various ways. Antibiotics are used when an infection is present or suspected. Antibiotics are powerful drugs and must be used carefully. Bacteria may become resistant, especially if the antibiotics are used when they are not needed. When a patient has been treated with many antibiotics for a long period of time they are at risk of developing a fungal or yeast infection, which may cause further problems.

Abnormal organ function in addition to the lung failure caused by ARDS can develop and may involve the liver, kidney, brain, blood or immune system. These organ dysfunctions may be related to the underlying illness, to treatment or may occur through the same inflammatory process which injured the lungs. If kidney failure occurs, the patient may be maintained with dialysis, which is treatment that removes waste products from the patient's blood by circulating the blood through a special machine.

Liver failure is a difficult problem to treat because there is no replacement for the many functions that the liver performs in our bodies. Ongoing infections, despite appropriate antibiotic therapy, may be due to dysfunction of the immune system. Patients may become unconscious or confused when they previously have been alert and oriented due to dysfunction of the brain or central nervous system. Blood transfusions or replacement of certain elements of the blood, such as platelets, which are needed for proper clotting of the blood, may be required.

IF YOU SURVIVE ARDS, WHAT HAPPENS TO YOUR LUNG FUNCTION,
AND HOW LONG DOES IT TAKE FOR YOUR LUNGS TO RECOVER?

Most patients who survive ARDS have a remarkable degree of recovery of lung function, given the severity of the initial injury. Recovery time for each patient is variable. Patients recover at different rates and have different end points. However, most patients recover the great majority of lung function in the first three to six months and then recovery levels off up to a year and beyond. Only a few patients have decreased lung function forever.

Recovery is defined as how well the patient is able to take a deep breath and how well oxygen is able to go from the lungs into the bloodstream. We measure these in tests called pulmonary function tests. Patients may receive these tests at discharge from the hospital and at three to six months after hospital discharge.

During the first three months after discharge from the hospital, some patients may feel short of breath, have a cough, produce phlegm, and feel fatigued. Some may be required to use supplemental oxygen for a period of time when they go home. Many patients experience hoarseness, which is due to irritation from the endotracheal tube used while they were on a ventilator. As the months go by, however, patients may feel like they can take a deep breath more easily, walk farther distances, or get tired less easily. Again all of these symptoms usually get better, usually within the first six months after leaving the hospital.

WHAT RESEARCH IS BEING DONE ON ARDS?

Research is being conducted all over the world in attempts to better understand the causes of ARDS, why some people get it and others do not, and on ways to improve the treatment of ARDS. Unfortunately, no magic cure for this illness has been found, but there are many new exciting possibilities being tested.

The major areas of ARDS research have revolved around four topics:

1. treatments to reduce the risk of developing ARDS;

2. therapy to reduce or reverse the inflammation in the lungs that is ARDS;

3. improvements in the supportive therapy for ARDS, such as better ventilators, and ways to minimize the risk of complications; and

4. therapy to improve the abnormal lung function that exists once ARDS occurs. 

Even a small but significant improvement in mortality could prevent many of the estimated 75,000 deaths each year from ARDS.  Continued research efforts increase the opportunity to provide improvements.

Harborview Medical Center, located in Seattle, Washington, is a national center for ARDS research and is involved in many of the above mentioned studies. It is supported by the Respiratory Distress Syndrome Foundation in Montgomeryville, Pennsylvania.