What
is ARDS anyway?
Acute
Respiratory Distress Syndrome (ARDS) is a form of sudden and often severe
lung failure. Lung failure means the lungs can no longer carry out their
normal functions of getting oxygen into the blood and removing carbon dioxide
from the body. To understand how ARDS can cause lung failure it is important
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. The bronchial tubes carry the air 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 blood stream into the air sacs. The carbon
dioxide is then exhaled. Unfortunately, ARDS interferes with the normal
processes in the lungs.
ARDS causes the lungs to become inflamed. The inflammation can be mild but
more often it is very severe. ARDS involves both lungs; in the early stages
of the illness the inflammation can start in one lung, but it usually spreads
to the other lung. When inflammation occurs in the lungs it causes a great
deal of 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 inflammation continues, the lung—like any other
part of the body—tries to heal itself and can become scarred as fluid
in the lungs is replaced by scar tissue (fibrosis). If extensive fibrosis
occurs, it permanently interferes with the exchange of oxygen and carbon
dioxide. Our treatment goal is to minimize the inflammation that causes
fibrosis as early as possible. ARDS often develops quickly. It can be easily
confused with pneumonia; however, pneumonia results from an infection in
the lungs whereas ARDS is inflammation with or without infections.
To summarize, ARDS occurs when there is severe inflammation in both lungs
resulting in an inability of the lungs to function properly.
What causes the widespread lung inflammation called ARDS?
ARDS
can be caused by a direct physical or toxic injury to the lungs. Examples
include the inhalation of vomited stomach contents (aspiration), inhalation
of smoke or other toxic fumes, pneumonia, or a severe “bruising”
of the lungs (usually one that occurs after a severe blow to the chest).
There is another common, though complicated mechanism that causes ARDS.
When a person is very sick or the body is severely injured, large quantities
of cytokines. (chemical signals) are released into the bloodstream. When
cytokines go through the lung blood vessels, the lungs react by becoming
inflamed, resulting in lung failure. Examples of sources for indirect lung
injury caused by these cytokine signals include the presence of a severe
infection somewhere in the body (sepsis), a severe injury to some part of
the body (trauma), severe bleeding that requires many units of blood (massive
transfusion), and some types of drug overdoses. There are several other
rare causes of ARDS, but the two most common causes are sepsis and severe
trauma. Fortunately, not everyone with the aforementioned, relatively common
problems develops ARDS. So why do some patients with sepsis or trauma develop
ARDS, but not others?
Only a small percentage of patients who are at risk for ARDS because of
other illnesses or injuries actually develop ARDS. We cannot predict with
any certainty who will develop ARDS or who will escape it. The unpredictable
nature of ARDS makes it a frustrating complication of other serious illnesses.
How
common is ARDS?
People
affected by ARDS often ask this question. Almost everyone knows about cancer,
strokes, and heart attacks, but most people have never 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 United States alone.
How
serious is ARDS/Incidence?
ARDS
is more common than most people think; and it is a very serious disease.
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 not survive the disease.
It is 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.
Very few cases of ARDS are alike. Some people get better quickly (within
several days), whereas others take weeks or months to improve. Some people
have no complications while others develop multiple complications of ARDS.
Dealing with the seriousness and the unpredictability of ARDS is frustrating
and can be emotionally devastating for patients, family, friends, and for
the patient’s doctors and nurses. Hopefully, current and future research
will make ARDS a more treatable, less serious, and more predictable illness
than it is now.
How
is ARDS treated?
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. While a person is on a ventilator, there is an
artificial airway or endotracheal tube in place to aid in breathing. The
endotracheal tube is positioned in the mouth, nose, or trachea. 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.
PEEP, which is positive end expiratory pressure, is adjusted through the
ventilator. It keeps some pressure in the alveoli, the tiny air sacs of
the lung. Additional pressure in the alveoli keeps them 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 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 on his or her own,
timing the set number of breaths to the patient’s own 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.
The ventilator support, which may be adjusted frequently by the doctors,
is designed to assist the patient’s body in performing the critical
lung functions of getting oxygen into the blood and carbon dioxide removed
from the body. One way to measure this is the oxygen saturation level, which
appears on a monitor at the patient’s bedside. It is important to know
what is going on with the entire body—the vital organs and body processes—because
while ARDS is a pulmonary condition involving the lungs, the effects of
ARDS strike throughout the body.
What
are some common complictions that occur with ARDS?
Bacterial
infections, a common complication of ARDS, contribute to continued lung
injury. The lung is the most common site of infection in the body. Lung
infection or pneumonia may be difficult to diagnose in a patient with ARDS
because the patient’s chest X-ray is already very abnormal. A secretion
specimen, obtained from deep in the lung using a procedure called a bronchoscopy,
can help to diagnose infection. The bronchoscope is a flexible, tube-like
instrument that contains a light and an eyepiece; it is inserted through
the patient’s endotracheal tube so the doctor can see inside the patient’s
airways. Sedative medications are used during the procedure to keep the
patient comfortable and the ventilator is adjusted so the patient’s
breathing continues without interruption.
Bacterial infections may also occur in other parts of the body such as the
bloodstream, the urinary tract, sinuses, skin, 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. A patient who has been
treated with many antibiotics for a long period of time may be at risk for
developing a bacterial infection resistant to antibiotics, or fungal (yeast)
infection, so antibiotic treatment is administered only when indicated.
In ARDS the lung is weakened and the combination of the high pressures of
the ventilator and the volume of air used to inflate the lungs pose a risk
of lung rupture called a pneumothorax. If pneumothorax occurs, it leads
to an accumulation of air in the pleural cavity and partial collapse of
the affected lung. 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 (air is present in the space between the pleura
and the lungs) 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. It is common for the chest tube to drain fluid that may
look bloody or amber in color. This fluid is collected in a device called
a chest tube drainage system. The chest tube is removed when the physician
determines that the lung is returning to normal function.
What
happens to your lung function as a survivor of ARDS, and how long does it
take your lungs to recover?
Most
ARDS survivors have a remarkable degree of recovery of lung function, despite
the severity of the initial injury. Recovery time for each patient is variable;
patients recover at different rates. Most patients recover the great majority
of lung function in the first three to six months, then recovery may slowly
continue for approximately a year.
