ACUTE LUNG INJURY (ALI);
ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

Critical illness, lung injuries and severe infections are examples of problems that can cause an inflammatory response within the lungs. Inflammation makes the blood vessels "leaky". When the blood vessels in the lung leak, fluid from the blood moves into the tissue around the lung. Fluid in the lung tissue is called "Pulmonary Edema".

The small air sacs at the end of the breathing passages are called "alveoli". These alveoli are surrounded by the smallest of our blood vessels, called capillaries. The word "pulmonary" means lung, therefore, these small vessels are called pulmonary capillaries. In order for oxygen to enter the pulmonary capillaries, the air must be able to reach the alveoli.

When the lung tissue becomes flooded with fluid, the alveoli have trouble staying open. As more and more alveoli begin to close down, it becomes harder and harder to breathe. In addition, the amount of oxygen able to get into the blood stream begins to fall.

When a patient develops pulmonary edema (lung water) and low oxygen levels as a result of lung inflammation, the disorder is called Acute Lung Injury (ALI) or Acute Respiratory Distress Syndrome (ARDS). The difference between the two disorders is the severity of the oxygen problem: we call the disorder ARDS when the drop in oxygen levels is more severe. Patients with ARDS often require mechanical ventilation with PEEP.

ABOVE: Normal air sacs or "alveoli", surrounded by pulmonary capillaries.

ABOVE: Acute Lung Injury (ALI) and Acute Respiratory Distress Syndrome (ARDS) cause large areas of alveoli to close. This makes it more difficult to breathe, and causes the level of oxygen in the blood to drop.