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From http://www.ards.org/learnaboutards/whatisards/faq/faq1.html

What is Adult Respiratory Distress Syndrome, commonly called simply: ARDS? What ARDS is not. What are the phases/stages of ARDS?

Adult (or Acute) Respiratory Distress Syndrome (ARDS) is a medical condition which prevents the normal breathing process from taking place. ARDS occurs when there is severe inflammation in both lungs resulting in an inability of the lungs to function properly. ARDS is a devastating, often fatal, inflammatory lung condition that usually occurs in conjunction with catastrophic medical conditions, such as pneumonia, shock, sepsis (or severe infection throughout the body, sometimes also referred to as systemic infection, and may include or also be called a blood or blood-borne infection), and trauma. It is a form of sudden and often severe lung failure. Lung failure means that the lungs can no longer carry out their normal function of getting oxygen into the blood and removing carbon dioxide from the body.

As will be seen, ARDS may result in a relatively short period of battle to recover characterized by what seems to be a remarkably speedy recovery with little if any damage to the lungs. More often, if death does not occur, there is a very protracted period of battle with varying levels of lung damage sometimes but not always leading to the need for extensive physical and pulmonary rehabilitation.

Thousands upon thousands of individuals suffer from ARDS (and the less severe precusor medical condition known as Acute Lung Injury - ALI) each year in the US alone. Worldwide, the cases of ARDS and ALI total in the hundreds of thousands. Patients, family members and friends, survivors, medical personnel and facilities, are severely tested emotionally, morally, financially, and significantly effected by the devastating consequences of ARDS each year.

The pathogenesis of ARDS generally has been characterized into three phases/stages. It is important to remember each individual is different and the path through battling ARDS may vary widely. There may usually are many ups and downs along the way.

1.) Exudative phase/stage. Characterized by accumulation of excessive fluid in the lungs due to exudation (leaking of fluids) and acute injury Characterized by accumulation of excessive fluid in the lungs due to exudation (leaking of fluids) and acute injury (Acute Lung Injury "ALI" often is a precusor injury medical condition to the more severe development of ARDS, although ALI is more prevalent and not all ALI cases develop into ARDS). Arterial oxygention is usually most severe during this phase of actute injury, including injury to the endothelium (lining membrane) and epithelium (surface layer of cells). Some individuals quickly recover from this first phase; many others progress after about a week into the second phase/stage.

2.) Fibroproliferative (or sometimes shortened to proliferative) phase/stage. Connective tissue and other structural elements in the lungs proliferate in response to the initial injury, including development of fibroblasts (cells giving rise to connective tissue). Under a microscope, lung tissue appears densely cellular. Terms "stiff lung" and "shock lung" have been known to characterize this phase/stage. Two to four weeks after the onset of lung injury, abnormally enlarged air spaces and fibrotic tissue (scarring) are increasingly apparent. There is ongoing danger of barotrauma or volutrauma causing a pneumothorax - a rupture allowing leakage of air from the damaged lung into surrounding spaces, driven by the high pressure (barotrauma) or the volume of air used (volutrauma) in mechanically assisted breathing. There is also danger of pneumonia and blood-born infection (sepsis) developing, or difficulty in resolving one or both of these conditions if they were the precipitating conditions leading to the ARDS. Many people die during this phase/stage because of Multiple Organ Failure (MOF) or infectious complications. The second phase/stage typically lasts 3-10 weeks. The first two phases/stages generally are the most critically severe stages from a life or death consequence standpoint.

3.) Fibrosis (or fibrotic) phase/stage-Repair and recovery (or the healing stage). The lung reorganizes and recovers during this phase/stage. Resolution of inflammation, excess cellularity, and fibrosis settles. Oxygenation improves to the point of liberation from mechanical ventilation becomes possible. Lung function may continue to improve for as long as 6 to 12 months after onset of respiratory failure, depending on the precipitating condition and severity of the initial injury. It is important to remember that there may be and often are different levels of pulmonary fibrotic changes between individuals who suffer from ARDS.