Acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS), also known as respiratory distress syndrome (RDS) or adult respiratory distress syndrome (in contrast with IRDS) is a serious reaction to various forms of injuries to the lung. This is the most important disorder resulting in increased permeability pulmonary edema. more...

ARDS is a severe lung disease caused by a variety of direct and indirect insults. It is characterized by inflammation of the lung parenchyma leading to impaired gas exchange with concomitant systemic release of inflammatory mediators causing inflammation, hypoxemia and frequently resulting in multiple organ failure. This condition is life threatening and often lethal. So it usually requires mechanical ventilation and admission to an intensive care unit. A less severe form is called acute lung injury (ALI).

ARDS formerly most commonly signified adult respiratory distress syndrome to differentiate it from infant respiratory distress syndrome in premature infants. However, as this type of pulmonary edema also occurs in children, ARDS has gradually shifted to mean acute rather than adult. The differences with the typical infant syndrome remain.

Definition

Historical background

Acute respiratory distress syndrome was first described in 1967 by Ashbaugh et al. Initially there was no definition, resulting in controversy over incidence and mortality. In 1988 an expanded definition was proposed which quantified physiologic respiratory impairment.

In 1994 a new definition was recommended by the American-European Consensus Conference Committee. It had two advantages: 1 it recognizes that severity of pulmonary injury varies, 2 it is simple to use..

ARDS was defined as the ratio of arterial partial oxygen tension (PaO₂) as fraction of inspired oxygen (FiO₂) below 200 mmHg in the presence of bilateral alveolar infiltrates on the chest x-ray. These infiltrates may appear similar to those of left ventricular failure, but the cardiac silhouette appears normal in ARDS. Also, the pulmonary capillary wedge pressure is normal (less than 18 mmHg) in ARDS, but raised in left ventricular failure.

A PaO₂/FiO₂ ratio less than 300 mmHg with bilateral infiltrates indicates acute lung injury (ALI). Although formally considered different from ARDS, ALI is usually just a precursor to ARDS.

Consensus after 1967 and 1994

ARDS is characterized by:

• Acute onset
• Bilateral infiltrates on chest radiograph
• Pulmonary artery wedge pressure < 18 mmHg (obtained by pulmonary artery catheterization)
• if PaO₂:FiO₂ < 300 acute lung injury (ALI) is considered to be present
• if PaO₂:FiO₂ < 200 acute respiratory distress syndrome (ARDS) is considered to be present

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