Definition

• A very severe form of acute lung injury defined as a ratio of PaO2:FiO2<200mmHg in the presence of bilateral alveolar infiltrates on chest X- ray and in the absence of let ventricular failure.

Epidemiology

• Incidence rates range from 17 to 34 per 100 000 people years.

 • 10– 15% of all intensive care patients meet criteria for ARDS.

Aetiology

 • Any severe injury to the lung may lead to ARDS.

 • Common causes include severe pneumonia, shock, trauma, multiple transfusions. • In many cases, multiple factors act together

Pathogenesis

• Severe damage to the lung causes widespread alveolar necrosis with severe impairment of normal gas exchange.

Presentation

 • Severe breathlessness, in addition to signs of the underlying cause.

Macroscopy

• Both lungs are typically markedly heavy and fluid- filled, often weighing > 1000 g each (a normal lung weighs 300– 400 g).

Histopathology

• the histopathological hallmark is diffuse alveolar damage, characterized by the presence of hyaline membranes lining alveolar spaces.

• Hyaline membranes are composed of a mixture of fibrin and necrotic alveolar epithelial cells

Prognosis

 • Severe condition with high mortality rates averaging between 30% and 50%, depending on the cause.

 • Survivors usually demonstrate residual pulmonary functional abnormalities