Acute lung injury/acute respiratory distress syndrome

Acute lung injury (ALI) and the more severe acute respiratory distress syndrome (ARDS) are defined as respiratory distress occurring with stiff lungs, diffuse bilateral pulmonary infiltrates, refractory hypoxaemia, in the presence of a recognised precipitating cause and in the absence of cardiogenic pulmonary oedema (i.e. no clinical evidence of left atrial hypertension)

Aetiology

The commonest precipitating factor is sepsis. Other causes include trauma, burns, pancreatitis, fat or amniotic fluid embolism, aspiration pneumonia or cardiopulmonary bypass.


Pathophysiology

The cardinal feature is pulmonary oedema as a result of increased vascular permeability caused by the release of inflammatory mediators. Oedema may induce vascular compression resulting in pulmonary hypertension, which is later exacerbated by vasoconstriction in response to increased autonomic nervous activity. A haemorrhagic intra-alveolar exudate forms, which is rich in platelets, fibrin, and clotting factors. This inactivates surfactant, stimulates inflammations and promotes hyaline membrane formation. These changes may result in progressive pulmonary fibrosis.


Clinical features

Tachypnoea, increasing hypoxia, and laboured breathing are the initial features. The chest X-ray shows diffuse bilateral shadowing, which may progress to complete 'white out'.


Management

This is based on the treatment of the underlying condition

Pulmonary oedema should be limited with fluid restriction, diuretics and haemofiltration. If these measures fail, aerosolized surfactant, inhaled nitric oxide and aerosolised prostacyclin are experimental treatments whose exact role in the management of ARDS is unclear. Repeated positional change, i.e. changing the patient from supine to prone may allow reductions in airway pressures and the inspired oxygen fraction in those with severe hypoxaemia.


Prognosis

Although the mortality has fallen over the last decade, it remains at 30-40% with most patients dying from sepsis. The prognosis is very dependent on the underlying cause, and rises steeply with age and with the development of multiorgan failure.