ARDS

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Restrictive lung diseases cause reduced compliance of the lungs, i.e. they are difficult to expand with respiration.
The main reason for this pattern of respiratory dysfunction is abnormality of alveolar walls that renders them rigid, usually by oedema or fibrosis.
Those affected complain of breathlessness (dyspnoea) as they feel the greater effort needed to inflate the lungs.
Fibrosis and oedema of alveolar walls causes a dffusional defect so that hypoxia develops.

Diffuse alveolar wall damage is the main feature of restrictive lung diseases

The restrictive lung diseases are characterized by damage to alveolar walls.
Injury to the alveolar walls leads to three main phases of reaction in the lung:

Haemorrhage and high protein exudation into alveoli (causes so-called Hyaline membranes).

Oedema and inflammation of the interstitium.

Fibrosis in the interstitium.

Two main clinical patterns of disease are recognized, depending on which phase of diffuse alveolar damage is most evident: acute restrictive lung disease, the main features of which are exudation and oedema; and chronic restrictive lung disease, the main features of which are inflammation and fibrosis.

Adult respiratory distress syndrome (ARDS) is an acute restrictive lung disease caused by diffuse alveolar damage

The adult respiratory distress syndrome is a manifestation of diffuse alveolar damage leading to widespread systemic metabolic derangements.
Many conditions predispose to this serious reaction in lung, most commonly systemic sepsis and severe trauma.

Systemic liberation of chemical mediators of inflammation, particularly cytokines, is important in progression of disease.

Pathophysiology of ARDS

The events that take place in the lung in ARDS are termed diffuse alveolar damage (DAD) and occur in two phases:

1. Acute exudative phase, with destruction of alveolar lining cells.

2. Late organization phase, with cell proliferation and fibrosis.

Following the damaging stimulus, there is a latent period of 4-24 hours before symptoms develop.
Dyspnoea develops before changes are visible on chest radiographs.
ARDS develops progressively over a period of 24-48 hours.

In the acute phase, there is:

Necrosis of alveolar epithelium.

Exudation of fibrin and fluid into alveolar spaces forming hyaline membranes.

Microthrombosis in alveolar capillaries.

Adherence and activation of neutrophils, with release of neutrophil enzymes.

Haemorrhage into alveoli.

In the organization phase the lungs are congested and ventilation is impaired by disturbance of alveolar walls with exudation into alveolar spaces There is re-generation of type 2 alveolar lining cells and organization of hyaline membranes with pulmonary fibrosis.
Organization may result in interstitial fibrosis with thickening of alveolar walls, or may cause fibrous obliteration of alveolar spaces.

If ARDS has been caused by septicaemia, endotoxin causes endothelial and neutrophil activation, with activation of complement.
Treatment with high concentrations of oxygen may, paradoxically, increase damage by generation of oxygen-derived free radicals.
ARDS is often complicated by secondary infection in the lungs.

In severe cases of ARDS from any cause, cytokines liberated from the lung vascular bed can enter the systemic circulation and may cause systemic endothelial activation with systemic neutrophil activation leading to multi-organ failure (systemic inflammatory response syndrome).

The clinical diagnosis of ARDS depends on:

Presence of a condition known to precipitate ARDS.

Refractory hypoxaemia (PaO2 <8.0 kPa on >40% O2).

Radiographic evidence of evolving diffuse pulmonary shadowing.

Clinical signs of lungs becoming abnormally rigid with low total compliance.

Treatment of ARDS is by continuous positive airway pressure ventilation, and intensive support of cardiac, circulatory and renal function.
Patients with ARDS usually die from systemic inflammatory response syndrome with multi-organ failure, mortality being around 70% Of those who survive ARDS, 20% have some permanent lung dysfunction due to organization of exudate and persisting restrictive defect.

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