Acute Respiratory Distress Syndrome
Syn: Adult Respiratory Distress
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The most important cause of diffuse alveolar damage is the acute respiratory distress syndrome (ARDS).
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In this syndrome, a patient with apparently normal lungs suffers an insult and then develops rapidly progressive respiratory failure, characterized by hypoxia and extensive radiologic opacities in both lungs.
Acute Respiratory Distress Syndrome (ARDS) is characterized by diffuse alveolar capillary damage, leading to severe pulmonary edema, respiratory failure and arterial hypoxemia refractory to oxygen therapy.
Respiratory failure, unresponsive to oxygen therapy, develops with diffuse bilateral infiltration on x-ray and frequent superimposed infections, resulting in death (more than 50 %).
Over the years there has been much debate about the term ARDS.
Some feel that the definition of rapidly progressive respiratory failure in a patient with apparently normal lungs is too broad, since it obviously encompasses many conditions in which a more specific diagnosis can be made, such as pneumonia of any sort, fat embolism, pulmonary thromboembolism, aspiration, inhalation of toxic gases etc.
Others have doubted the existence of ARDS related to trauma and have held that the pulmonary lesions are due to known causes, such as oxygen toxicity or hemodynamic pulmonary edema .
However, there is now little doubt that ARDS is a useful term and that it is not simply due to oxygen or hemodynamic pulmonary edema.
It should be recognized that the syndrome of ARDS exists and is multifactorial and that it is appropriate to qualify the term as ARDS due to specific conditions, such as nonthoracic trauma, inhalation of toxic gases, aspiration etc. In addition, oxygen toxicity produces similar lesions, and an idiopathic category should also be recognized.
The overall medical costs are enormous, since mangement requires high technology and intensive care.
The syndrome was first recognized as an entity during the Vietnam War as a result of effective resuscitation techniques for seriously injured combatants.
Typically nonthoracic trauma or infection leads to hemodynamic shock, from which the patient is resuscitated.
However, recovery is inturrupted by respiratory symptoms (Eg. tachypnea, dyspnea, and hypoxemia) and a chest radiograph shows diffuse bilateral infiltrates, which progresses to virtually complete opacification.
The patient requires ventilatory assistance and increasing amounts of oxygen.
The lungs become stiff (decreased compliance), and increasing end-expiratory pressures are required, until the patient needs 100% oxygen to maintain tissue oxygenation.
Half of all patients with Acute Respiratory Distress Syndrome (ARDS) die.
ARDS has been studied in patients and in animal models.
The important early event is leakiness of endothelial capillaries, with morphologic loosening of the intercellular junctions. At this stage respiratory failure is not apparent.
Then, 24 to 48 hours after the initial insult, pulmonary edema and resultant hypoxemia ensue in the exudative phase.
The next stage is diffuse alveolar damage, in which necrosis of type I epithelial cells and hyaline membranes that line the air spaces are prominent.
In the proliferative phase, type II cells multiply to reconstitute the alveolar lining and an interstitial inflammatory infiltrate of mononuclear cells is accompanied by proliferation of fibroblasts.
All these conditions are present 4 to 7days after the insult, and the patient usually dies in severe respiratory failure.
If the patient survives, the lesions may heal with resorption of the alveolar exudates and hyaline membranes and restitution of the normal alveolar epithelium.
Fibroblastic proliferation ceases and the extra collagen is metabolized.
It is well documented that patients with ARDS who recover have normal pulmonary function.
Alternatively, more fibrous tissue is laid down and the lung then becomes remodeled to produce the “honeycomb lung” - multiple cyst-like spaces throughout the lung, separated from each other by fibrous tissue and lined by type II cells, bronchiolar epithelium, or squamous cells . The resemblance of the honeycomb lung to the end stage of fibrosing alveolitis is rarely more than superficial.
Following ARDS there is more active fibroblastic proliferation and less dense scarring. Bronchiolar epithelium lining is less promonent and secretions are not present in the spaces.
Occasionally, an appearance similar to bronchopulmonary dysplasia develops.
In ARDS produced by the inhalation of toxic gases or near-drowning, the damage occurs primarily at the alveolar epithelial surface.As indicated, the alveolar epithelial junctions are usually very tight ; damage to the epithelium results in exudation of fluid and proteins from the interstitium into the alveolar spaces. Endothelial damage may or may not occur in ARDS that is due to inhalation of toxic substances, but the sequence of events is similar to that due to endothelial damage in ARDS that follows trauma or septicemia.
Mechanism of injury:
1. Oxygen-derived free radicals, especially in the toxicity induced by prolonged exposure (e.g. in respirators) to high concentrations of oxygen or other toxins (e.g.- paraquat).
2. Aggregation of activated neutrophils in the pulmonary vasculature. Clinical studies have shown that a reduced number of neutrophils in the blood of patients with risk factors for ARDS is a good predictor for the development of the syndrome. This finding suggests that increased numbers of neutrophils are sequestered in the pulmonary capillary bed. These neutrophils damage epithelium by secreting several types of injurious factors, including oxygen-derived free radicals and lysosomal enzymes (proteases). It also secretes arachidonic acid metabolites that induce neutrophil aggregation.
3. Activation of lung macrophages, which release oxidants, proteases and proinflammatory cytokines (e.g. IL-8).
4. Loss or damage to surfactant, contributing to atelectasis, which (in combination with pulmonary edema) results in the stiff lungs characteristic of ARDS.