Formerly known as adult respiratory distress syndrome, it is a form of acute respiratory failure in which a variety of different disorders give rise to lung injury. The condition has a high mortality rate (about 70 per cent); it is a complex clinical problem in which an overstimulated immune system plays a major role. (See IMMUNITY.)
Chemical mediators produced by cells of the immune system spread and sustain an inflammatory reaction. CYTOTOXIC substances (which damage or kill cells) such as oxygen-free radicals and PROTEASE damage the alveolar capillary membranes (see ALVEOLUS). Once this happens, protein-rich fluid leaks into the alveoli and interstitial spaces of the lungs. SURFACTANT is also lost. This impairs the exchange of oxygen and carbon dioxide between air and blood, which takes place in the lungs so gives rise to acute respiratory failure.
The typical patient with ARDS has rapidly worsening hypoxaemia (lack of oxygen in the blood), often requiring mechanical ventilation. There are all the signs of respiratory failure (see TACHYPNOEA; TACHYCARDIA; CYANOSIS), although the chest may be clear apart from a few crackles. Radiographs show bilateral, patchy, peripheral shadowing. BLOOD GASES will show a low PaO2 (concentration of oxygen in arterial blood) and usually a high PaCO2 (concentration of carbon dioxide in arterial blood). The lungs are ‘stiff’ – they are less effective because of the loss of surfactant and the PULMONARY OEDEMA.
The causes of ARDS may be broadly divided into the following:
- Viral, bacterial and fungal PNEUMONIA
- Lung trauma or contusion
- Inhalation of toxic gases or smoke
- ASPIRATION of gastric contents
- Near-drowning
- Septic, haemorrhagic and cardiogenic SHOCK
- METABOLIC DISORDERS such as URAEMIA and pancreatitis (see PANCREAS, DISORDERS OF)
- Bowel infarction
- Drug ingestion
- Massive blood transfusion, transfusion reaction (see TRANSFUSION OF BLOOD), CARDIOPULMONARY BYPASS, disseminated intravascular coagulation.
The underlying condition should be treated if that is possible. Mechanical ventilation of the lungs is used while attempts are made to reduce the formation of PULMONARY OEDEMA (outpouring of inflammatory fluid into the lungs) by careful management of how much fluid is given to the patient (fluid balance). Infection is treated if it arises, as are the possible complications of prolonged ventilation of stiff lungs (e.g. PNEUMOTHORAX). Giving SURFACTANT through a nebuliser or aerosol use of ANTIOXIDANTS and inhalation of NITRIC OXIDE (NO) have all been employed to improve breathing. In severe cases, EXTRA-CORPOREAL MEMBRANE OXYGENATION (ECMO) can be used in specialist centres able to perform this technique. (See also RESPIRATORY DISTRESS SYNDROME; HYALINE MEMBRANE DISEASE; SARS.)