ADULT RESPIRATORY DISTRESS SYNDROME (ARDS) DEFINITION A state of life-threatening respiratory distress due to an acute lung injury with an impaired oxygenation (PaO2/FiO2 < 200), bilateral pulmonary infiltrates and a normal pulmonary arterial occlusive pressure (< 18 mmHg). TOXIC CAUSES Aspiration of hydrocarbons Inhalation of irritants (e.g. chlorine, NO2, smoke, ozone, high concentrations of oxygen, metal fumes, mustard gas) Paraquat Opiods (e.g. heroin, morphine, dextropropoxyphene, or methadone). NON-TOXIC CAUSES Pulmonary aspiration (frequently occurs in the context of poisoning) Severe acute systemic disease such as infection, trauma or shock. CLINICAL FEATURES The earliest signs are increased respiratory rate, dyspnoea, and cyanosis. On physical examination, a few fine inspiratory crackles may be audible. Chest x-ray shows diffuse bilateral pulmonary infiltrates, though these may initially be minimal or absent. Later the patient becomes more cyanosed and increasingly dyspnoeic and tachypnoeic. Crackles become more prominent. The chest x-ray may show complete opacification. DIFFERENTIAL DIAGNOSIS Acute cardiogenic pulmonary oedema Pneumothorax Bacterial or viral pneumonia RELEVANT INVESTIGATIONS Arterial blood gases - reduced PaO2 with a ratio of PaO2/FiO2 below 200, Chest x-ray - appearance varies from bilateral pulmonary infiltrates to complete opacification. Haemodynamic investigation by Swan-Ganz catheter - pulmonary arterial occlusive pressure (PAOP) below 18 mmHg. TREATMENT Patients with ARDS must be admitted to an Intensive Care Unit. Treatment includes : Mechanical ventilation with a volume controlled mode (tidal volume = 6 to 10 mL/kg) and a high concentration of oxygen. The FiO2 should be sufficient to achieve adequate oxygenation (PaO2 >90 mmHg or SaO2 > 92%). Positive end expiratory pressure may be used (5 to 10 cm H2O), but caution is necessary because of the risk of barotrauma. Fluid restriction in order to decrease PAOP and pulmonary oedema. Lateral decubitus or prone position and extracorporeal respiratory support have been proposed in patients unresponsive to adequate mechanical ventilation. New methods of treatment currently under investigation include: Inhaled Nitrous Oxide which may be beneficial by improving pulmonary hypertension and gas exchange. IV N-Acetyl-Cysteine which may increase the lung surfactants. The usefulness of corticosteroids has not been established. CLINICAL COURSE AND MONITORING ARDS is a serious syndrome with a mortality rate of from 40 to 60%. Numerous complications may occur leading to multiorgan failure. ARDS requires close monitoring of : Arterial blood gases, Respiratory parameters, Haemodynamic parameters, especially pulmonary arterial occlusive pressure (Swan-Ganz catheter) Pulse oximetry Chest x-ray. Monitoring of other parameters may be indicated according to the cause or to the occurrence of other organ failures. LONG TERM COMPLICATIONS Chronic pulmonary fibrosis may occur. AUTHOR(S)/REVIEWERS Author: Dr. A. Jaeger, Director, Service de Réanimation Médicale et Centre Anti-Poisons, HÔpital Civil de Strasbourg, Strasbourg, France. Reviewers: Sao Paulo 9/94, Cardiff 3/95, Berlin 10/95: J. Szajewski, A. Jaeger.