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.