Acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is a form of acute lung injury and occurs as a result of a severe pulmonary injury that causes alveolar damage heterogeneously throughout the lung. It can either result from a direct pulmonary source or as a response to systemic injury.

This is a distinct entity from neonatal respiratory distress syndrome, which is caused by surfactant deficiency in premature babies.

Terminology 

ARDS has a similar clinical presentation and histological features to those seen in acute interstitial pneumonitis (AIP), showing extensive diffuse alveolar damage (DAD). Both conditions likely represent the same pathology, with AIP probably accounting for some of the idiopathic cases of ARDS. 

Pathology

Lung damage results in leakage of fluid into alveoli, leading to non-cardiogenic pulmonary oedema and decreased arterial oxygenation.

The diagnosis is based on mainly clinical criteria set forth by the American-European Consensus Conference ⁴. Acute respiratory distress syndrome is characterised by the following criteria ⁷:

• lung injury of acute onset, within one week of an apparent clinical insult and with the progression of respiratory symptoms

• bilateral opacities on chest imaging not explained by other pulmonary pathology (e.g. pleural effusion, pneumothorax, or nodules)

• respiratory failure not explained by heart failure or volume overload

• decreased arterial PaO₂/FiO₂ ratio

  • mild ARDS: 201-300 

  • moderate ARDS: 101-200 

  • severe ARDS: ≤100 

It is of note that the clinical diagnosis of ARDS using internationally accepted guidelines and chest radiographs has been demonstrated to correlate poorly with histopathological diagnosis at autopsy ^8,9.

Aetiology

The causes of ARDS can result from a direct lung injury, termed pulmonary ARDS, or extrapulmonary where the triggering insult is outside of the lungs. These two aetiological subtypes respond in different ways to mechanical ventilation. Some authors have described distinct early phase radiological appearances between the two.

Pulmonary causes

• fat embolism

• drowning

• infection

  • viral pneumonia

  • bacterial pneumonia

• pulmonary irritants ¹⁶

  • chlorine, chloramines, ammonia

  • phosgene

  • oxides of nitrogen (silo-filler's disease)

• oxygen toxicity

• smoke inhalation

  • polymer fume fever ¹⁴

• disseminated intravascular coagulopathy

• aspiration

  • gastric contents

  • barium contrast

• thoracic trauma

  • lung contusion

Extrapulmonary causes

• systemic inflammation

  • pancreatitis

  • burns

  • trauma

  • sepsis

• neurogenic pulmonary oedema

• transfusion-related lung injury (TRALI)

• post-cardiopulmonary bypass

• abdominal compartment syndrome

• systemic toxicologic exposures

  • medications

    • colchicine, amiodarone, salicylates

  • drugs

    • opioids, sedative-hypnotics, cocaine

  • bipyridyl herbicides

    • paraquat ¹⁵

Radiographic features

Plain radiograph

Chest radiographic findings of acute respiratory distress syndrome are non-specific and resemble those of typical pulmonary oedema or pulmonary haemorrhage. There are diffuse bilateral coalescent opacities (the only radiological criterion defined by the Consensus Conference). The time course of ARDS may help in differentiating it from typical pulmonary oedema.

Chest x-ray features usually develop 12-24 hours after initial lung insult as a result of proteinaceous interstitial oedema. Within one week, alveolar pulmonary oedema (hyaline membrane) occurs due to type 1 pneumocyte damage.

In contrast to cardiogenic pulmonary oedema, which clears in response to diuretic therapy, ARDS persists for days to weeks. Also, as the initial radiographic findings of ARDS clear, the underlying lung appears to have a reticular pattern secondary to type 2 pneumocyte proliferation and fibrosis ⁴.

CT

Features depend on the phase of the disease ¹⁰.

Early phase

• pulmonary opacification: often demonstrates an anteroposterior density gradient within the lung, with dense consolidation in the most dependent regions, merging into a background of widespread ground-glass attenuation and then normal or hyperexpanded lung in the non-dependent regions (described as a classical appearance ¹⁰)

• the typical CT presentation of bilateral symmetrical changes is more common in extrapulmonary ARDS, whereas in pulmonary ARDS the opacities tend to be asymmetrical ¹¹

• ground-glass opacification: a non-specific sign that reflects an overall reduction in the air content of the affected lung. In acute ARDS likely represent oedema and protein within the interstitial and alveolar spaces

• bronchial dilatation within areas of ground-glass opacification

• some publications also report pulmonary cysts in the early phase ¹¹

Postulated reason for inhomogeneity of appearances:

• increased weight of overlying lung causing compressive atelectasis posteriorly, which produces dense opacification

• supported by the fact that with the positional change from supine to prone, the density gradient can quickly redistribute accordingly

In the non-dependent portions, the lung may be of normal attenuation, or it may be lower if mechanically ventilated.

Late phase and appearances in long-term survivors

CT appearances can be variable in this phase:

• complete resolution: may occur in some cases

• coarse reticular pattern and ground-glass opacification in the anterior (non-dependent) part of the lungs: considered more typical later-stage CT appearances

• areas of reticular and ground-glass opacification

• pulmonary cysts of varying sizes and bullae (probably develop as a result of prolonged ventilation)

Classification

One described method is the Ichikado CT scoring of acute respiratory distress syndrome.

Ultrasound

Point of care ultrasound may be used to complement more traditional imaging modalities to differentiate cardiogenic from non-cardiogenic pulmonary oedema, as well as potentially assess dynamic responses to therapy (e.g. recruitment manoeuvres, ventilator changes) at bedside.

Sonographic findings consistent with a diagnosis of ARDS include:

• alveolar-interstitial syndrome

  • defined by the presence of bilateral, diffuse lung rockets ¹⁹

    • three or more B-lines per intercostal sonographic field

    • non-specific

  • heterogenous distribution

    • interposed "spared areas" of normal sonographic lung

    • commonly more prominent in anterior fields

  • pleural interface abnormalities

    • thickening and/or irregularity of visceral-parietal pleural interface

    • subpleural consolidations may be present ¹⁸

    • normal "lung sliding" may be diminished

      • presence of a pneumothorax excluded by underlying artifacts (e.g. B-lines, lung pulse)

  • absence of significant pleural effusions

    • common in cardiogenic pulmonary oedema

  • absence of elevated cardiac filling pressures ¹⁷

    • a pulmonary capillary wedge pressure may be semiquantified with spectral Doppler derived diastolic indices such as an E/e' ²⁰

    • absence of a severely abnormal left ventricular systolic function also supportive

Treatment and prognosis

Acute respiratory distress syndrome carries high mortality of around 50% ² and many survivors develop chronic lung disease, with the damaged lung healing by fibrosis. However, a minority do make a full recovery.

History and etymology

It was first described in 1967 by David G Ashbaugh et al ¹³.