Acute respiratory distress syndrome
Acute respiratory distress syndrome (ARDS) is a form of acute lung injury (ALI) 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.
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. ARDS 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 mmHg (≤39.9 kPa)
- moderate ARDS: 101-200 mmHg (≤26.6 kPa)
- severe ARDS: ≤100 mmHg (≤13.3 kPa)
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.
The causes of ARDS can result from 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.
- fat embolism
- viral pneumonia
- oxygen toxicity
- smoke inhalation
- disseminated intravascular coagulopathy
- aspiration of gastric contents
- lung contusion
- hypovolaemic shock
- head injury
- transfusion reaction
- cardiopulmonary bypass
Chest radiographic findings of ARDS 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.
Features depend on the phase of the disease 10.
- 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 nondependent portions, the lung may be of normal attenuation, or it may be lower if being 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 (nondependent) 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)
One described method is the Ichikado CT scoring of acute respiratory distress syndrome.
Treatment and prognosis
ARDS carries a high mortality of around 50% 2 and many survivors develop chronic lung disease, with damaged lung healing by fibrosis. A minority recover fully.
History and etymology
It was first described in 1967 by D G Ashbough et al.
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