Acute respiratory distress syndrome (ARDS)
Approach
Because the diagnosis of ARDS is based on clinical criteria rather than a pathological diagnosis, ARDS should be considered in all critically ill patients regardless of whether invasive mechanical ventilation is required. As many as 40% of patients who meet the criteria for ARDS are never diagnosed with the condition.[4][42] If patients develop new bilateral infiltrates on chest x-ray (CXR), they may have or may be developing ARDS. The importance of evaluating patients for the development of ARDS stems primarily from the survival benefit gained by ventilating with a low tidal volume, plateau-pressure-limited ventilator strategy.
History
The history should be directed at determining whether there is an underlying condition associated with ARDS, such as sepsis, pneumonia, aspiration of gastric contents, pancreatitis, blood transfusions, severe trauma, or e-cigarette use/vaping. The underlying cause can be an important determinant of outcome; patients with ARDS due to sepsis generally have the highest mortality. Specific treatments directed at the underlying cause are warranted, with particular attention to source identification and treatment in the context of sepsis. Symptoms that suggest ARDS include the acute onset of shortness of breath and hypoxaemia leading to acute respiratory failure requiring high flow nasal oxygen or non-invasive or invasive mechanical ventilation, and cough with expectoration of frothy pulmonary oedema. The history should also collect information that might suggest an alternative diagnosis of an ARDS mimic, such as pulmonary oedema secondary to heart failure, diffuse alveolar haemorrhage due to pulmonary vasculitis, collagen vascular disease, or acute eosinophilic pneumonia.[43]
Examination
Physical examination findings that support the diagnosis of ARDS are acute hypoxic respiratory failure requiring high levels of oxygen and/or positive end-expiratory pressure to maintain an oxygen saturation >90%. In ventilated patients, both peak inspiratory pressure and end-inspiratory plateau pressure are also increased. Lung examination may reveal basilar or diffuse rales.[44] Particular attention should be put on identifying the source of infection if sepsis is suspected to be the underlying cause of ARDS.
Investigation
Key tests include arterial blood gas analysis for calculation of the partial pressure of oxygen, arterial (PaO₂)/inspired oxygen ratio. In screening for ARDS, the oxygen saturation to inspired oxygen fraction (SpO₂/FiO₂) can also be used as long as the SpO₂ is less than 97% (below the plateau on the oxyhaemoglobin dissociation curve). An SpO₂/FiO₂ ratio of 315 has been shown to correlate with PaO₂/FiO₂ of 300.[45] Use of the SpO₂/FiO₂ ratio to diagnose ARDS identifies patients with similar clinical outcomes to patients diagnosed using the PaO₂/FiO₂ ratio and is now included in the new global definition of ARDS.[1][46]
A CXR should be performed to look for bilateral infiltrates that are consistent with pulmonary oedema and not fully explained by atelectasis or pulmonary effusions. In resource-limited settings, lung ultrasound by an experienced operator to look for evidence of bilateral B lines and/or consolidations may be substituted if chest radiography is not available.[1] Brain natriuretic peptide (BNP) levels should be considered if heart failure is a potential cause in patients with bilateral infiltrates on radiography. BNP levels <100 nanograms/L (100 picograms/mL) make heart failure unlikely, whereas BNP levels >500 nanograms/L (>500 picograms/mL) make it likely. An echocardiogram should be ordered if heart failure is still a possible diagnosis after BNP levels are available, particularly if there are no risk factors for ARDS present. If the BNP and echocardiogram are inconclusive, insertion of a pulmonary artery catheter (to estimate left ventricular end-diastolic pressure) may be helpful to differentiate heart failure from ARDS. However, routine insertion of a pulmonary artery catheter in all patients is not indicated.[47][Figure caption and citation for the preceding image starts]: Chest x-ray image of bilateral infiltrates in a patient with ARDSFrom the personal collection of Dr Lorraine Ware; used with permission [Citation ends].
Blood, sputum, and urine cultures should be performed to investigate for the presence of sepsis. Viral testing should be considered in the appropriate clinical setting (e.g., influenza, SARS-CoV-2). Bronchoalveolar lavage (BAL) or endotracheal aspiration for Gram stain and cultures is also recommended in patients with ARDS due to suspected pneumonia and those without a defined predisposing condition.[48] However, bronchoscopy should be avoided in patients with suspected SARS-CoV-2 (COVID-19)-related ARDS due to high risk of provider exposure during aerosolising procedures.[49] BAL can also be helpful for identifying other causes of acute respiratory failure with bilateral radiographic infiltrates that mimic ARDS, such as diffuse alveolar haemorrhage or acute eosinophilic pneumonia.
Serum lipase and amylase tests should be requested in patients with suspected acute pancreatitis. Both tests have similar sensitivity and specificity, but lipase levels remain elevated for longer (up to 14 days after symptom onset vs. 5 days for amylase).[50]
Computed tomography (CT) scanning of the thorax is not routinely required to diagnose or manage ARDS. It is more sensitive than a plain CXR and may be helpful in some patients for diagnosing pneumonia or underlying lung disease.[51] CT scanning has shown that ARDS affects the lung parenchyma heterogeneously, with dependent portions of the lung being the most affected.[44] However, routine chest CT scanning in ARDS to assess the heterogeneity of infiltrates is not currently indicated.
Open lung biopsy can be helpful in the setting of continued diagnostic uncertainty.[52][53] However, this is not routinely performed in critically ill patients because of the high risk of morbidity and mortality.
How to obtain an arterial blood sample from the radial artery.
How to perform a femoral artery puncture to collect a sample of arterial blood.
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