Acute Respiratory Distress Syndrome (ARDS) is a severe, life-threatening medical condition characterised by widespread inflammation in the lungs and sudden failure of the respiratory system. In ARDS, due to direct or indirect inflammatory response, the endothelial barrier of a vast network of capillaries surrounding the alveoli is injured. The alveoli are then filled with fluid and are unable to perform the normal oxygenation of the blood and the patient suffers from overall low oxygen levels, i.e. hypoxia. The Berlin definition is used to diagnose ARDS and evaluate the presence of acute onset, bilateral pulmonary edema, and the degree of hypoxia.
Common causes of ARDS include bacterial pneumonia, viral pneumonia (e.g. influenza, avian flue, swine flue etc.), other severe infections known as sepsis, aspiration of fumes, food or stomach contents going into the lung, or significant direct trauma to the lungs or severe indirect trauma causing a systemic inflammatory response. The condition is rare, but could evolve into a widespread problem in an outbrake of an aggressive respiratory viral infection, e.g. MERS or SARS. It is ARDS that causes the death of these patients.
The international consensus criteria for ARDS were updated in 2012 and are known as the Berlin Definition. The former definition of acute lung injury (ALI) was incorporated into mild ARDS and its use is no longer recommended. ARDS is divided into mild, moderate and severe depending on the degree of hypoxia the patient is suffering from depicted by a oxygenation measurement of PaO2/FiO2 ratio (arterial oxygen partial pressure to fractional inspired oxygen), commonly this is also named the P/F ratio.
According to the Berlin definition, ARDS is characterized by:
· lung injury of acute onset, within 1 week of an apparent clinical insult and with progression of respiratory symptoms
· bilateral opacities on chest imaging (chest radiograph or CT) not explained by other lung pathology (e.g. effusion, pneumothorax, or nodules)
· respiratory failure not explained by heart failure or volume overload
· decreased (a decreased PaO2/FiO2 ratio indicates reduced arterial oxygenation from the available inhaled gas):
· mild ARDS: 201 - 300 mmHg (≤ 39.9 kPa)
· moderate ARDS: 101 - 200 mmHg (≤ 26.6 kPa)
· severe ARDS: ≤ 100 mmHg (≤ 13.3 kPa)
Note that the Berlin definition requires a minimum positive end expiratory pressure (PEEP) of 5 cmH2O for consideration of the PaO2/FiO2 ratio. This degree of PEEP may be delivered non-invasively with CPAP to diagnose mild ARDS.