Acute respiratory distress syndrome (ARDS) is a life-threatening disease, characterized by acute onset of hypoxia and pulmonary infiltrates, and incited by conditions such as sepsis, pneumonia, trauma, burns, pancreatitis and blood transfusion. ARDS causes diffuse lung inflammation which leads to increased pulmonary vascular permeability, pulmonary edema, and alveolar epithelial injury.

The diagnosis of ARDS is made based on the following criteria:

• acute onset,
• bilateral lung infiltrates of a non-cardiac origin on chest x-ray or tomographic (CT) scan, and
• moderate to severe impairment of oxygenation.

In the absence of drug therapy to treat or prevent ARDS, the treatment strategy consists of positive pressure ventilation and supportive care. Severe ARDS carries a mortality rate of 45% (Ranieri et al., 2012).

The severity of the ARDS is defined by the degree of hypoxemia, which is calculated as the ratio of arterial oxygen tension to fraction of inspired oxygen (PaO₂/FiO₂). ARDS can be mild, moderate or severe as clarified by the Berlin definition of ARDS, outlined in the table below (Ranieri et al., 2012).
 

Berlin Definition of ARDS (Ranieri et al., 2012)
ARDS Severity	PaO2/FiO2
Mild	200 - 300
Moderate	100 - 199
Severe	< 100
*on positive end expiratory pressure (PEEP) ≥ 5 cm H2O

Determining the PaO₂/FiO₂ requires arterial blood gas (ABG) analysis. To calculate the PaO₂/FiO₂ ratio, the PaO₂ is measured in mmHg and the FiO₂ is expressed as a decimal between 0.21 and 1. As an example, if a patient has a PaO₂ of 100 mmHg while receiving 80 percent oxygen, then the PaO₂/FiO₂ ratio is 125 mmHg (ie, 100 mmHg/0.8).

The PaO₂/FiO₂ ratio is a valuable clinical measure of the patient's respiratory status while receiving supplemental oxygen. It enables bedside clinicians to monitor the degree of hypoxemia, quickly detect early progression of respiratory failure, and intensify treatment. For example, proning the patient may improve oxygenation when the ARDS patient progresses to from mild to moderate ARDS. Treatment of severe ARDS might include neuromuscular blockade to reduce oxygen consumption, extracorporeal membrane oxygenation (ECMO), or inhaled nitric oxide (Ramanathan et al., 2020).

References: 
ARDS Definition Task Force, Ranieri V., Rubenfeld G., Thompson B., Ferguson N., Caldwell E., Fan, E., Camporota, L., & Slutsky A. (2012). Acute respiratory distress syndrome: the Berlin Definition. JAMA,.307(23). doi: 10.1001/jama.2012.5669.

Matthay, M., Ware, L., & Zimmerman G. (2012). The acute respiratory distress syndrome. The Journal of Clinical Investigation, 22. doi: 10.1172/JCI60331 

Ramanathan, K., Antognini, D., Combes, A., Paden, M., Zakhary, B., Ogino, M., MacLaren, G., Brodie, D., & Shekar, K. (2020). Planning and provision of ECMO services for severe ARDS during the COVID-19 pandemic and other outbreaks of emerging infectious diseases. The Lancet Respiratory Medicine.   doi: https://doi.org/10.1016/S2213-2600(20)30121-1