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. Patients with ARDS require continuous pulse oximetry (SaO2) monitoring and intermittent arterial blood gas analysis to obtain partial pressure of oxygen (PaO₂) values. Such monitoring facilitates rapid titration of the fraction of inspired oxygen (FiO2) or positive end expiratory pressure (PEEP) for patients with hypoxia.

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, tomographic (CT) scan, or ultrasound, and
• moderate to severe impairment of oxygenation, as defined by PaO₂/FiO₂ less than or equal to  300 mm Hg or oxygen saturation as measured by pulse oximetry SaO₂/FiO₂ less than or equal to 315 (if oxygen saturation as measured by pulse oximetry is less than or equal to 97%).

In the absence of drug therapy to treat or prevent ARDS, the treatment strategy consists of high flow nasal cannula oxygen or positive pressure ventilation and supportive care. Severe ARDS carries a mortality rate of 45% (Matthay et al., 2024).

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	less than 100

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 mm Hg (e.g., 100 mm Hg/0.8).

The PaO₂/FiO₂ (or SaO₂/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 warrants mechanical ventilation and 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. A., Arabi, Y., Arroliga, A. C., Bernard, G., Bersten, A. D., Brochard, L. J., Calfee, C. S., Combes, A., Daniel, B. M., Ferguson, N. D., Gong, M. N., Gotts, J. E., Herridge, M. S., Laffey, J. G., Liu, K. D., Machado, F. R., Martin, T. R., McAuley, D. F., Mercat, A., Moss, M., … Wick, K. D. (2024). A New Global Definition of Acute Respiratory Distress Syndrome. American journal of respiratory and critical care medicine, 209(1), 37–47. https://doi.org/10.1164/rccm.202303-0558WS

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

 
 

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