Calculating the severity of acute respiratory distress syndrome (ARDS)

Acute respiratory distress syndrome (ARDS) is a life-threatening disease characterised by the acute onset of hypoxia and pulmonary infiltrates, attributable to conditions akin to sepsis, pneumonia, trauma, burns, pancreatitis, and blood transfusion. ARDS causes diffuse lung inflammation, which results in increased pulmonary vascular permeability, pulmonary edema, and damage to the alveolar epithelium. Patients with ARDS require continuous monitoring with pulse oximetry (SaO2) and periodic evaluation of arterial gases to acquire the partial pressure of oxygen (PaO₂) values. Such monitoring facilitates rapid titration of the fraction of inspired oxygen (FiO2) or positive end-expiratory pressure (PEEP) in patients with hypoxia.

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

• sharp start,
• bilateral pulmonary infiltrates of non-cardiac origin visible on chest X-ray, computed tomography (CT) or ultrasound,
• moderate to severe impairment of oxygenation, as defined by PaO₂/FiO₂ lower than or equal to 300 mm Hg or oxygen saturation measured by pulse oximetry SaO₂/FiO₂lower than or equal to 315 (if oxygen saturation measured by pulse oximetry is lower than or equal to 97%).

In the absence of pharmacotherapy to treat or prevent ARDS, the treatment strategy is to make use of high-flow nasal oxygen or positive pressure ventilation and supportive care. Severe ARDS is related to a mortality rate of 45% (Matthay et al., 2024).

The severity of acute respiratory distress syndrome (ARDS) is decided by the degree of hypoxia, which is calculated because the ratio of arterial oxygen tension to the fraction of inspired oxygen (PaO₂/FiO₂). ARDS might be mild, moderate or severe, in accordance with the Berlin ARDS definition presented within the table below (Ranieri et al., 2012).

Determining PaO₂ /FiO₂requires arterial blood gas evaluation (ABG). To calculate PaO₂ /FiO₂ PaO coefficient₂ is measured in mmHg and FiO₂ is expressed as a decimal number from 0.21 to 1. For example, if a patient has PaO₂100 mmHg while receiving 80 percent oxygen concentration, then the PaO₂ /FiO₂the ratio is 125 mm Hg (e.g. 100 mm Hg/0.8).

PaO₂ /FiO₂(or SaO₂/FiO₂) index is a useful clinical indicator of the patient’s respiratory status while receiving supplemental oxygen. It allows bedside clinicians to watch the degree of hypoxemia, rapidly detect early progression of respiratory failure, and intensify treatment. For example, prone positioning can improve oxygenation as a patient with ARDS transitions from mild to moderate ARDS. Treatment for severe ARDS requires mechanical ventilation and should include neuromuscular blockade to cut back oxygen consumption, extracorporeal membrane oxygenation (ECMO), or inhaled nitric oxide (Ramanathan et al., 2020).

ARDS Definition Group, Ranieri V., Rubenfeld G., Thompson B., Ferguson N., Caldwell E., Fan E., Camporota L., and Slutsky A. (2012). Acute respiratory distress syndrome: the Berlin definition. (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 brand new global definition of acute respiratory distress syndrome. , (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., and Shekar, K. (2020). Planning and delivery of ECMO services for severe ARDS through the COVID-19 pandemic and other emerging infectious disease outbreaks. . doi: https://doi.org/10.1016/S2213-2600(20)30121-1