According to the Berlin Criteria for ARDS severity
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"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)."
https://www.nursingcenter.com/ncblog/march-20...ty-of-ards