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High Flow Nasal Oxygen for Severe Hypoxemia: Oxygenation Response and Outcome in COVID-19 Patients. RATIONALE: The "Berlin definition" of acute respiratory distress syndrome (ARDS) does not allow inclusion of patients receiving high-flow nasal oxygen (HFNO). However, several articles proposed that criteria for defining ARDS should be broadened to allow inclusion of patients receiving HFNO. OBJECTIVE: To compare the proportion of patients fulfilling ARDS criteria during HFNO and soon after intubation, and 28-day mortality between patients treated exclusively with HFNO and patients transitioned from HFNO to IMV. METHODS: From previously published studies we analyzed COVID-19 patients who had PaO2/FiO2 ≤300 while treated with HFNO ≥40 L/min, or NIV with PEEP ≥5 cmH2O (comparator). In patients transitioned from HFNO/NIV to invasive mechanical ventilation (IMV), we compared ARDS severity during HFNO/NIV and soon after IMV. We compared 28-day mortality in patients treated exclusively with HFNO/NIV vs. transitioned to IMV. MEASUREMENTS AND MAIN RESULTS: We analyzed 184 and 131 patients receiving HFNO or NIV, respectively. 112 HFNO, and 69 NIV patients transitioned to IMV. 104 (92.9%) HFNO patients and 66 (95.7%) NIV patients continued to have PaO2/FiO2 ≤300 under IMV. 28-day mortality in patients who remained on HFNO was 4.2% (3/72) while in patients transitioned from HFNO to IMV it was 28.6% (32/112) (p<0.001). 28-day mortality in patients who remained on NIV was 1.6% (1/62), while in patients who transitioned from NIV to IMV it was 44.9% (31/69) (p<0.001). Overall mortality was 19.0% (35/184) and 24.4% (32/131) for HFNO and NIV, respectively (p=0.2479). CONCLUSIONS: Broadening ARDS definition to include HFNO patients with PaO2/FiO2 ≤300 may identify patients at earlier stages of disease but with lower mortality. This article is open access and distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives License 4.0 (http://creativecommons.org/licenses/by-nc-nd/4.0/).