High flow nasal oxygen therapy (HFNO) is an established modality in the supportive treatment of patients suffering from acute hypoxemic respiratory failure. The high humidified gas flow supports patient's work of breathing, reduces dead space ventilation, and improves functional residual capacity while using an unobtrusive patient's face interface \[Mauri et al, 2017; Möller et al, 2017\]. As hyperoxia is considered not desirable \[Barbateskovic et al, 2019\] during any oxygen therapy, the inspired O2 concentration is usually adapted to a pre-set SpO2 target-range of 92-96% in patients without hypercapnia risk, and of 88-92% if a risk of hypercapnia is present \[O'Driscoll et al, 2017; Beasley et al, 2015\]. In most institutions, the standard of care is to manually adapt the FiO2, although patients frequently have a SpO2 value outside the target range. A new closed loop oxygen controller designed for HFNO was recently developed (Hamilton Medical, Bonaduz, Switzerland). The clinician sets SpO2 targets, and the software option adjusts FiO2 to keep SpO2 within the target ranges. The software option offers some alarms on low and high SpO2 and high FiO2. Given the capability, on the one hand, to quickly increase FiO2 in patients developing sudden and profound hypoxia, and, on the other hand, of automatically preventing hyperoxia in patients improving their oxygenation, such a system could be particularly useful in patients treated with HFNO. A short-term (4 hours vs 4 hours) crossover study indicated that this technique improves the time spent within SpO2 pre-defined target for ICU patients receiving high-flow nasal oxygen therapy \[Roca et al, 2022\]. Due to its simplicity, HFNO is increasingly used outside the ICU during transport and in the Emergency Room (ER). This environment poses specific challenges, as patients may deteriorate very quickly and depending on patient's flow, healthcare providers can easily be overwhelmed. We thus propose to evaluate closed loop controlled HFNO in ER patients. The hypothesis of the study is that closed loop oxygen control increases the time spent within clinically targeted SpO2 ranges and decreases the time spent outside clinical target SpO2 ranges as compared to manual oxygen control in ER patients treated with HFNO.