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.

Inclusion Criteria:

• Patient admitted to the ER
• Requiring NHFO
• Requiring FiO2 ≥ 30% to keep SpO2 in the target ranges defined by the clinician
• Aged over 18 years
• Written informed consent signed and dated by the patient or one relative in case that the patient is unable to consent, after full explanation of the study by the investigator and prior to study participation
• In case that the consent is given by the relative, patient consent will be requested as soon as the patient will be able to provide informed written consent

Exclusion Criteria:

Patients who fulfil any of the following exclusion criteria are not eligible for study participation:

• Patient with indication for immediate CPAP, NIV, or invasive mechanical ventilation
• Hemodynamic instability defined as a need of continuous infusion of epinephrine or norepinephrine > 1 mg/h
• Low quality on the SpO2 measurement using finger and ear sensor (quality index below 60% on the Massimo SpO2 sensor, which is displayed by a red or orange color bar)
• Severe acidosis (pH ≤ 7.30)
• Pregnant woman
• Patients deemed at high risk for need of mechanical ventilation within the next 12 hours
• Chronic or acute dyshemoglobinemia: methemoglobin, CO poisoning, sickle cell disease
• Tracheotomized patient
• Formalized ethical decision to withhold or withdraw life support
• Patient under guardianship
• Patient deprived of liberties
• Patient included in another interventional research study under consent
• Patient already enrolled in the present study in a previous episode of acute respiratory failure

Post enrollment exclusion criteria

• Apparition of a persistent low quality SpO2 signal
• Need for an emergent intubation
• Discharge from ER