Brain death is a clinical and paraclinical diagnosis established in a known etiological context. It involves the irreversible destruction of cerebral functions in a beating-heart individual.

In the context of organ and tissue donation from a critically ill patient, the clinical diagnosis of brain death is confirmed through a mandatory apnea test. This test is conducted by disconnecting the patient from the ventilator for a duration of 8 to 10 minutes. The test is considered validated when there is no spontaneous respiratory movement and the presence of hypercapnia. Hypercapnia is defined as a PaCO2 (carbon dioxide partial pressure) greater than 60 mmHg or an increase of more than 20 mmHg compared to the pre-ventilator disconnection capnia. These thresholds are meant to provide a strong stimulus to the respiratory centers, objectively confirming the absence of spontaneous ventilation.

In practice, the apnea test is performed under CPAP (Continuous Positive Airway Pressure) or by administering oxygen at 6-10L/min through the endotracheal tube.

High-flow oxygenation could be a simple alternative for the apnea test during the validation of the clinical diagnosis of brain death while reducing the risk of lung derecruitment and compromising organ viability and/or lung donation.

However, due to the high gas flow used (60 L/min), high-flow oxygenation can lead to a "washout" effect in the anatomical dead space, which may lower the carbon dioxide partial pressure. Consequently, even in a patient in a state of brain death, where spontaneous ventilation is absent by definition, it is not certain that the carbon dioxide partial pressure will increase to the threshold of 60 mmHg or more than 20 mmHg in 10 minutes during an apnea test conducted with high-flow oxygenation.

The purpose of this study is, therefore, to examine the feasibility of clinically diagnosing brain death through an apnea test performed with high-flow oxygenation.

To determine whether the validation criterion which involves observing an arterial capnia > 60 mmHg or an increase > 20 mmHg (compared to the capnia at the beginning of the test) after 10 minutes can be achieved during an apnea test conducted under high-flow oxygen therapy in patients with clinically confirmed brain death by standard apnea test (under standard oxygen therapy)

The protocol is based on performing two consecutive apnea tests on a patient in clinical brain death in intensive care unit. The first apnea test will be conducted under standard oxygen therapy and confirms clinical brain death (standard procedure). The second apnea test (for the study) will be conducted under high-flow oxygen therapy. Only patients whose apnea test is initially validated under standard oxygen therapy will have the second apnea test under high-flow oxygen therapy.

After a standardized preoxygenation, the 1st apnea test will be performed under standard oxygen therapy through the endotracheal tube (flow at 8L/min) for 10 minutes. An arterial blood gas sampling will be sampled every 2 minutes during the1st apnea test.

The confirmation of clinical brain death by validating the apnea test under standard oxygen therapy will be done if the following two conditions are met: 1) absence of a spontaneous respiratory movements AND 2) an arterial capnia > 60 mmHg or an increase > 20 mmHg (compared to the capnia at the end of preoxygenation).

Then the patient will be reconnected to the ventilator for 15 minutes. A second standardized preoxygenation will be performed before the 2nd apnea test. The second apnea test will be performed under high-flow oxygen therapy through the endotracheal tube (flow at 50L/min, FiO2 100%) for 10 minutes. An arterial blood gas sampling will be sampled every 2 minutes during the1st apnea test.

Then the patient will be reconnected to the ventilator with the same settings before the start of the study.

Inclusion Criteria:

• All patients in critical care with clinical brain death and potential organ and tissue donors for whom an apnea test is indicated.

Exclusion Criteria:

• Minor patients.
• Person under guardianship
• Protected Majors
• Not affiliated to French social security
• Pregnant women.
• Patients with a history of Chronic Obstructive Pulmonary Disease with a GOLD score of III or IV.
• Patients with at least one absolute contraindication to organ donation.
• Patients whose relatives have spontaneously expressed the patient's refusal to donate organs for therapeutic purposes.
• Patients on VA ECMO and, in general, any patient with a contraindication to performing the apnea test (DV, significant hemodynamic instability...).
• Patients whose relatives have refused the patient's participation in the study.