Validation of the Apnea Test Performed by High-flow Oxygen Therapy in Patients With Clinical Brain Death

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 crit