Prospective evaluation of a modified apnea-test without disconnection from the ventilator in brain death candidates

Abstract Background: The apnea test (AT) is the central clinical component in the diagnosis of brain death (BD) and normally entails disconnecting the patient from the ventilator followed by tracheal oxygen sufflation to ensure adequate oxygenation. Most international guidelines state that a lack of spontaneous breathing must be demonstrated when PaCO2 ≥ 60mmHg to rate the test as positive. However, the loss of positive end-expiratory pressure (PEEP) that is associated with disconnection from the ventilator may cause rapid desaturation, frequently leading to cardiopulmonary instability (especially in ARDS-patients) and generally putting patients at risk. This prospective study therefore aimed to investigate whether a modified version of the AT (mAT) in which the patient remained connected to the ventilator, is potentially less harmful yet remains a valid test. Methods: mAT was performed in all 140 BD candidates registered between January 2019 and December 2022: After 10 min of pre-oxygenation, (i) PEEP was increased by 2mbar, (ii) ventilation mode was switched to CPAP and (iii) apnea back-up mode was turned off (flow trigger 10l/min). mAT was considered positive when spontaneous breathing occurred upon increasing PaCO2 to ≥ 60 mmHg (baseline 35–45 mmHg). Clinical complications during/after mAT were documented. Results mAT was possible in 139/140 patients and had a median duration of 15 min (IQR 13–19). Severe complications did not occur. In n = 51 patients, the post-mAT PaO2 was lower than that during pre-mAT, while it was equal or higher in n = 88 cases. In ARDS-patients, apneic oxygenation during mAT improved PaO2. In n = 123 cases, there was a transient post-mAT drop in blood pressure, in n = 12 to a MAP < 60 mmHg. Conclusions The mAT is a safe and gentle means for identifying patients in whom the central respiratory drive has not failed.