Prospective Evaluation of a Modified Apnea Test in Brain Death Candidates that Does Not Require Disconnection from the Ventilator

The apnea test (AT) is an important component in the determination of brain death/death by neurologic criteria (BD/DNC) and often entails disconnecting the patient from the ventilator followed by tracheal oxygen insufflation to ensure adequate oxygenation. To rate the test as positive, most international guidelines state that a lack of spontaneous breathing must be demonstrated when the arterial partial pressure of carbon dioxide (PaCO₂) ≥ 60 mm Hg. However, the loss of positive end-expiratory pressure that is associated with disconnection from the ventilator may cause rapid desaturation. This, in turn, can lead to cardiopulmonary instability (especially in patients with pulmonary impairment and diseases such as acute respiratory distress syndrome), putting patients at increased risk. Therefore, this prospective study aimed to investigate whether a modified version of the AT (mAT), in which the patient remains connected to the ventilator, is a safer yet still valid alternative.

The mAT was performed in all 140 BD/DNC candidates registered between January 2019 and December 2022: after 10 min of preoxygenation, (1) positive end-expiratory pressure was increased by 2 mbar (1.5 mm Hg), (2) ventilation mode was switched to continuous positive airway pressure, and (3) apnea back-up mode was turned off (flow trigger 10 L/min). The mAT was considered positive when spontaneous breathing did not occur upon PaCO₂ increase to ≥ 60 mm Hg (baseline 35–45 mm Hg). Clinical complications during/after mAT were documented.

The mAT was possible in 139/140 patients and had a median duration of 15 min (interquartile range 13–19 min). Severe complications were not evident. In 51 patients, the post-mAT arterial partial pressure of oxygen (PaO₂) was lower than the pre-mAT PaO₂, whereas it was the same or higher in 88 cases. In patients with pulmonary impairment, apneic oxygenation during the mAT improved PaO₂. In 123 cases, there was a transient drop in blood pressure at the end of or after the mAT, whereas in 12 cases, the mean arterial pressure dropped below 60 mm Hg.

The mAT is a safe and protective means of identifying patients who no longer have an intact central respiratory drive, which is a critical factor in the diagnosis of BD/DNC.

Clinical trial registration DRKS, DRKS00017803, retrospectively registered 23.11.2020, https://drks.de/search/de/trial/DRKS00017803