Apnea Test

Apnea testing is mandatory for the determination of brain death, but there have been controversies about it. Its safety is a major issue. During the apnea test, severe changes in vital signs, marked hypotension, severe cardiac arrhythmias, and pneumothorax may occur,^{[64] [65]} and the intracranial pressure can increase markedly. The apnea test should be performed as the last test after the other tests fulfill the criteria of brain death. Goudreau and coworkers^[66] reported that during the apnea test, complications such as a marked blood pressure decrease or the development of ventricular arrhythmia developed in 27 (39%) of 70 patients with pretest unfavorable factors but developed in only 11 (15%) of 74 patients without them. They pointed out that inadequate preoxygenation and acid-base or electrolyte abnormalities were the major unfavorable pretest factors. The American Academy of Neurology recommends preoxygenation with 100% oxygen for 10 minutes before apnea testing if PaO₂ is less than 200 mm Hg.^[63]

Another issue is the appropriate level of PaCO₂ to be achieved. After equipment for analyzing blood gases became widespread, the importance of the value of PaCO₂ rather than the time for observation of apnea was confirmed. An increase in PaCO₂ decreases pH in the cerebrospinal fluid, which stimulates the medullary respiratory chemical center. The ventilatory response to the increase in PaCO₂ rises linearly to 80 mm Hg, and then the slope becomes less steep and peaks at about 150 mm Hg in dogs.^[67] Damage in the medullary respiratory center may affect the level of PaCO₂ for development of spontaneous respiration. The level of PaCO₂ at which spontaneous respiration resumes is modified by PaO₂ . Although the exact level of PaCO₂ to be achieved is still unknown, the American Academy of Neurology criteria define various prerequisites and the procedure of apnea testing and adopt the level of PaCO₂ ≥ 60 mm Hg.^{[10] [63]}