Apnea is one of the three cardinal findings in brain death (BD). Apnea testing 
(AT) is physiologically and practically complex. We sought to review described 
modifications of AT, safety and complication rates, monitoring techniques, 
performance of AT on extracorporeal membrane oxygenation (ECMO), and other 
relevant considerations regarding AT. We conducted a systematic scoping review 
to answer these questions by searching the literature on AT in English language 
available in PubMed or EMBASE since 1980. Pediatric or animal studies were 
excluded. A total of 87 articles matched our inclusion criteria and were 
qualitatively synthesized in this review. A large body of the literature on AT 
since its inception addresses a variety of modifications, monitoring techniques, 
complication rates, ways to perform AT on ECMO, and other considerations such as 
variability in protocols, lack of uniform awareness, and legal considerations. 
Only some modifications are widely used, especially methods to maintain 
oxygenation, and most are not standardized or endorsed by brain death 
guidelines. Future updates to AT protocols and strive for unification of such 
protocols are desirable.