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End Tidal CO2 continuous capnography is being utilized more in the ED for procedural sedation. One of the best studies is a randomized control trial using propofol that showed you could see signs of hypoventiliation prior to hypoxia by about 60 seconds - which can be plenty of time to get your BVM and airway cart ready.
References
Deitch K, Miner J, Chudnofsky CR, Dominici P, Latta D.
Ann Emerg Med. 2010 Mar;55(3):258-64. Epub 2009 Sep 24.