You have a 50 y/o male with bad bilateral pneumonia. BP 108/70, HR 96, RR 28. He is delirious, agitated, and looks sick, sick, sick! Saturation is 70% on a nasal cannula; when you try to place the patient on a non-rebreather (NRB) he just swats your hand away and rips off the mask. It is obvious to everyone in the room that this patient needs intubation, but the question is how are you going to do it?
Your first impulse may be to perform RSI, maybe with some bagging during the paralysis period. This is essentially a gamble. If you have first pass success, you (and your patient) may just luck out, allowing you to get the tube in and start ventilation before critical desaturation and the resultant hemodynamic instability. However, the odds are against you: bagging during RSI predisposes to aspiration, conventional BVM without a PEEP valve is unlikely to raise the saturation in this shunted patient, and if there is any difficulty in first-pass tube placement your patient will be in a very bad place.