Answer: Great question! The “once you are out, you are out” phrase really came to exist in relation the nitroglycerin (NTG) chest pain medical directive where patients would get NTG, become hypotensive, then get improve. Occasionally, a paramedic would then attempt to administer NTG again (and sometimes did with the same result…hypotension).
To answer your above question, we would suggest taking a step back from the “once you are out, you are out” dogma and consider the clinical situation.
In your above example, your patient needs positive pressure ventilation (PPV) assistance period. You are treating a severely unwell patient in need of PPV appropriately with a BVM, and then your patient improves. Great. Now, do they still require PPV? Are they too awake for BVM but they still remain in respiratory distress? If the patient “improves” to the point where the indications for CPAP (which to this point has not been initiated) are met, then please go ahead.
It could also be that your patient has improved to the point where PPV is no longer required at all (like perhaps in your case example of SaO2 99% and GCS 15 and if the patient was no longer in respiratory distress). In this case, perhaps only high flow oxygen is indicated.
If in doubt, patching to a BHP for advice and direction is an option. Bottom line, consider the patient, if they need positive pressure ventilation assistance, you have two options. If the conditions for CPAP are met, please treat as indicated.