Preoxygenation is not just placing oxygen near the patient's face.
The goal is to increase the oxygen reservoir before apnea.
In critically ill patients, that reservoir may be small and fragile.
Healthy patients can often tolerate apnea for several minutes after effective preoxygenation. Critically ill patients may desaturate quickly because of shunt, low functional residual capacity, high oxygen consumption, obesity, pregnancy, pneumonia, ARDS, pulmonary edema, or shock.
Oxygen masks can deliver high FiO2, but they do not provide positive pressure.