First-- and I'm sure you weren't suggesting this, but I feel I need to mention-- it's extremely dangerous to pressurize a person's lungs above the surrounding environment[1]. Not to mention that pressurizing the inside of the throat could pressurize the middle ear and blow out your eardrums.
Second, I'm not a physician, I'm a physicist. What follows is for curiosity's sake.
I suspect that the goal is to maximally enrich the patient's airstream in oxygen, whenever it is that they happen to breathe in. In a patient with fluid-filled alveoli, the surface area available for diffusion of oxygen into the bloodstream is greatly diminished. Additionally, the distance that oxygen needs to diffuse before it reaches hemoglobin is increased: rather than just the lining of alveoli and capillaries, it has to first dissolve into the fluid gunk filling the space, then diffuse through the fluid, then pass through the lining of the alveolus and capillary. To top it off, water doesn't have great solubility for oxygen, and atmospheric air is mostly nitrogen anyways.
Each of these passive transport phenomena occurs at a rate that depends on the gradient (roughly...) of available O2. This concentration is greatest in the air, and lowest in the bloodstream adjacent to the alveoli, where hemoglobin binds up oxygen. One way to increase the rate of dissolution and diffusion is to increase the concentration gradient. That means enriching the airstream in O2.
Second, I'm not a physician, I'm a physicist. What follows is for curiosity's sake.
I suspect that the goal is to maximally enrich the patient's airstream in oxygen, whenever it is that they happen to breathe in. In a patient with fluid-filled alveoli, the surface area available for diffusion of oxygen into the bloodstream is greatly diminished. Additionally, the distance that oxygen needs to diffuse before it reaches hemoglobin is increased: rather than just the lining of alveoli and capillaries, it has to first dissolve into the fluid gunk filling the space, then diffuse through the fluid, then pass through the lining of the alveolus and capillary. To top it off, water doesn't have great solubility for oxygen, and atmospheric air is mostly nitrogen anyways.
Each of these passive transport phenomena occurs at a rate that depends on the gradient (roughly...) of available O2. This concentration is greatest in the air, and lowest in the bloodstream adjacent to the alveoli, where hemoglobin binds up oxygen. One way to increase the rate of dissolution and diffusion is to increase the concentration gradient. That means enriching the airstream in O2.
[1] https://en.wikipedia.org/wiki/Barotrauma#Pulmonary_barotraum...