Oxygen has to do three things. It has to move through air, cross a membrane, and load onto a red cell that's actually moving. Respiratory failure breaks down the same three ways, and each one fails a different step.
Ventilation failure is an airflow problem. Air is not getting in and out. Asthma, COPD, chest wall injury, a spontaneous pneumothorax, neuromuscular disease, all of those block the mechanics of breathing.
Diffusion failure is a membrane problem. Air gets in fine, but oxygen can't cross into the blood. Pulmonary edema, pneumonia, ARDS, inhalation injury, and drowning all thicken or flood that membrane.
Perfusion failure is a blood problem. The membrane is fine and the air is fine, but there's no blood there to pick the oxygen up. Hypovolemia, anemia, and a pulmonary embolism blocking flow all do this.
You sort which one you're looking at before you treat, because the fix for one does nothing for the others. A bronchodilator opens airways. That's a ventilation fix. CPAP pushes oxygen across a wet membrane. That's a diffusion fix. Neither one touches a clot. That's why the assessment always comes before the intervention, not the other way around.
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