Concentration of oxygen in a gas mixture is described by which term?

• A. FiO2
• B. Oxygen saturation
• C. PaO2
• D. O2 content

Answer: (A)

The main idea here is the fraction of inspired oxygen, FiO2. This term describes how much of the gas a person is breathing is oxygen, i.e., the concentration of oxygen in the inhaled mixture. It’s expressed as a decimal or a percentage (for example, room air is about 0.21 or 21%). When you give supplemental oxygen, FiO2 rises depending on the delivery method, indicating the oxygen content of the gas itself, not what happens after it enters the lungs.

Oxygen saturation is about the percent of hemoglobin binding sites that are actually loaded with oxygen in the blood, not the gas mixture you’re breathing. PaO2 is the arterial oxygen tension, the pressure of oxygen in arterial blood, measured in mmHg. O2 content is the total amount of oxygen carried in the blood, combining both dissolved oxygen and what’s bound to hemoglobin. So the term that describes the concentration of oxygen in the gas mixture is FiO2.