For FiO2 greater than 35%, a substitute oxygen concentration of what percent is used in Venturi calculations?

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In Venturi calculations, oxygen from the source mixes with room air to achieve a target FiO2. When the desired FiO2 is higher than about 35%, precise math becomes unwieldy due to entrainment dynamics, so a conventional substitution is used to keep calculations practical and consistent. The standard substitute for the portion of gas that comes from entrained air is 20%, reflecting the approximate oxygen content of the air-entraining mix in this high-FiO2 range. This makes the math workable and aligns with common teaching tables and rules of thumb.

Using 20% as the substituted oxygen concentration helps simplify the calculation without greatly distorting the result. The other options don’t fit this convention: using 100% would ignore air entrainment, using 35% would simply mirror a mid-range FiO2 rather than a calculation shortcut, and using 21% would approximate room air but isn’t the standard substitute adopted for high FiO2 Venturi calculations.