Mechanical Vent Test 3 Practice 2026 - Free Mechanical Ventilator Practice Questions and Study Guide

The main idea is to adjust FiO2 in proportion to the change you want in PaO2, assuming the relationship between FiO2 and PaO2 is roughly linear for the current situation. If you know the current FiO2 (FiO2 known) that gives a current PaO2 (PaO2 known) and you want to achieve a new PaO2 (PaO2 desired), you scale FiO2 by the ratio of the target to the current PaO2. So the formula is:

Desired FiO2 = FiO2 known × (PaO2 desired / PaO2 known)

This reflects the idea that if you need a higher PaO2, you increase FiO2 proportionally, and if you need a lower PaO2, you decrease FiO2. The other formulations either invert the ratio or use a subtraction, which don’t represent how FiO2 relates to PaO2 in this context.