In a mixture with nitrous oxide to oxygen ratio of 2:1, which of the following is true about the inspired gas composition?

• A. Oxygen is one third of the mix
• B. Oxygen is two thirds of the mix
• C. Oxygen is half of the mix
• D. Oxygen is all of the mix

Answer: (A)

Interpreting gas mix ratios by volume shows you how much of each gas is present in the total mix. A nitrous oxide to oxygen ratio of 2:1 means the mixture is three equal parts in total: two parts N2O and one part O2. The oxygen portion is 1 out of 3 parts, so oxygen makes up one third of the mix (about 33%). The remaining two thirds are nitrous oxide. Hence oxygen being one third of the mix correctly describes the inspired gas composition. If you had oxygen as two thirds or half, you’d need a different ratio, and if oxygen were all of the mix, it would be pure oxygen with no nitrous oxide.