How can the ppO2 of a breathing gas be calculated?

To calculate the partial pressure of oxygen (ppO2) in a breathing gas, you need to consider the total ambient pressure and the fraction of oxygen in that gas mixture. The formula for partial pressure is derived from Dalton's Law of Partial Pressures, which states that in a gas mixture, the total pressure is the sum of the partial pressures of the individual gases.

When using this principle, the appropriate calculation involves multiplying the ambient pressure (which is the total pressure exerted by the atmosphere or the pressure in a diving environment) by the fraction of oxygen (FO2) in the breathing gas. This results in the ppO2, providing the effective pressure of oxygen available for respiration.

In contrast, dividing ambient pressure by the fraction of oxygen would yield an incorrect value, as it does not reflect the direct relationship needed for calculating partial pressure. Similarly, multiplying cylinder pressure by the fraction of nitrogen does not pertain to calculating ppO2, as this would factor in nitrogen instead of oxygen. Lastly, multiplying air pressure by the fraction of oxygen confuses the context since air pressure typically refers to ambient pressure, emphasizing the necessity of clarifying which pressures and fractions are being used in the calculation. Thus, the correct method is to multiply ambient pressure by