What is the equation for calculating P(A-a)O2 in mm Hg?

The equation for calculating P(A-a)O2, which represents the difference between the alveolar oxygen partial pressure (PAO2) and arterial oxygen partial pressure (PaO2), is based on the principles of gas exchange in the lungs. The correct formulation notably takes into account the inspired fraction of oxygen (FiO2), the atmospheric pressure (Pb), and the partial pressure of water vapor (PH2O), which is essential for accurately determining the effective partial pressure of oxygen in the alveoli.

The equation in option B reflects these considerations accurately: PAO2 is calculated by adjusting the fraction of inspired oxygen (FiO2) with the barometric pressure (Pb) corrected by the partial pressure of water vapor (PH2O), and then subtracting the contribution of carbon dioxide (PaCO2) adjusted for the respiratory exchange ratio (R). This formulation allows for a more precise assessment of how well oxygen is being transferred from the alveoli into the blood, accounting for the presence of water vapor in the airways which reduces the partial pressure available for gas exchange.

In this context, the adjustments in this equation help clarify how changes in inspired oxygen and environmental pressures influence alveolar oxygen levels, thus making it critical for clinicians to understand and