Which statement correctly describes what the alveolar gas equation estimates?

Which statement correctly describes what the alveolar gas equation estimates?

• A. It estimates PaO2 directly.
• B. It uses barometric pressure to compute PaCO2.
• C. It estimates PAO2 from PB, PH2O, FiO2, and PaCO2.
• D. It replaces the need for calculating A-a gradient.

Answer: (C)

The main idea is that the alveolar gas equation estimates the alveolar oxygen tension (PAO2), not the arterial PaO2. It does this by using the barometric pressure to determine the inspired oxygen tension (PIO2) through FiO2 and the water vapor pressure of inspired air (PH2O), and then it accounts for CO2 using the measured PaCO2 divided by the respiratory quotient. In formula terms, PAO2 ≈ FiO2 × (PB − PH2O) − (PaCO2 / R). Because it provides PAO2, you can compare PAO2 to the arterial PaO2 to assess the A-a gradient, which reflects gas exchange efficiency. The equation thus relies on PB, PH2O, FiO2, and PaCO2, making that statement the best fit. It does not give PaO2 directly, and it does not replace the need to calculate or interpret the A-a gradient.

The main idea is that the alveolar gas equation estimates the alveolar oxygen tension (PAO2), not the arterial PaO2. It does this by using the barometric pressure to determine the inspired oxygen tension (PIO2) through FiO2 and the water vapor pressure of inspired air (PH2O), and then it accounts for CO2 using the measured PaCO2 divided by the respiratory quotient. In formula terms, PAO2 ≈ FiO2 × (PB − PH2O) − (PaCO2 / R). Because it provides PAO2, you can compare PAO2 to the arterial PaO2 to assess the A-a gradient, which reflects gas exchange efficiency. The equation thus relies on PB, PH2O, FiO2, and PaCO2, making that statement the best fit. It does not give PaO2 directly, and it does not replace the need to calculate or interpret the A-a gradient.