Alveolar gas equation example: with FiO2 0.60, PaCO2 45, and R=0.8 at sea level, PAO2 is approximately?

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Alveolar gas equation example: with FiO2 0.60, PaCO2 45, and R=0.8 at sea level, PAO2 is approximately?

The alveolar gas equation links the amount of oxygen reaching the alveoli to the inspired oxygen, accounting for barometric pressure, water vapor, and CO2 with the respiratory quotient. The key is PIO2 minus the CO2 term: PAO2 = PIO2 − (PaCO2 / R). First compute PIO2: at sea level, PB − PH2O = 760 − 47 = 713 mmHg. PIO2 = FiO2 × (PB − PH2O) = 0.60 × 713 = 427.8 mmHg. Next subtract the CO2 term: PaCO2 / R = 45 / 0.8 = 56.25 mmHg. So PAO2 ≈ 427.8 − 56.25 = 371.55 mmHg, about 371.6 mmHg. This matches the expected value. Using PaCO2 × (1 − R) would give a different result (around 419 mmHg), which is not correct for this equation.

The alveolar gas equation links the amount of oxygen reaching the alveoli to the inspired oxygen, accounting for barometric pressure, water vapor, and CO2 with the respiratory quotient. The key is PIO2 minus the CO2 term: PAO2 = PIO2 − (PaCO2 / R).

First compute PIO2: at sea level, PB − PH2O = 760 − 47 = 713 mmHg. PIO2 = FiO2 × (PB − PH2O) = 0.60 × 713 = 427.8 mmHg.

Next subtract the CO2 term: PaCO2 / R = 45 / 0.8 = 56.25 mmHg.

So PAO2 ≈ 427.8 − 56.25 = 371.55 mmHg, about 371.6 mmHg.

This matches the expected value. Using PaCO2 × (1 − R) would give a different result (around 419 mmHg), which is not correct for this equation.

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