Which equation is used to calculate PAO2 from inspired oxygen and gas pressures?

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Which equation is used to calculate PAO2 from inspired oxygen and gas pressures?

Estimating PAO2 from inspired oxygen and gas pressures uses the alveolar gas equation. This equation ties together the oxygen you breathe, the pressure environment, and the CO2 present in the alveoli to give the oxygen tension inside the alveoli. PAO2 is calculated as the inspired O2 pressure minus the portion of CO2 that lowers the alveolar O2 gradient. The inspired O2 pressure, PIO2, is FiO2 times (barometric pressure minus water vapor pressure): PIO2 ≈ FiO2 × (Pb − PH2O). Then you subtract PaCO2 divided by the respiratory quotient (R) to account for the CO2 that’s equilibrated with the alveolar gas: PAO2 ≈ PIO2 − (PaCO2 / R). A small correction term may be added for non-ideal mixing, but it’s often negligible. Typical values at sea level: Pb ≈ 760 mmHg, PH2O ≈ 47 mmHg, R ≈ 0.8, so with room air FiO2 ≈ 0.21 and PaCO2 ≈ 40 mmHg, PAO2 is about 100 mmHg. This is the best choice because it directly converts inspired oxygen and gas pressures into an alveolar oxygen tension. The Henderson-Hasselbalch equation relates pH to CO2 and bicarbonate, not PAO2. The Bohr equation deals with dead space ventilation. Fick’s law describes diffusion across membranes, not how to compute PAO2 from inspired oxygen.

Estimating PAO2 from inspired oxygen and gas pressures uses the alveolar gas equation. This equation ties together the oxygen you breathe, the pressure environment, and the CO2 present in the alveoli to give the oxygen tension inside the alveoli.

PAO2 is calculated as the inspired O2 pressure minus the portion of CO2 that lowers the alveolar O2 gradient. The inspired O2 pressure, PIO2, is FiO2 times (barometric pressure minus water vapor pressure): PIO2 ≈ FiO2 × (Pb − PH2O). Then you subtract PaCO2 divided by the respiratory quotient (R) to account for the CO2 that’s equilibrated with the alveolar gas: PAO2 ≈ PIO2 − (PaCO2 / R). A small correction term may be added for non-ideal mixing, but it’s often negligible. Typical values at sea level: Pb ≈ 760 mmHg, PH2O ≈ 47 mmHg, R ≈ 0.8, so with room air FiO2 ≈ 0.21 and PaCO2 ≈ 40 mmHg, PAO2 is about 100 mmHg.

This is the best choice because it directly converts inspired oxygen and gas pressures into an alveolar oxygen tension. The Henderson-Hasselbalch equation relates pH to CO2 and bicarbonate, not PAO2. The Bohr equation deals with dead space ventilation. Fick’s law describes diffusion across membranes, not how to compute PAO2 from inspired oxygen.

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