What is PAO2 when FiO2 = 0.21, PaCO2 = 40, Pb = 760, PH2O = 47, R = 0.8?

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What is PAO2 when FiO2 = 0.21, PaCO2 = 40, Pb = 760, PH2O = 47, R = 0.8?

The alveolar gas equation is used to estimate the oxygen pressure in the alveoli (PAO2) from the inspired oxygen and ventilation status. First find the inspired oxygen tension: PIO2 = FiO2 × (Pb − PH2O). With FiO2 = 0.21, Pb = 760 mmHg and PH2O = 47 mmHg, PIO2 = 0.21 × (760 − 47) ≈ 0.21 × 713 ≈ 150 mmHg. Then account for CO2 using PAO2 = PIO2 − (PaCO2 / R). With PaCO2 = 40 mmHg and R = 0.8, PaCO2 / R = 40 / 0.8 = 50 mmHg. Thus PAO2 ≈ 150 − 50 = 100 mmHg. This aligns with the normal alveolar oxygen tension on room air. The other options reflect either the inspiratory tension alone (about 150) or values not reduced by the CO2 term, which is why they are not correct.

The alveolar gas equation is used to estimate the oxygen pressure in the alveoli (PAO2) from the inspired oxygen and ventilation status. First find the inspired oxygen tension: PIO2 = FiO2 × (Pb − PH2O). With FiO2 = 0.21, Pb = 760 mmHg and PH2O = 47 mmHg, PIO2 = 0.21 × (760 − 47) ≈ 0.21 × 713 ≈ 150 mmHg. Then account for CO2 using PAO2 = PIO2 − (PaCO2 / R). With PaCO2 = 40 mmHg and R = 0.8, PaCO2 / R = 40 / 0.8 = 50 mmHg. Thus PAO2 ≈ 150 − 50 = 100 mmHg. This aligns with the normal alveolar oxygen tension on room air. The other options reflect either the inspiratory tension alone (about 150) or values not reduced by the CO2 term, which is why they are not correct.

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