Which expression correctly represents the alveolar gas equation PAO2?

Which expression correctly represents the alveolar gas equation PAO2?

• A. PAO2 = FiO2(Patm - PH2O) - PaCO2/Respiratory quotient
• B. PAO2 = FiO2Patm - PH2O - PaCO2/Respiratory quotient
• C. PAO2 = Patm - PH2O - PaCO2/Respiratory quotient
• D. PAO2 = FiO2(Patm - PH2O) + PaCO2/Respiratory quotient

Answer: (A)

Alveolar oxygen tension is determined by the amount of oxygen delivered in inspired air, adjusted for humidification, minus the amount of oxygen consumed to exhale CO2. The correct expression is PAO2 = FiO2 (Patm − PH2O) − PaCO2 / R. This reflects that the inspired O2 fraction (FiO2) times the available driving pressure for gas exchange (Patm minus the water vapor pressure PH2O) sets the oxygen entering the alveoli, while the CO2 partial pressure in blood (PaCO2) divided by the respiratory quotient (R, about 0.8) accounts for the O2 used in producing CO2. The other forms either omit the FiO2 factor, omit the division by R, or use the wrong sign, which would misrepresent the balance between oxygen entering the alveoli and oxygen consumed to eliminate CO2. For example, with typical values (Patm 760 mmHg, PH2O ≈ 47 mmHg, FiO2 0.21, PaCO2 40 mmHg, R ≈ 0.8), PAO2 ≈ 0.21 × (760 − 47) − 40/0.8 ≈ 150 − 50 ≈ 100 mmHg.

Alveolar oxygen tension is determined by the amount of oxygen delivered in inspired air, adjusted for humidification, minus the amount of oxygen consumed to exhale CO2. The correct expression is PAO2 = FiO2 (Patm − PH2O) − PaCO2 / R. This reflects that the inspired O2 fraction (FiO2) times the available driving pressure for gas exchange (Patm minus the water vapor pressure PH2O) sets the oxygen entering the alveoli, while the CO2 partial pressure in blood (PaCO2) divided by the respiratory quotient (R, about 0.8) accounts for the O2 used in producing CO2. The other forms either omit the FiO2 factor, omit the division by R, or use the wrong sign, which would misrepresent the balance between oxygen entering the alveoli and oxygen consumed to eliminate CO2. For example, with typical values (Patm 760 mmHg, PH2O ≈ 47 mmHg, FiO2 0.21, PaCO2 40 mmHg, R ≈ 0.8), PAO2 ≈ 0.21 × (760 − 47) − 40/0.8 ≈ 150 − 50 ≈ 100 mmHg.