In the alveolar gas equation, which term represents the effect of CO2 elimination relative to metabolism?

In the alveolar gas equation, which term represents the effect of CO2 elimination relative to metabolism?

• A. FiO2
• B. Patm
• C. PaCO2/Respiratory quotient
• D. PH2O

Answer: (C)

The alveolar gas equation estimates PAO2 by starting with the portion of inspired oxygen available after diluting with water vapor, then subtracting the amount of CO2 that must be eliminated relative to metabolism. The term that captures this CO2 effect is PaCO2 divided by the respiratory quotient. The respiratory quotient (RQ) is the ratio of CO2 produced to O2 consumed during metabolism, so dividing PaCO2 by RQ translates metabolic CO2 load into its impact on alveolar O2. Thus PAO2 = FiO2*(Patm − PH2O) − PaCO2/RQ. The other terms—FiO2 (inspired oxygen fraction), Patm (atmospheric pressure), and PH2O (water vapor pressure)—do not represent CO2 elimination relative to metabolism.

The alveolar gas equation estimates PAO2 by starting with the portion of inspired oxygen available after diluting with water vapor, then subtracting the amount of CO2 that must be eliminated relative to metabolism. The term that captures this CO2 effect is PaCO2 divided by the respiratory quotient. The respiratory quotient (RQ) is the ratio of CO2 produced to O2 consumed during metabolism, so dividing PaCO2 by RQ translates metabolic CO2 load into its impact on alveolar O2. Thus PAO2 = FiO2*(Patm − PH2O) − PaCO2/RQ. The other terms—FiO2 (inspired oxygen fraction), Patm (atmospheric pressure), and PH2O (water vapor pressure)—do not represent CO2 elimination relative to metabolism.