Alveolar Gas Equation Questions will appear on many exams throughout your respiratory therapy schooling and a high probability on your Therapist Multiple-Choice (TMC) Examination. Use our free online Alveolar Gas Equation calculator to help with the following test examples.

## Alveolar Gas Equation Units

**PaO2**: Alveolar partial pressure of oxygen (mmHg/Torr)

**FiO2**: Inspired oxygen (expressed as a decimal)

**Patm**: Atmospheric pressure (mmHg/Torr)

**pH2O**: Water vapour pressure, 47 mmHg saturated at 37℃

**PaCO2**: Arterial partial pressure of carbon dioxide (mmHg/Torr)

**RQ**: Respiratory exchange ratio

## Alveolar Gas Equation Questions

1. Calculate the alveolar gas tension given the following FiO2 = 0.21, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PaCO2 = 40 mmHg.

PAO2 = FiO2(Patm – PH2O) – PaCO2/RQ

PAO2 = 0.21(760 mmHg – 47 mmHg) – 40 mmHg/0.8

PAO2 = 0.21(713 mmHg) – 50 mmHg

PAO2 = 149.73 mmHg – 50 mmHg

PAO2 = 99.73 mmHg

2. Calculate the alveolar gas tension given the following FiO2 = 0.40, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PaCO2 = 50 mmHg.

PAO2 = FiO2(Patm – PH2O) – PaCO2/RQ

PAO2 = 0.40(760 mmHg – 47 mmHg) – 50 mmHg/0.8

PAO2 = 0.40(713 mmHg) – 40 mmHg

PAO2 = 285.2 mmHg – 40 mmHg

PAO2 = 245.2 mmHg

3. Calculate the alveolar gas tension given the following FiO2 = 0.21, Patm = 700 Torr, pH2O = 47 Torr, RQ = 0.8, and PaCO2 = 38 Torr.

PAO2 = FiO2(Patm – PH2O) – PaCO2/RQ

PAO2 = 0.21(700 Torr – 47 Torr) – 38 Torr/0.8

PAO2 = 0.21(653 Torr) – 47.5 Torr

PAO2 = 137.13 Torr – 47.5 Torr

PAO2 = 89.63 Torr

## Algebra and the Alveolar Gas Equation

Using algebra you should be able to solve for other variables within the Alveolar Gas Equation.

1. Calculate the FiO2 given the following PAO2 = 99.73 mmHg, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PaCO2 = 40 mmHg.

PAO2 = FiO2(Patm – PH2O) – PaCO2/RQ

99.73 = FiO2(760 mmHg – 47 mmHg) – 40 mmHg/0.8

99.73 = FiO2(713 mmHg) – 50 mmHg

99.73 + 50 mmHg = FiO2(713 mmHg)

149.73 = FiO2(713 mmHg)

149.73 / 713 mmHg = FiO2

FiO2 = 0.21