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
