Alveolar Gas Equation
Alveolar Gas Equation is used to calculate the partial pressure of oxygen in the alveoli being it is not yet possible to collect gases straight from the alveoli. The main function of the alveolar gas equation is in calculating Alveolar Arterial Oxygen Gradient (A-a gradient).
- PaCO2: Partial pressure of Carbon Dioxide, normal physiological conditions around 40 to 45 mmHg.
- FiO2: Fraction of inspired oxygen (usually as a fraction 0.21, but entered here as a percentage for ease of use 21%).
- Patm: Ambient atmospheric pressure, which is 760 mmHg at sea level.
- PH2O: Vapour pressure of water at 37°C and is equal to 47 mmHg (760 mmHg – 47 mmHg).
- RQ: Respiratory quotient or respiratory coefficient, it is the ratio of CO2 eliminated divided by the O2 consumed, and its value is typically 0.8.
Alveolar Gas Equation Calculator
PAO2:
Using Normals – Alveolar Gas Equation
PAO2 = (FiO2 ( Patm -PH2O)) – (PaCO2 / RQ)
PAO2 = (0.21 ( 760 mmHg – 47 mmHg)) – (40 mmHg / 0.8)
PAO2 = 97.73 mmHg
Practice Exam Questions
Study from our Alveolar Gas Equations practice exam questions, our test bank has examples you may see on your Therapist Multiple-Choice (TMC) Examination or needed while doing Clinical Simulation Examination (CSE), furthermore these questions maybe appear on CBRC National Respiratory Therapy Examination.
1. Given the following: FiO2 = 0.21, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PaCO2 = 40 mmHg. What is the patient’s Alveolar Partial Pressure of Oxygen?
A. 103.95 mmHg
B. 99.73 mmHg
C. 80.0 mmHg
D. 112.35 mmHg
Answer: B.
2. Given the following FiO2 = 0.40, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PaCO2 = 60 mmHg. Calculate the Alveolar Gas Tension?
A. 310.2 mmHg
B. 99.73 mmHg
C. 40.2 mmHg
D. 210.2 mmHg
Answer: D.
3. What does the Alveolar Gas Equation mean?
A. The Alveolar Gas Equation relates to the alveolar concentration of oxygen, equivalently the partial pressure of oxygen in the alveoli.
B. The Alveolar Gas Equation relates to the alveolar concentration of oxygen, equivalently the partial pressure of oxygen in the artery.
C. The Alveolar Gas Equation relates to the arterial concentration of oxygen, equivalently the partial pressure of oxygen in the alveoli.
D. The Alveolar Gas Equation relates to the arterial concentration of oxygen, equivalently the partial pressure of oxygen in the artery.
Answer: A.
4. Given the following FiO2 = 0.21, Patm = 760 mmHg, pH2O = 47 mmHg, PQ = 0.8, and PAO2= 120 mmHg. Calculate the partial pressure of Carbon Dioxide ( PaCO2).
A. 23.78 mmHg
B. 29.13 mmHg
C. 65.22 mmHg
D. 14.23 mmHg
Answer: A.
5. Calculate the FiO2 given the following: PaCO2 = 50 mmHg, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PAO2 = 100 mmHg.
A. 0.23
B. 0.25
C. 0.27
D. 0.20
Answer: A.
6. During rounds the Doctor asks you to calculate the patient’s Alveolar Partial Pressure of Oxygen? The Doctor gives you the following information: FiO2 = 0.50, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PaCO2 = 70 mmHg.
A. 221.28 mmHg
B. 269.00 mmHg
C. 80.0 mmHg
D. 112.35 mmHg
Answer: B.
7. A fellow Respiratory Therapy Student wants to know what the PaCO2 is given the following: FiO2 = 0.50, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and the Alveolar Gas Tension is 244.00 mmHg?
A. 90 mmHg
B. 76 mmHg
C. 34 mmHg
D. 102 mmHg
Answer: A.
8. Given the following: FiO2 = 0.35, Patm = 760 mmHg, pH2O = 47 mmHg, RQ = 0.8, and PAO2 = 168.30 mmHg. Calculate the partial pressure of Carbon Dioxide.
A. 90 mmHg
B. 76 mmHg
C. 34 mmHg
D. 65 mmHg
Answer: D.
9. Given the following: FiO2 = 0.21, Patm = 760 mmHg, pH2O = 47 mmHg, PQ = 0.8, and PAO2= 109.73 mmHg, and PaCO2 = 40. Calculate the Respiratory quotient (RQ).
A. 0.8
B. 0.9
C. 1.0
D. 1.1
Answer: C.
10. Calculate the Patm given the following: PaCO2 = 40 mmHg, pH2O = 47 mmHg,FiO2 = 0.21, RQ = 0.8, and PAO2 = 100.78 mmHg.
A. 765 mmHg
B. 755 mmHg
C. 745 mmHg
D. 735 mmHg
Answer: A.
11. Calculate the Vapor pressure given the following: PaCO2 = 40 mmHg, Patm = 760 mmHg,FiO2 = 0.21, RQ = 0.8, and PAO2 = 100.78 mmHg.
A. 765 mmHg
B. 755 mmHg
C. 745 mmHg
D. 735 mmHg
Answer: A.
