# Laboratoy Diagnostics: A-A Gradient & Alveolar Gas Equation

by Carlo Raj, MD
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Finally, we are dealing with the gases, this puts us into yet another formula and this is one in which you can’t, there is no way you can circumvent this topic. A-a gradient is what we are looking at and it is the big 'A' that you are paying attention to and the little a. Literally. Have to. The big 'A' represents PAO2. In another words, this is the oxygen in your alveoli and then you subtract from this, the oxygen that is in your little 'a' and that then represents the artery. If you take a look at the first bullet point here, you have, PaO2 and from hence forth, you are not going to hear me say PaO2 anymore. I am just going to refer to as being PO2, but to make sure that we are clear and that our teaching points are coming across. The PaO2 is a partial pressure of oxygen in the artery and you will tell me that is approximately PO2, there you go, of 100. 95, depending on the little bit of shunt. Now, the PAO2 would be the alveoli and that is obtained from the alveoli, obviously, but more importantly, let’s just talk about what A-a gradient means to you. What does that gradient even refer to, the gradient coming out of the alveoli or going into the alveoli? Put yourself in the alveoli right now. There you are. You are sitting in that sack. Are you there? Nice to see you. Okay, now, in that sack, you are trying to get that oxygen through the alveolar membrane, through the interstitium and into the pulmonary capillary. That is the gradient. Who are you? Oxygen, okay. So, the gradient there should normally be, well how much? What is your PO2 in the alveoli,...

The lecture Laboratoy Diagnostics: A-A Gradient & Alveolar Gas Equation by Carlo Raj, MD is from the course Pulmonary Diagnostics. It contains the following chapters:

• Assessing Lung Function
• Alveolar Gas Equation

### Included Quiz Questions

1. PAO2 - PaO2
2. PaO2 - PAO2
3. PAO2 - PCO2
4. PaO2 - PCO2
5. PCO2 - PAO2
1. ABG, alveolar gas equation
2. ABG, FEV1/FVC
3. Venous blood gas, alveolar gas equation
4. PaO2, PAO2
5. PaO2 - PAO2, alveolar gas equation
1. 40 mmHg
2. 100 mmHg
3. 60 mmHg
4. 80 mmHg
5. 120 mmHg
1. 0.2
2. 0.8
3. 1.0
4. 1.2
5. 2.2
1. VCO2/VO2 = 0.8
2. PiO2/VO2 = 0.8
3. VCO2/VO2 = 0.2
4. VO2/VCO2 = 1.0
5. VO2/VCO2 = 1.2
1. PAO2 = PiO2 - (PaCO2/R)
2. PAO2 = FiO2 - (PaCO2/R)
3. PaO2 = PiO2 - (PACO2/R)
4. PaO2 = FiO2 - (PaCO2/R)
5. PAO2 = (PB - PH2O) - (PaCO2/R)

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