# Alveolar Gas Equation – Laboratory Diagnostics

by Carlo Raj, MD

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00:56 Now, if you are ahead of the game, then this little part right here is really good review and just positive reinforcement. So, let's begin. Your PAO2 equals the following. So, why is it only say PAO2. Where is little 'a'? Because while let’s say that you are doing a question on a computer screen, really difficult for you to do an ABG on a computer screen. You can’t take a syringe and put the needle in. As much as you would like to, you can’t.

01:24 So, the little 'a', they will give that to you. So, of the A-a gradient, all you are focusing on right now is the big 'A', is that clear? So, Dr. Raj, this entire formula is only for the big 'A'? Yep. That is exactly what I am telling you. Now, if you are lucky, maybe they will just give it to you. But, chances are not. So, someone has to break that to you and you are responsible for calculating it. ........., as I said, they will do an ABG and they will give you the PO2 in the arterial side.

01:52 Let's talk about the alveoli now. So, you have an FiO2 of 0, what does 760 mean to you? Good. That is you barometric pressure at sea level. Take that air, oxygen, put it into your trachea. What is the trachea responsible for? It has cartilage we talked about. We talked about cilia, mucous. This is then introducing water vapour. So, that partial pressure of water is 47 mmHg. You subtract this from the 760 and you get approximately 713 or let’s say about 700 to keep things rather simple. Where you are at this point? You are in the trachea, right, in the breathing area, basically. Breathing meaning strictly the conducting zone. What does that conducting zone mean to you? Do you see as to how that anatomy now is coming into play? That conducting zone is going to take this oxygen from the ambient air at sea level and put it into the alveoli. What about that alveoli? Your oxygen has a roommate. And what about these roommates? Well, they don’t exactly get along. And actually, when one is in, the other one is out. That way, they actually maintain homeostasis. What am I talking about? Carbon dioxide and oxygen in the alveoli, keep it simple. So, whenever there is carbon dioxide, inverse relationship with oxygen.

03:13 So, now, let's start way back. Let's start back up in the ambient air and we have 760.

07:26 Now, we go to systemic veins. Systemic veins, they have carbon dioxide being added to my 40 to give you 46. Are we clear? The ratio of carbon dioxide production at the level of tissue to oxygen consumption at the level of the tissue. It makes no sense for you to say there is oxygen consumption at the lung. All you are doing at the lung is to deliver the oxygen for Pete’s sakes. I will give you exception for the carbon dioxide, but that statement right here, do you understand that? Consumption and production at the level of tissue.

The lecture Alveolar Gas Equation – Laboratory Diagnostics by Carlo Raj, MD is from the course Pulmonary Diagnostics.

### Included Quiz Questions

1. PAO2 - PaO2
2. PaO2 - PAO2
3. PAO2 - PCO2
4. PaO2 - PCO2
5. PCO2 - PAO2
1. Arterial blood gas, and alveolar gas equation.
2. Alveolar gas equation, and arterial blood gas.
3. Venous blood gas, and alveolar gas equation.
4. PaO2 and PAO2.
5. PaO2 - PAO2, and 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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Dr Raj is awesome
By Sima Fatima u. on 27. October 2019 for Alveolar Gas Equation – Laboratory Diagnostics

Dr Raj is absolutely incredible; respiratory physio/patho is a subject I find very difficult to understand, but when I started to watch Dr Raj's lectures, everything started to slowly make sense. He makes the concepts simple and easy to understand, and he always relates the concepts to their clinical purposes....I just LOVE this professor!