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Alveolar Gas Equation – Laboratory Diagnostics

by Carlo Raj, MD
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    00:00 that means to you clinically? Ah, that is what we are getting at. Let’s talk about the equation, shall we? Now, every step of the way, I want you to be clear about what we are looking at. Your PAO2, is your PiO2, which you are inspiring and from this you subtract your PaCO2 divided by aO2, excuse me, CO2 and divided by the R quotient. We are going to put all this together. What is your PiO2? This then represents your FiO2 and your barometric pressure and the water vapour pressure. Let's go even one step further. Your FiO2 is, for the most part, going to remain pretty constant on planet Earth at approximately 0.2. Please use 0, that will make your life easier. Now, what is this whole thing that I have been repeating over and over again about high altitude? What does that mean? Now, you will see.

    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.

    03:20 And all we have done with 760 multiply by 0.2. What do you get? 160. Is it 160 of PO2 that you have in the alveoli? Not at all. That is ambient air. What’s next? You put in the trachea. Once you put in the trachea, the PO2 in your inspiratory air will be 760 minus 47 which gives you approximately 700 and you multiply that by 0.2. That will give you 150, isn’t it? Physiologically, remember this conversation? That 150, is that PO2 of alveoli? No. What do we say PO2 alveoli is? 100. So, what’s left? Oh, the carbon dioxide. What’s your carbon dioxide approximately in the alveoli? Remember, you are taking the carbon dioxide from the pulmonary capillary, putting into alveoli and you are blowing it off. So, CO2 approximately 46-47. Hmm, what are you going to do with this? You subtract it from the 150 and you get approximately 100. Stick to that and you will be fine. So, PiO2 equals the partial pressure of oxygen in the central airways. Isn’t that interesting? So, take a look at that equation that we see, the second bullet point there. PiO2 is only taken into consideration the air that is in the airway. What is the difference between the second and the third equation? That third big equation which is the complete equation for alveolar gas equation is the actual oxygen in the alveoli. Do you see how beautifully that worked? You literally are moving from compartment to compartment to compartment, from the outside world externally to the alveoli and every step of the way, you want to be asked questions. So, what is this fraction of inspired oxygen fraction? 0, I told you about, room air. I don’t care for up in the mountains or if you are down in the sea level. PaCO2, value from your ABG, they will give that to you. What’s normal? Approximate please. We just had a huge discussion forever on acid-based disturbances, 40. Now, the only thing is just make sure a week from today, you come back and you still say, “Oh, yeah, PCO2 is 40.” Yes, you want to test your short term memory. But, you don’t rely upon your short term memory exclusively. Every so often, you keep coming back and you review, and it’s the only way. It’s about reinforcement. Here is the barometric pressure at sea level, 760. Your water vapour pressure, 47 at 37 degree Celsius. Your respiratory quotient, about 0.8. You can use 1.0 for calculation purposes. And the ratio of carbon dioxide production to oxygen consumption is what this is. May I ask you something? I want you to go down to tissue. Are you there? Tissue. Really, how do you get there? You have the aorta, systemic arteries and you end up getting into arterioles and the capillaries and I am at the tissue. Okay, what are you doing at the level of tissue? That dissolved oxygen, or first diffused through, followed by? Good. That oxygen that is coming off of my haemoglobin. Are we clear? One oxygen comes off the haemoglobin and all of a sudden, my PO2 drops down to 40. How many oxygen have on your haemoglobin? 4. So, Dr. Raj, you are saying 1 comes off, I have 3 left on my haemoglobin, there is 75% and that is equivalent to a PO2 of 40? Yep, amazing. That is still lot of oxygen on your haemoglobin, isn’t it? And that's still, you call that deoxygenated? Yes, you do. That oxygen is being consumed by the tissue to do what? Daily activity. How about just to breathe, huh? Daily activity. And in the mean time, if that oxygen allows for you to go through aerobic glycosis, TCA cycle, electron transfer chain and you are producing carbon dioxide. There you go.

    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.


    About the Lecture

    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. 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)

    Author of lecture Alveolar Gas Equation – Laboratory Diagnostics

     Carlo Raj, MD

    Carlo Raj, MD


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