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Approach to Acid Base Status: Step 3 – Laboratory Diagnostics

by Carlo Raj, MD
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    00:02 Finally, with dealing with the gases, this puts us into yet another formula and this is one in which you cannot... you can’t... there is no way you can circumvent this topic.

    00:15 A-a gradient is what we are looking at and it is a 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.

    01:19 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, do you remember from physio? 100. Good! And that PO2 that is being carried by my pulmonary artery and pulmonary capillary into the alveoli, the PO2 is how much? A measly 40, right? So, that will be the blood on the...

    02:11 or oxygen content on the venous side, isn’t it? Deoxygenated blood, pulmonary artery and company. Therefore, the difference between the two would then give you a gradient of 60. That is a pretty big gradient for the oxygen to hop out of the alveoli and diffuse through the membranes into the pulmonary capillary and then bind to whom? Good. Haemoglobin.

    02:33 Hmm. Speaking of which, the PO2, the partial pressure of oxygen, is this actually referring to the haemoglobin attached or the oxygen attached to the haemoglobin? Not at all. The oxygen bound to haemoglobin, give me a test that you are going to use for its percentage.

    02:50 That is your saturation of oxygen, isn’t it? So, your saturation of oxygen approximately 97% being the optimal percentage of the oxygen bound to haemoglobin. Right? But, here, we are referring to PO2, that little bit of oxygen is dissolved in your blood. I hope that is clear. I hope that you understand the diffusion. I hope you understand the gradient and what 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, big “A”, it is the PiO2, which you are inspiring and from this you subtract your PaCO2 divided by aO2, excuse me, CO2 and divided by the RR 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. 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.

    04:13 Now, if you are ahead of the game, then this little part right here is really good review and just possibly enforce this again. Your PAO2 equals the following. So, why is it only say PAO2. What 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. As much as you would like to, you can’t.

    04:41 So, the big... 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, may be 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. Literally, as I said, they will do an ABG and they will give you the PO2 in the arterial side.

    05:08 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 celia, mucous, huh? Hah, 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 we 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 kick this oxygen from the ambient air at sea level and put it into the alveoli. Hmm, what about that alveoli? The 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.

    06:30 So, now, let’s start way back. Let’s start back up in the ambient air and we have 760 and all we are done with 760 multiply by that 0.2. What do you get? 160. Is it 160 of PO2 as 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 what...

    07:07 is that PO2 of alveoli? No, no, 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 it is... 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? The 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 asking 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.” That’s... It’s not... 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 70... 47 at 37 degree Celsius. 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? Hmm? I want you to go down to the tissue. Are you there? Tissue? Really? How do you get there? I have the aorta, systemic arteries and you end up getting into arterials and the capillaries and I am at the tissue. Okay, what are you doing at the level of tissue? They dissolve the oxygen or first diffuse through, followed by...? Good. Now, that oxygen is coming off of my haemoglobin. Are we clear? One oxygen comes after haemoglobin and all of a sudden, my


    About the Lecture

    The lecture Approach to Acid Base Status: Step 3 – Laboratory Diagnostics by Carlo Raj, MD is from the course Pulmonary Diagnostics.


    Included Quiz Questions

    1. Immediate respiratory compensation that will not correct pH.
    2. Delayed respiratory compensation that will correct the pH.
    3. Immediate metabolic compensation that will not correct pH.
    4. Immediate respiratory compensation that will correct the pH.
    5. Delay metabolic compensation that will correct the pH.
    1. Bicarbonate binds to H+ ions and acts as a buffer.
    2. Bicarbonate is excreted faster in acidic environments.
    3. Metabolic acidosis leads to decreased production of bicarbonate.
    4. Bicarbonate is lost, either through diarrhea or vomitting.
    5. Acidic pH denatures bicarbonate.
    1. Metabolic acidosis.
    2. Respiratory acidosis.
    3. Metabolic alkalosis.
    4. Respiratory alkalosis.
    5. PCO2 does not effect pH.
    1. PCO2 = 1.5 x [HCO3-] + 8
    2. PCO2 = 15 x [HCO3-] + 8
    3. PCO2 = 0.9 x [HCO3-] + 9
    4. PCO2 = 0.9 x [HCO3-] + 16
    5. PCO2 = 0.9 x [HCO3-] + 8
    1. 60mmHg
    2. 10mmHg
    3. 40mmHg
    4. 80mmHg
    5. 30mmHg

    Author of lecture Approach to Acid Base Status: Step 3 – Laboratory Diagnostics

     Carlo Raj, MD

    Carlo Raj, MD


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