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Hello and welcome to the third video in our respiratory snapshot series. And this is part one of two on ABG interpretation. So the aim of this video is more to provide context and, and and a foray into the basic sciences that are involved in asset basing myasis. Whereas our second video is going to be an entirely case based um video. So it's just going to be cases and questions that you can use to practice your interpretation before exams, before we get started. I really wanna make sure we understand just a bit of a background on what acid base metasis is. We're looking at the PH of arterial blood primarily but close be applied to venous blood. Normal arterial blood range for PH is 7.35 to 7.45. This range is slightly lower for venous blood if the PHPH is lower than 7.35 that's a sign of acidemia. If greater than 7.45. It's alkalemia. The ph of arterial and venous blood is tightly regulated because acid base homeostasis is critical for the functioning of enzymes involved in a wide variety of functions like regulating the functions of the kidney and absorption of electrolytes, regulating the heart. And so you can see clearly why this is very important. So we need to first define what an acid is and what a basis. And so from our chemistry, um prest we might understand that an acid is any molecule that contains a hydrogen atom and that can release these hydrogen atoms in the form of a hydrogen ion or proton. Whereas the base is any molecule that can accept hydrogen ions. And the difference between a strong acid versus a weak acid is largely dependent on number one, how fast they release and dissociate and release those hydrogen ions or protons and the amount of protons or hydrogen ions that they release. Whereas a strong base is generally a molecule that can accept hydrogen ions or protons very quickly and remove it from solution. A weaker base can perform this task. But at a slower rate or maybe they are able to bind less hydrogen problems. This is the formula that's used to calculate the Ph where Ph is equal to the PK A which is the dissociation constant of the molecule. Typically the acid plus the log of a minus. So a minus stands for the conjugate base. So whenever you release a half proton, the acid will then form a conjugate base, which is what will then accept the hydrogen ion to then return back to its normal acidic state. And in the bicarbonate buffer system. This is usually the bicarbonate hydrogen. The um H stands for the acid concentration and the acid that releases the hydrogen ion in the bicarbonate buffer system, which is a major buffer system in buffer system for the body but not the only buffer system, but it is the major buffer system for the body that is determined by primarily the concentration of dissolved CO2. Now, this is because CO2 reacts with water to form carbonic acid. Um and that associates to form hydrogen ions and bicarbon ions. And its reaction is catalyzed by the presence of a carbonic and hydrate enzyme usually found on the border of the red blood cells or erythrose. The total amount of dissolved carbon dioxide is equal to the PCO two in the arterial blood. And that is a direct reflection on the PCO two of the partial pressure of carbon dioxide bound within the alveoli in arterial. But in venous blood, this is associated also with the tissues that also produce CO2 via anaerobic respiration. And that partial pressure of CO2 or the sorry, the the amount of dissolved CO2 is equal to the partial pressure of CO2 multiplied by its solubility constants. Therefore, the um the partial pressure of CO2 in the arterial blood is determined by the partial pressure in the alveoli. And that's determined by the rate and depth of your breathing. So, ultimately, by varying the how fast you breathe. And how deeply you breathe, you can regulate the amount of dissolved CO2 and therefore the amount of acid produced. And therefore you can regulate the PH of the body using the breathing and the respiratory system. This is the Henderson Hassleback equation um which is used to calculate the PH of the blood. And that is the dissociation constant for the PK, which is 6.1 plus the log of the bicarbonate iron concentration, which is a conjugate base. And the um essentially acid is the solubility, constant constant of CO2 multiplied by its partial pressure. And that's determined by the alveolar concentration of CO2, primarily in arterial blood. So you can see how an increase in bicarbonate ions would therefore increase the Ph making it more alkaline. Whereas an increase in the partial pressure of CO2 commonly because of hyperventilation or difficulties in um removing CO2 from the body that would actually decrease the PH and therefore make it more acidic. And that also identifies how the lungs have a role in regulating the partial pressure of CO2 in the alveoli, therefore regulating the amount of CO2 in the arterial blood, therefore regulating the ph of the arterial blood. And this is just once again, emphasized on this slide where when the amount of CO2 increases, the forward reaction increases, you produce more higher than s but you also produce more bicarbonate. S as we said, that's the conjugate base. Um Whereas if you increase the amount of bicarbonates, there's a reverse reaction to the left and that will result in an increase in the amount of CO2 being produced. And CO2 usually exists either bound to carb amino, sorry, bound to hemoglobin in the form of carb amino compounds or it's found dissolved in the plasma or most commonly, it's also found in the form of bicarbon ions in the blood or in the plasma. So, this is a um a pictorial representation of what I've been discussing for the last couple of slides. And this is at the level of the tissues. So the tissues will produce CO2 um via either anaerobic or aerobic respiration as a byproduct of um metabolism that CO2 will dissolve across the membrane of the cell into the tissue and then eventually into the plasma. Once it enters the plasma, the idea that CO2 can do a few things. Erythrocytes have carbonic anhydrate, which is an enzyme that catalyzes the reaction of CO2 with water to form carbonic acid that associates to form bicarbonate ions. Those bicarbonate ions in this reaction here will be exchanged at the membrane using a chloride bicarbonate ion exchanger. Therefore, resulting in chloride ions being poured into the red blood cell and bicarbonate ions being pumped into the plasma. So that's one way in which CO2 is stored. Now CO2 at the level of the tissue is also able to directly bind to hemoglobin to form carb amino hemoglobin. Hemoglobin has a higher affinity of carbon dioxide when it has a low level of oxygen. Because when there's a low and posh pressure of oxygen in the environment, its affinity for oxygen is reduced. Um and so hemoglobin will change its state from its relaxed state into a 10 state. CO2 can then bind to the hemoglobin to form carbamine, hemoglobin. So actually, hemoglobin can actually directly carry CO2 as well in the plasma. There's also the slow reaction to form the bicarbon ions and the hydrogen ions and these hydrogen ions will bind to plasma proteins but can also directly react or bind to hemoglobin as well. So high can react with hemoglobin directly as well. Um And the reason this is important is because once these red blood cells reach the lungs and this is what we're talking about from the perspective of the venous blood, the partial pressure of oxygen in the environment will increase because the lungs have a high partial pressure environment for oxygen. The affinity for hemoglobin for oxygen increases and the affinity for carbon dioxide decreases. So, carbon dioxide is directly offloaded from hemoglobin in that form. Moreover, the affinity for hemoglobin for hydrogen I also decreases in a high oxygen environment. And so hydrogen hours will be released. These hydrogen ions will then enter the plasma and react with bicarbon S in the plasma to form in the reverse reaction CO2, you form even more CO2 in that way as well. Um And finally, the CO2 that's in the plasma, the bicarbonate as, as we mentioned, also reacts with hydrogen S as well. So this is just AAA basic review of the Basic Sciences. If you want to get a better understanding of this, I would recommend reviewing Tatiana Christi's lectures from uh second year in the Respi module. The bicarbonate ions are primarily regulated by the um production. It's product is produced in red blood cells. And that's as we mentioned through that reaction using the chronic anhydrous enzyme and then it's exchanged to CHS. However, it's regulated primarily by the kidneys. The kidneys regulate number one, how much bicarbonate hours are reabsorbed? That's usually across the tubular epithelium. Bicarbonate hours are then are reacting with hydrogen hours that pumped out using the sodium hydrogen exchanger. And those hydrogen ions in the tubular lumen reacted by carbon ions to form carbonic acid that reacts from that associates from CO2 and water that CO2 dissolves across the membrane of the tubal epithelium reacts internally within the tubular cells um with water and therefore forms bicarbon an acid that associates with bicarbonate irons again, and those bicarbonate irons are finally pumped into or exchanged into the ti of the kidney. And that's how bicarbonate I are reabsorbed primarily. Um This will also vary based on acidic or um alkaline states. So when you have a more acidic ph the kidney in it's a long term process, but it compensates by um increasing the amount of bicarbonate irons, reabsorb. Um That's because you get more hydrogen pumped out to react to those bicarbonate irons. If an interfer in a more alkaline state, the kidney will re re reabsorb less bicarbonates and excrete more bicarbonate as well. So that's how the kidneys regulate the Ph. So, ultimately, the kidneys regulate the bicarbonate iron concentration and the lungs regulate the P CO2. So, therefore, the kidneys and the lungs are primarily the main organs or the main areas responsible for acid base homeostasis. The ABG is a very common diagnostic test you performed as a foundation in, but it's also commonly tested in third year and fifth year. As a clinical skill, you take the sample usually from the radial artery. After obviously explaining the procedure to the patient and gaining the informed consent, you will check the blood flow as appropriate to the ulnar artery. Using the ac you squeeze both the ra and ulnar arteries and get them to pump their hands. When the, when the palm goes down, you release the um occlusion on the ulnar artery and see if there's a return of blood to the palm and fingers. And that would identify that the older artery is providing a good blood flow after you take your sample, provide appropriate aftercare, explain that it's going to be a slightly uncomfortable procedure. Have some s of ready apply pressure over the area to make sure that they don't bleed um too much. And then um make sure to explain the process and explain to patients if they feel any significant pain. Please inform a uh a local healthcare worker, you would insert the blood into the machine. Um obviously, then the machine would produce a, a paper report that would contain the key features. You need to, the key things you want to know are confirm, patient identity, confirm an arterial and venous sample. The F IO two patient and their temperature. This is usually what you see on a, an ABG report. So it have a few details of the patient ID, date of birth if I two temperature. But the main things you want to focus on on the ph the partial pressure of CO2, you want to know that po two actually the bicarbon iron concentration and the base excess is quite useful. It does provide a quick um sodium and potassium. So if you want to check electrolytes quickly, that's also important. A lactate and glucose are very important as well. It can help you identify something who's hyperglycemic, somebody who's got lactic acidosis going on, it might be a sign of sepsis or tissues. And ultimately, you interpret the ABG um first looking at the PH by remembering that a normal ph does not mean they have a normal acid base status. You need to check the P CO2 and their bicarbonate and concentration. And compare between the two of them to check for compensation. So the five step approach really is number one clinically, what does the patient look like? How the patient and what are their clinical features? So, if they're septic, drowsy decreased consciousness, then that will give you an idea of what the ABG might look like. Assess the oxygenation are they hypoxic and then determine the ph are they acidemic or alkalemic? If they are acidemic is their CO2 raised. That's a sign of respirator acidosis. If they are alkalemic is their CO2 low, that's a sign of respiratory alkalosis. So check their respiratory component and then check them with the metabolic component. If their PH is low or their bicarbonate, iron level is low, that's a risk of metabolic acidosis because there's some other acid that's being produced that's reacting with the bicarbon I locally in the plasma, reducing it. If the PH is high greater than 7.45 then if the bicarbonate is high, that is a sign of metabolic alkalosis as well. Finally, this is a table that covers all of the key values of PH PC two and bicarbonate. When you're looking at the ABG and will help you look at the interpretation um of these different findings. For example, focusing on metabolic acidosis, we can see that you have an acidic Ph so great less than seven point P five. The PC two is normal, but the bicarbon archest administration is decreased suggesting some other form of acid is being produced. Now, to take it a step further, you can look at the anion gap and then you can subdivide metabolic acidosis based on whether they have a normal anion gap or whether they have a increased anion gap, suggesting some of the folic acid being produced. Now, for all of these different um labels, there are different, wide variety of, there's a wide variety of different causes that can result in these conditions. We'll cover this in our case based study where I'll try and incorporate as many different presentations um and their common interpretations in the form of SBA questions. So hopefully this video has been useful. And before I finish, I'd just like to say, thank you very much to Dr Aman Verma for reviewing these slides. The GE in Hall medical textbook also has a great chapter on asset based interpretation. So this would be a really good resource to consider when reviewing this as well and that you get a free link to that University of Leicester Library. And finally, another amazing resource to consider for this topic is Dr Titanic's lectures on the Basic Science of um ABG interpretation, but also the Basic Sciences of CO2 and ana and regulation. Thank you for now and stay tuned for part two in this um mini series on ABG interpretation. I would also like to apologize if it's been unclear or if you feel like my voice has been slightly unclear in this video, I have a cold at the moment, so hopefully it hasn't impacted me learning too much. Thank you for now.