Davenport Diagram – Laboratory Diagnostics

by Carlo Raj, MD

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    00:01 On this graph look at a very important concept and it’s dealing with acid base and the way that the questions are repost to you and the way that the patient will appear to you are all intermixed with this graph and the way that it’s modified but if you lay down the foundation, anything that they throw at you, in terms of modification of this graph, well, you won’t be confused and you won’t feel rattled.

    00:30 And what I mean by that is at first, what we’ll do is take a look at the graph and dissect it, line by line by line, and then we’ll start putting in information.

    00:41 Where as if the same protocol that we’re gonna utilize that we have been which is starting off with the pH, the arterial pH of the patient, and then from there fill in the gaps – welcome to the Davenport Diagram.

    00:55 Now, the hyperbolic curve that you’re seeing there is the carbon dioxide level.

    01:00 And just like you had done with many graphs in physiology, whenever you remain on that curve anywhere along the point on that curve, it will always remain the same.

    01:12 The last time we did something like this was dealing with cardiology in which you did the cardiac output curve and anywhere that you remain on that line the cardiac output remained exactly the same, do you remember that? The same concept can be and should be and has to be, used with the carbon dioxide curve.

    01:31 Anywhere that you are on that curve, please understand, your PCO2, is always going to be 40.

    01:39 Now, something that I wish for you to take a look at to make your life a heck of a lot easier, I’d like for you to begin with our patient being perfectly normal at 7.4. At 7.4 we have a patient who is at normal arterial pH.

    01:53 Close your eyes and think about what the carbon dioxide should be? The carbon dioxide for that patient should be at approximately 40 and so we have a perfectly homeostatic respiratory status of our patient.

    02:05 Next, if you’re referring to bicarb, now the bicarb we will call this the buffering line, we have to, as bicarb is alkaline, and so therefore you should know that the more that you have bicarb in your system, either it is contributing to a primary disorder – some type of metabolic alkalosis, or maybe perhaps, there’s compensation taking place.

    02:28 That buffering line that you see here, all that represents, it is not given a value, do you see that? Unlike carbon dioxide, that curve for carbon dioxide was given a value of exactly 40 and that is something that you will stick to anywhere on that curve, but the bicarb buffering line, what it represent, as you move from high left to lower right, your bicarb level, which is on the y axis is diminishing.

    02:59 Now, let me ask you a question, at normal levels your bicarb should be between 22- 26, we, at a pH for patient at 7.4 or at 25 and that is your steady state and that is your point of reference as being in normal patient, is that clear? Next, I want you to start moving to the right on your x axis.

    03:21 You tell me what is happening to the pH.

    03:24 Why has it moved to the right, please understand, that the pH is increasing.

    03:29 What does that mean to you? Alkalosis, correct? Now, we don’t know what kind yet, if it is respiratory or it’s metabolic, at this point all that I want you to take away from, are x axis while moving to the right is alkalemia, whereas if you move to the left, if you move to the left you’ll notice that your pH is diminishing below 7.4 and technically speaking, below 7.35, thus your patient is in the state of acidosis, is that understood? But we still don’t know if your patient has metabolic or respiratory acidosis What you'll do next and what I wish for you to predict next few slides in which we will further fill in the details for the graph include the following: if you move to the right, you have alkalosis.

    04:15 If I ask you to plot in -- plot in the carbon dioxide level as you get more alkalotic, what will you tell me that PCO2 should do? Are you telling me that that PCO2 should decrease? If that’s the case, you are spot on and that’s what you’re seeing here.

    04:35 As you move to the right, then I assume, in as to the boards, that you know that your PCO2 is diminishing, is that understood? Okay, now, as to what how to utilize the bicarb buffering line – well, we’ll get into that later.

    04:51 Whereas if you move to the left from the PCO2, tell me about your pH? You see as to how everything is perfectly coordinated, you have to make sure that you maintain composure and you organize your thoughts, otherwise, if you started approaching this at all angles at the same time, you will confuse the heck out of yourself and you can’t do that right now.

    05:12 So as you move to the left your pH is decreasing.

    05:15 You know that it’s becoming acidotic, then, therefore, what should your carbon dioxide level be in your plasma for it to be acidotic? It should be increasing, thus you’ll see a shift of that curve to the left representing exactly that.

    05:31 Let us now move on and what we’re gonna do is then shade in those parts of this graph that then represent – now what I like for you to do -- in the very beginning we began this algorithm where I began with making sure that you look at your pH at first and then we divide it into alkalosis and acidosis and they move down the flow chart, I want you to go back to that flow chart, right now, take a look at the colors of the shades of the pathologies.

    06:01 For example, on the side of the acidosis you'll take a look at metabolic acidosis and respiratory acidosis and take a look at those colors respectively for example, the metabolic acidosis was orange and then you’ll take a look at the same one on the right and the reason I want you to take a look at those shaded colors, they’re done on purpose because as you move to the graph I’m going to take those shades and then put them into the graph where it then it coordinates perfectly to that particularly acid base disorder.

    06:30 Let us begin.

    06:34 On this graph, as you move to the left, what happens to your pH? Step by step by step – you keep doing this as a protocol and before you know it, you’ll be that quick with diagnosing your patient, otherwise, you’ll be as I said, you’ll be looking at three or four different things at the same time and you know, your eyes don’t even know what to focus on.

    06:51 I’m showing you what your eyes should focus on, stick with me.

    06:56 Number one, take a look at the pH, you’re moving to the left.

    06:59 I want you to work through this example for me.

    07:02 As you move to the left, what happens to your pH? It decreases.

    07:05 Can we all agree that we’re in the state of acidosis? Good. Next, well what do you want to do now? Well you wanna try to figure out as to what your primary is.

    07:14 Is your primary a metabolic acidosis or respiratory acidosis? How can you tell? Well, in the upper left quadrant where you see a red shaded area, you’ll notice that here, if you had this curves of carbon dioxide moving to those left, if you actually shifted it, what must your carbon dioxide do? Increase, that’s exactly what happens, and now, your carbon dioxide is increasing.

    07:41 Please tell me as to what your breathing pattern is in a patient, so that you could then give rise to such an increase in carbon dioxide.

    07:50 Good, a decrease in respiratory rate, clear? So the patient has hypoventilation or dyspnea, therefore resulting in retaining of carbon dioxide – welcome to hypercarbia or hypercapnea, so far so good, we have respiratory acidosis.

    08:05 Next, as a general rule, how do you want to think of this as being carbon dioxide and bicarb? When you one that is increasing, as a method of compensation which is in this case, bicarb, it should always be increasing – both moving the same direction.

    08:20 As a rule of thumb, when both are moving in the same direction, what do you have going on? Compensation. What's my pH right now? Stick to it like an anchor – it’s acidotic.

    08:32 So if your bicarb is increasing that could never be metabolic acidosis, right? Because if you’re bicarb is increasing then that would put you in the realm of metabolic alkalosis which makes no sense in this patient because you’re moving to the left with acidosis, so why is this bicarb increasing? This bicarb is increasing as a method of compensation, so what is the kidney doing? Increasing the reclaiming threshold for bicarb – remember that entire topic for renal threshold, forgot bicarb and where as to your increasing reclamation and that’s what you do in here.

    09:08 If you take a look at the y axis which is bicarb in the zone that’s red, which represent respiratory acidosis because carbon dioxide is increased, what your patient doing? Hypoventilating. What is your bicarb doing? Is this minutes or days that your next step of really, protocol? Days, it takes days for the kidney to eventually get rid of your bicarb, do you remember that? So therefore what we’ll do even further is then divide our respiratory issues into acute and chronic. We must, we must.

    09:43 I’m laying down the foundation now.

    09:46 Acute, well, maybe bicarb’s rising but then chronically, please understand your bicarb is well, drastically increased. Gonna do what with that pH? Ah, bring it back to normal to about 7.4. Are you ever going to reach perfect compensation? Never, never – that the red shaded area, this will be a good time for you to go back and take a look at the algorithm were under respiratory acidosis, we look at the various causes of what – hypoventilation.

    10:16 The one big one that I’ve given you was opioid overdose resulting in respiratory center shut down.

    10:23 Let’s move on to the other side here.

    10:25 The blue shaded area – the blue shaded area represents, well, before we move on, let’s predict.

    10:31 Close your eyes, pretend that you didn’t even see that blue shaded area but you definitely know that your right at 7.4.

    10:39 If your right at 7.4, what kind of environment are you creating? Good, alkalosis.

    10:46 Your next question is well, if I’m moving to the right and I'm all the way up there, right now, if you were to deal with your carbon dioxide curve, your control is 40, you move to the left – what did I say happens when you shift your curve to the left for carbon dioxide? An increase in carbon dioxide which makes no sense if you’re thinking about alkalosis – how do you know? I told you to stick to that page like an anchor, never let go.

    11:17 So now you move to the right, you know you're alkalotic.

    11:20 It cannot be respiratory acidosis. Ahh, not where are you. We’ll take a look at the y axis.

    11:27 What's your normal bicarb? Oh, about 25? Where are you with that blue violet type of shade? You are at 35 maybe 40 maybe 45 for bicarb. Is this safe to say now that that alkalosis is being caused by the bicarb? Sure it is. What do you have now, ladies and gentlemen? Metabolic alkalosis – good! Now, what's your only method of compensation? Obviously, respiratory. What is it doing? Now, before we go on when you have compensation, which direction would it be moving in when compared to the primary? Same direction, bicarb is increasing, what do you want for carbon dioxide to do – increase.

    12:08 Is that what's happening here? Yes. Why? The curve there in that blue shaded area would be shifting in which direction? To the left thus increasing carbon dioxide. Spend some time laying down the values and the foundation.

    12:24 Now, if they’re both moving in the same direction, is this minutes or days the respiratory compensation? Minutes, within the snap of a finger, you’re hypoventilating, you're maintaining your carbon dioxide because you’re in the state of metabolic alkalosis, I slow down my breathing for you, did you see that? Okay.

    12:48 Now, let’s move on to another two shades, well, we’ll do the same thing and then now I’ll finally walk you through a mix.

    12:54 You only do a mixed once you get these four down first.

    12:59 Let’s do the green shaded area. Let’s move to the right. Let’s go quicker.

    13:03 What's your pH? Increased, so therefore alkalosis, stop.

    13:08 Next step, what’s my primary, is it respiratory or is it metabolic? If you’re ahead of the game well hold on for one second, hold on.

    13:17 Let’s go step by step – I have to. So here, I want you to take a look at the bicarb.

    13:22 My bicarb in the green region is below 25.

    13:26 If your bicarb is decreased, could this ever be metabolic alkalosis? Never – that’s metabolic acidosis, that’s not your primary pathology, it’s not your diagnosis, get away from that.

    13:38 So what about your carbon dioxide, where is my 40? See the curve, good.

    13:44 If that curve shifts to the right, shifts to the right, shift to the right -- tell me what happens to carbon dioxide level? Good, decreases, excellent. So now what are you doing? Hyper or hypoventilating? And faster and faster and faster -- therefore I’m blowing off carbon dioxide. Huh? This is a primary respiratory alkalosis. Tell me about compensation. Kidney.

    14:11 Is it immediate or days? Days. Therefore, what kind of changes might you have – acute and chronic.

    14:19 Dr. Raj, why do you keep breaking it down like that? You’ll see. I haven’t told you anything that isn’t important for you, trust me.

    14:26 Acute chronic, acute chronic if it's compensation.

    14:30 Now, what should that bicarb be doing when you’re compensating? A decrease in carbon dioxide, therefore, a decrease in bicarb. Safe to assume? Very much so.

    14:42 Then we take a look at the orange, as I've said these shades are corresponding to the algorithm in the previous discussion, keep them side by side.

    14:51 Let’s take a look at metabolic acidosis.

    14:53 First, pH – decrease, good. If it’s acidosis and you want to check out the primary, what do you expect the bicarb to be? Decreased, anywhere in that orange area. The y axis, is it below 25? That it is, good, when? Metabolic acidosis. Your next question is what? Compensation.

    15:13 So if your bicarbs decreased, what should your carbon dioxide do? Can you predict it, can you see the actual curve? Shift it to the right, it has to.

    15:24 Anywhere in that orange area, the carbon dioxide curve has been shifted to the right.

    15:29 What does that mean to you? A decrease in carbon dioxide, there you go.

    15:33 How quickly – minutes. For example, think about DKA, if you don’t believe me.

    15:39 Think about DKA, oh, smell that breath – you want more of it, don’t you? It is oh so sweet.

    15:45 What are we talking about? What is all these thing I’m doing right here? It’s just silliness, its Kussmaul’s breathing. Kussmaul’s breathing, DKA, diabetic ketoacidosis.

    15:54 You know that is being not just a metabolic acidosis that is a severe anion gap metabolic acidosis so therefore why is their breath so sweet in that DKA patient? Because of -- very good, acetone – your ketone bodies.

    16:09 Are we clear about the shades and how to approach the graph? Once you're done, you're gonna take a look at the mixed disorder. I'll do both with you real quick.

    16:18 So let’s take a look at mixed alkalosis. That entire area, alkalosis, entire area in that region.

    16:24 First and foremost, always go to your anchor.

    16:26 What’s the anchor here? Anchor, you drop it into the ocean, my anchor here is going to be my increased pH.

    16:34 Let’s take a look at 7.7. Say if you find your 7.7 that’s to bit by bit by bit – if you expect your alkalosis to be primary and it’s you expect your metabolic, you expect your bicarb to be increased.

    16:47 Okay, going to that region of mixed alkalosis, is your bicarb above 25? That it is, that it is.

    16:56 So that arrives us to metabolic alkalosis, okay.

    17:00 Now, if you expect your bicarb to be increased and its compensation, once again tell me, what do you expect your carbon dioxide to do? Increase as well.

    17:09 Be careful here, take a look at your carbon dioxide at 40, you’re shifting to the right, what does that mean? It’s decreased. Uh-oh, I've have an increase in bicarb and I have a decrease in carbon dioxide, what did I tell to as a general rule of thumb? When we have compensation both should be moving in the same direction – we have opposite direction, no doubt you have a mixed issue, mixed alkalosis.

    17:38 It’s the same thing, mixed acidosis are things coming together now? They should be, if they’re not, go back and review.

    17:44 You’ll get this and if you don’t get it the first time around, go back again, go back again, you have to, it’s medicine.

    17:51 Now, as to mixed acidosis – same thing pH, that’s your anchor, wrap it.

    17:57 What's my anchor saying? Acidosis, your pH less than 7.4.

    18:01 Okay, great. Now, if you expect it to be bicarb, what are you expected to do? Decrease. Is it below 25? That it is, stop there. Next, what should be compensation? A decrease in bicarb means a decrease in carbon dioxide, you're in that mixed acidosis area.

    18:20 What is your carbon dioxide curve doing? Oh, look, it’s shifting to the left. Wow.

    18:26 What does that mean? An increase. What is this? Opposite direction, we have a mixed acidosis.

    18:32 How beautifully does this graph illustrate everything that we did there on the flow chart – that flow chart algorithm is of utmost importance at your priority.

    18:42 Once that flow chart is in your head solid, and then you come back and take a look at these graphs, you build and build and build.

    18:50 Let’s continue.

    18:52 Alright, ABG definition. What are you looking for? Conditions in which you’re looking for, well, acid base disorders.

    18:58 You’re gonna build and build and more some more.

    19:01 Normal pH, I memorized 7.35-7.45, you’ll be fine, technically, yes – 36 and 44.

    19:08 If you’re above 7.45 give me a definition, alkalosis; below 7.35 acidosis. Good.

    19:15 Normal PCO2, what is this? What’s the title of this topic? Arterial. What’s my PCO2 – 40.

    19:26 What if it said, 46 – obviously, you’re not on the arterial side. Let's go on.

    19:30 Normal bicarb, it goes at 24, technically 22-26. Normal PO2 80-100.

    19:37 I’d be closer to 100 if I were you cuz what 60 mean? Hypoxia. What’s the topic – arterial.

    19:43 Move on.

    About the Lecture

    The lecture Davenport Diagram – Laboratory Diagnostics by Carlo Raj, MD is from the course Pulmonary Diagnostics.

    Included Quiz Questions

    1. 40 mmHg
    2. 44 mmHg
    3. 14 mmHg
    4. 20 mmHg
    5. 50 mmHg
    1. HCO₃
    2. PCO₂
    3. PO₂
    4. H₂O
    5. H⁺
    1. Compensation
    2. Hyperventilation
    3. Desaturation
    4. Hypoventilation
    5. Hydroxylation
    1. Days
    2. Minutes
    3. Hours
    4. Weeks
    5. Months
    1. PCO₂ is decreasing in minutes.
    2. PCO₂ is increasing in minutes.
    3. PCO₂ is decreasing in hours.
    4. PCO₂ is increasing in hours.
    5. PCO2 remains the same.
    1. Mixed alkalosis
    2. Respiratory acidosis
    3. Metabolic alkalosis
    4. Mixed acidosis
    5. Respiratory alkalosis
    1. pH 7.41, PCO₂ 40 mm Hg, HCO₃ 25 mmol/L, PO₂ 99 mm Hg
    2. pH 7.31, PCO₂ 40 mm Hg, HCO₃ 25 mmol/L, PO₂ 99 mm Hg
    3. pH 7.44, PCO₂ 40 mm Hg, HCO₃ 25 mmol/L, PO₂ 66 mm Hg
    4. pH 7.38, PCO₂ 40 mm Hg, HCO₃ 45 mmol/L, PO₂ 60 mm Hg
    5. pH 7.40, PCO₂ 60 mm Hg, HCO₃ 25 mmol/L, PO₂ 90 mm Hg

    Author of lecture Davenport Diagram – Laboratory Diagnostics

     Carlo Raj, MD

    Carlo Raj, MD

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