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ABSITE Review: Fluids, Electrolytes, Acid/Base
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ABSITE Review: Fluids, Electrolytes, Acid/Base
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Segment:0 .
DR. MCCLELLAN: Alright, welcome back to Behind the Knife. We are continuing today with fluids, electrolytes, and acid/base as part of our ABSITE series. These questions on the ABSITE and Boards tend to be pretty straightforward, especially if you know the content. As a reminder, this chapter is also in our ABSITE series published on Amazon. Be sure to give it a look.
DR. BINGHAM: Okay, so let's get started. Let's just jump right into it, with some fluids, electrolytes and acid/base. [Dr. Martin] Yeah, and we'll try and make this as least painful as possible. I actually didn't realize how much of a percentage of the exam was fluid, electrolytes, acid/base. So, we'll try and hit the high points. So, we'll start setting our baseline of fluid stats. So, what is our percentage of total body water by weight in an adult?
DR. BINGHAM: So typically I think it's different for men or women. It depends on how much of you is body fat, but I think a typical equation is you take your body weight and times 0.5 for men and 0.6 for women. [Dr. Martin] Yeah and I think generally the exam will use 0.6, I think that's the most widely accepted. So for a 70 kg male, that would be 42 liters of total body water.
DR. BINGHAM: And what percentage of that is intracellular?
DR. BINGHAM: Most of it, so about two thirds of it, generally speaking is intracellular, I believe. [Dr. Martin] Okay, good, and then we have the extracellular compartment, and that we can break down into intravascular and extravascular. And so how much of extracellular water is intravascular?
DR. BINGHAM: It's a lower percentage of it, so 20-25%, I believe. [Dr. Martin] Yeah, so about a quarter is intravascular and the other three quarters are extravascular. And then what's the blood volume for our 70 kg man?
DR. BINGHAM: I think on average, you're looking at about five liters. [Dr. Martin] Yeah, so by body weight, it's about 7%, and that's the way I remember it, five liters in a 70 kg male gives you about 7%. How about in a pediatric patient?
DR. BINGHAM: Oh gosh, I don't know, John?
DR. MCCLELLAN: I don't remember the equation. Yeah, I'm thinking of the equation, I can't think of it, I don't remember. [Dr. Martin] So for a pediatric patient, it's 80 cc's per kilogram. And when we talked about transfusion, we generally consider massive transfusion, a half of their total blood volume, which is 40 cc's per kilogram. But 80 cc's per kilogram would be their entire blood volume.
DR. MCCLELLAN: Okay, so I think we can break our fluids down, into maintenance fluids and resuscitative fluids. So, what would be our common maintenance fluids?
DR. MCCLELLAN: Well, D5 half-normal saline at 20 mEq of potassium is our typical one. [Dr. Martin] Okay, so generally it would be a D5 and I'd say our most common D5 half-normal, anybody we would use D5 quarter-normal on?
DR. MCCLELLAN: Kids, you would use it in kids. [Dr. Martin] Yeah, babies. So, babies after seven days, you want to use quarter normal. And then how about our resuscitative fluids?
DR. MCCLELLAN: That's your typical, your LR normal saline fluids. [Dr. Martin] Okay, so your crystalloids would be LR or NS. And what other types of resuscitative fluids do we have?
DR. MCCLELLAN: You can also use your colloids and plasmanates as well. [Dr. Martin] Okay, so then we have our colloids and we'll break those down in a little bit later. And of course, you always have to remember, the electrolyte concentration of these fluids. So, the easiest way is just start with normal saline. So, what does normal saline have in it?
DR. BINGHAM: Sodium and chloride. [Dr. Martin] Yeah, what are the concentrations?
DR. BINGHAM: 154 mEq per liter, each. [Dr. Martin] Good, 154 for both of them. So, then you automatically can do the math and half is 77. How about LR?
DR. BINGHAM: So, LR has a lower sodium concentration. The composition is 130 of sodium, 109 of chloride, there's 4 mEq of potassium, and 4 mEq of calcium. And then some bicarb, I think 28 are bicarb. [Dr. Martin] Good and the way to remember that is it's closer to physiologic. The other thing to remember about, it does have potassium, so if you get the patient who's hyperkalemic and they give you a choice of fluids, LR would be the wrong choice in a patient who's already hyperkalemic.
DR. BINGHAM: Okay, so we always have to calculate maintenance fluid. So, what's our easy way of calculating maintenance fluid for a patient who -- and this is the euvolemic patient - who is NPO?
DR. MCCLELLAN: The answer you could use is 4-2-1 rule. [Dr. Martin] Okay.
DR. MCCLELLAN: But, I think, the easier way of doing it, I think is the 150/20 rule, that's seems-- [Dr. Martin] Okay, I think those are both equally as easy. So, 4-2-1 gives you the hourly rate. 50-20 gives you the daily amount you want to give them. So, four cc's per kilo for the first 10, take two cc's for the next 10, and then for every kilo after 20 it's one cc, and that gives you your hourly rate.
DR. MCCLELLAN: Is there a quicker and easier method for anybody in normal weight range?
DR. BINGHAM: You just add 40 to their weight and it should give you your minutes rate. [Dr. Martin] Yeah, once you get to 20 kilos and above, weight plus 40 is the easy way to remember that. Okay, so that replaces your water that you need for the day. And then we also want to replace our electrolytes, mainly sodium and potassium. So, how much sodium generally does a person need per day, or do they lose per day?
DR. MCCLELLAN: 70 to 140 mEq a day. [Dr. Martin] Yeah, so you do that by weight. So generally one to two mEq per kg. So for the 70 kg man, it'd be 70 to 140. So, a liter of half-normal would have 77 mEq per liter, so when you give them about two liters or more of half-normal saline you've met their sodium requirements for the day. How about potassium?
DR. BINGHAM: Potassium it's about the same, it's a little bit less, 0.5 to one mEq per kilogram per day. [Dr. Martin] Yeah, so it's about half of what you need for sodium. So, for 70 kg person again, it's 70 mEqs per day, which can easily be met with 20 to 30 mEq per liter of potassium added to your fluids. If you're having high losses from some other source you might need to increase that.
DR. BINGHAM: So, sugar or salt? So, when would you use a D5 solution?
DR. BINGHAM: Somebody who's hypernatremic.
DR. MCCLELLAN: Yeah, you need free water. [Dr. Martin] I know, I mean, when you can give a person half-normal saline or you can give them D5 half-normal saline.
DR. BINGHAM: Oh, I see.
DR. MCCLELLAN: On children, you would use it on children-
DR. BINGHAM: Also people who are NPO you can be protein sparing, as they say, if you add a little dextrose into it. [Dr. Martin] Good, yeah that's the whole rationale is, you give them some amount of calories, that theoretically should prevent them from breaking down their own proteins. So, how much dextrose would that be in a liter of D5? D5 anything?
DR. BINGHAM: So let's see. Well, we've got 3.4 calories per gram, so in a liter-- I don't know how to do the math, how much is in a liter? [Dr. Martin] So it's 5%. So, it's 50 grams per liter.
DR. BINGHAM: Okay. [Dr. Martin] So, that gives you a little over 150 kcals per liter, which should give you protein sparing. So, anyone on maintenance we'll usually want to put them on D5 solution. Okay, so we have a 25-year-old male who's 80 kg who had an appy, he's NPO, and you want to put him on maintenance fluid. So, what fluid and rating are you going to write him for?
DR. MCCLELLAN: I think the easiest way is to use the weight plus 40. So, I would put him on 120 mLs an hour. And I guess you could consider doing LR versus D5 half-normal saline with 20 of K. [Dr. Martin] Good, so D5 half-normal 20 K is fine. 120 cc's an hour. Okay, now you have a 14 kg child.
DR. BINGHAM: For that one you'd use the 4-2-1, so 40 and then plus 8 so, 48. so 40 and then plus 8 so, 48. [Dr. Martin] Okay, and what fluid?
DR. BINGHAM: I mean D5 half-normal, if it's an infant or at 14 kg, it should be D5 half-normal plus 20 K. [Dr. Martin] Good, yeah D5 half or D5 quarter if it was an infant, but this one's a little bigger. Okay, so now you have a 48-year-old female, who has been vomiting for three days, [can't tell her NPO] she comes in, she's tachycardic, blood pressure is okay, and she has low urine output. So, what do you want to do for this person? Start her on D5 half at 100 cc's per hour?
DR. MCCLELLAN: No.
DR. BINGHAM: No,
DR. BINGHAM: this person needs resuscitation. So, they need resuscitative fluid, it's the gastric contents, you should probably replace that with normal saline. [Dr. Martin] Okay, so this is a patient we would give resuscitative boluses. And in general, how much fluid would you bolus this person with?
DR. BINGHAM: Probably a liter or so. [Dr. Martin] Yeah, exactly.
DR. BINGHAM: See how they respond. [Dr. Martin] Yeah, for standard crystalloid, usually about a liter at a time, and how much of that will stay intravascular?
DR. BINGHAM: About a quarter of it. [Dr. Martin] Good, so 200-250 cc's. Okay, then we have our colloid solutions, and we won't spend a lot of time on this. You may not get a question about colloids. They've generally been shown to have no benefit over crystalloids, in almost every indication. There's still some controversy for sepsis, but otherwise, generally they're just more expensive. But how do colloids increase your intravascular volume?
DR. MCCLELLAN: So, protein causes an oncotic pressure inside the vessel and it draws fluid back in. [Dr. Martin] Okay, good. But what will happen in an inflammatory condition, where you have increased capillary permeability?
DR. MCCLELLAN: Well, I think of it as those proteins won't stay in the vessel, they'll actually leak out, and then it draws the fluid with it. [Dr. Martin] Good, and in general what are our colloids that we have currently available?
DR. MCCLELLAN: So, ones that I've seen or use a lot are the albumin and plasmanate, I'm sure there's a bunch of other ones out there, too. [Dr. Martin] So, we have our natural and we have our synthetics. Our naturals would be the albumin products, and our synthetics?
DR. MCCLELLAN: Synthetics would be like your hetastarch. [Dr. Martin] Yeah, it would be the starch compounds, hetastarch, Hespan, any adverse effects of giving those fluids? Any adverse effects of these synthetic colloids? Hextend, Hespan?
DR. BINGHAM: I think it can result in coagulopathy. [Dr. Martin] Good, so one is, if you give enough of it, and why do they get coagulopathic? Or what aspect does it affect?
DR. BINGHAM: Is it that you dilute your coagulation factors? [Dr. Martin] No, it actually affects platelet function.
DR. BINGHAM: Oh, okay. [Dr. Martin] So they negatively affect platelet function. The other adverse impact, especially of the starches, has been acute kidney injury in ICU patients. And I guess that's enough reason why we've gone away from them.
DR. MCCLELLAN: Does that platelet dysfunction come with even the albumins? [Dr. Martin] No, that's what with the synthetics.
DR. MCCLELLAN: Just with the synthetic, okay. [Dr. Martin] So if they give you a dextran, or a hetastarch, Hespan, Hextend product, and they ask you, you give a large volume of it, what's its effect, it's going to be platelet inhibition.
DR. MCCLELLAN: Okay. [Dr. Martin] Okay, we also worry about insensible losses. Sometimes there's a general question on this. What are the things that increase insensible losses in patients we would be taking care of?
DR. MCCLELLAN: Prolonged operations, patients who have burns- [Dr. Martin] With an open cavity.
DR. MCCLELLAN: Yeah, burns are obviously a huge one.
DR. BINGHAM: And then fevers, and people who are on vents, would be insensible losses. [Dr. Martin] Fever's a huge one and then people on vents. And generally each degree Celsius increase in your temperature increases your insensible losses by about 10%. So, let's go through replacing some ongoing losses. So, we have the patient who's got high NG tube output. And you want to-- Normal saline. [Dr. Martin] You want to replace that, good, normal saline.
DR. BINGHAM: How about the patient with a high volume bile leak?
DR. BINGHAM: LR. [Dr. Martin] Okay, and why do you choose LR instead of NS?
DR. BINGHAM: Because for an ongoing bile leak they're losing bicarb, so you'd want to replace it with bicarb. [Dr. Martin] So it could either be LR, or you could give them a D5 solution with bicarb added. How about the patient with colitis and severe diarrhea?
DR. BINGHAM: For that one, I think also you would choose LR. [Dr. Martin] You definitely want to give them something with bicarb. What other electrolyte abnormality will they get?
DR. BINGHAM: Hypokalemia? [Dr. Martin] Yeah.
DR. BINGHAM: Or potassium loss. [Dr. Martin] Huge potassium losses in stool. And so, if we look at our electrolyte concentrations in our GI secretions, generally most of them, pass the stomach, look like normal saline, so usually you want to use normal saline. The bile, as you said, bile and pancreatic you are also going to lose a lot of bicarb, and colon you're going to lose a lot of potassium.
DR. BINGHAM: I've seen that question a couple of times actually from the test banks, "Where does your largest GI loss of potassium come from?" It's the colon. [Dr. Martin] Yep, that's the colon. And that's the patient you'll get significant hypokalemia. Okay, well, as long as we're talking about electrolytes let's talk about electrolyte abnormalities. So, we'll start with sodium. So we have a 37-year-old female, had an open coli, she's post-op day two, she's NPO, her sodium is 129.
DR. BINGHAM: She's awake, alert and afebrile. So, what do we think is the cause of her hyponatremia, and what are our treatment options?
DR. MCCLELLAN: Yeah, definitely seen questions like this and most likely it's iatrogenic, so you've gotten a lot of crystalloid from LR in the operating room. [Dr. Martin] Okay, so it could be dilutional. What are some other causes?
DR. BINGHAM: So like SIADH, that would be a common problem post-op, or post-traumatic. [Dr. Martin] Good, so, yeah. And on the exam, I think it's either going to be excess free water, or it's going to be SIADH.
DR. MCCLELLAN: Or she could be diabetic. [Dr. Martin] What's that?
DR. MCCLELLAN: She could also be diabetic, if you think of like a pseudo. [Dr. Martin] And that's the thing you're trying to figure out. And then you can get the pseudohyponatremia from?
DR. MCCLELLAN: Hyperglycemia.
DR. BINGHAM: From hyperglycemia, high triglycerides. [Dr. Martin] Anything with high proteins. So hyperproteinemia and hyperglycemia. Okay, so how do we make the diagnosis in this hyponatremic patient?
DR. BINGHAM: So we'll calculate their serum osms,
DR. BINGHAM: So we'll calculate their serum osms, usually you'd figure out their serum osms, and their measured serum osms versus their calculated serum osms. [Dr. Martin] Good.
DR. BINGHAM: It's a good place to start. [Dr. Martin] Okay, and how do we calculate serum osm?
DR. BINGHAM: So you look at your chemistry, and your three main components are sodium, glucose, or your three components are sodium, glucose and your BUN. or your three components are sodium, glucose and your BUN. So, the equation is two times the sodium, plus the glucose divided by 18, and then plus the BUN divided by 2.8. So the sodium is the main contributor. [Dr. Martin] Good, so the quick and dirty is you multiply sodium by two, and that gets you in the ballpark.
DR. BINGHAM: The simple way I remember it is, sodium times two, and then plus 10, for someone who's got normal glucose, and normal renal function. So, unless they give you a patient whose glucose is out of whack, or who's in renal failure, and has an elevated BUN it's generally going to be twice your sodium plus 10. And then what do we want to compare that to to figure out if this is excess free water or if this is SIADH?
DR. BINGHAM: So their urine osms. [Dr. Martin] Good. So if it's SIADH, what's going to be the serum osms and the urine osms?
DR. BINGHAM: So, I got to think this out. So, if it is SIADH your serum osm is going to be lower than your urine osm. So, your urine is going to be inappropriately concentrated. [Dr. Martin] Yeah. So, serum osms are going to be low, and urine osms are going to be high, which is inappropriate, because if it's excess free water, then what's that relationship going to be?
DR. BINGHAM: If it's excess free water, you're going to be trying to get rid of free water, so your urine osms are going to be low. [Dr. Martin] Yeah, so just remember your serum osm and urine osm should always be in the same direction. So, if they're both low, that's normal and you're talking about excess free water. If they're in the opposite direction, then you have SIADH. And same thing if they're high. If the serum osms are high, urine osms should be high, and if they're not high, then you're probably talking about a pathologic entity like diabetes insipidus.
DR. BINGHAM: Okay, so we made our diagnosis, how do we want to treat this patient? Let's say serum osms are low and urine osms are low.
DR. BINGHAM: Fluid restrict. [Dr. Martin] Okay, let's say serum osms are low, urine osms are high.
DR. BINGHAM: Well, I think you'd still fluid restrict that patient. [Dr. Martin] Okay, and then if that doesn't work? So that's SIADH.
DR. BINGHAM: If that doesn't work, you can-- [Dr. Martin] So first off, let's say, she's now symptomatic, sodium's 120.
DR. BINGHAM: You can give hypertonic saline. [Dr. Martin] So, you can start replacing sodium. Usually you want to start with an isovolemic like an NS, because that will raise the sodium or you can go to hypertonic. You do need to be careful though, because how quickly do you want to correct the sodium?
DR. BINGHAM: Generally like a half or to one mEq per hour. [Dr. Martin] Yeah, no more than one mEq per hour. Okay, and then are there any other treatments we can give for this patient with hyponatremia? Any other drugs we can treat them with?
DR. MCCLELLAN: The vaptans, I can't remember them off the top of my head. [Dr. Martin] Yeah, so there's two drugs you can use. One is demeclocycline which is an old antibiotic, but it also treats SIADH. It essentially will cause a diabetes insipidus. And then you mentioned the vaptans, so Conivaptan, Tolvaptan, those are the newer drugs for hyponatremia. What do those do?
DR. MCCLELLAN: I assume they've some type of diuretic effect. [Dr. Martin] So, they're vasopressin antagonists.
DR. MCCLELLAN: Oh, okay. [Dr. Martin] And so they antagonize vasopressin receptors in the kidney. And do you know which type of vasopressin receptors those are?
DR. BINGHAM: V1?
DR. MCCLELLAN: No, it's V2. [Dr. Martin] V2.
DR. BINGHAM: V2, yeah.
DR. MCCLELLAN: V1 is your arterial. [Dr. Martin] V1 are the systemic ones, when you give vasopressin for blood pressure, and V2 are the kidney ones where essentially, it'll work like ADH. Okay, so we have this hyponatremic patient and we want to figure out, how much sodium we need to give them. So we always have to calculate our sodium deficit, right? So how do we calculate sodium deficit?
DR. MCCLELLAN: So, you have your equation for this, or typically what I do is just guess in the middle of the ABCDs, but no, for your calculation of your sodium deficit you take your desired sodium and you subtract your actual sodium, and then you take that times your total body weight. [Dr. Martin] Yeah, so it's easy to confuse the calculation for sodium deficit and for free water deficit. Both of them involve taking the normal sodium and subtracting the actual, just remember, you always want to put the higher of those sodiums first.
DR. MCCLELLAN: So for this one, your sodium's going to be low, so you want your desired sodium minus your actual sodium, multiply that by total body water, which should be again, for a 70 kg man or woman 42 liters, and that will give you your sodium deficit. The other way I remember how to differentiate these equations, remember one of them you have to divide by the desired sodium and one you don't, and the sodium deficit is the one where you don't have a denominator.
DR. MCCLELLAN: And I just remember that sodium deficit is NAD, or no denominator.
DR. MCCLELLAN: Okay.
DR. BINGHAM: Oh, nice.
DR. BINGHAM: [Dr. Martin] And then the free water deficit is the one where you have to have the denominator. So sodium deficit's the easier one, desired minus actual times total body weight, and that will give you your mEqs of sodium that you have to give them back. And then you can just calculate how to give them back from the content of whichever fluid you're using.
DR. MCCLELLAN: Or we can just call nephrology, too. [Dr. Martin] Yeah, or you can plug it into Medi-Cal which, not going to have on your Boards.
DR. BINGHAM: I don't think you can use that on the ABSITE. [Dr. Martin] Not yet. Okay, let's talk about hypernatremia then. So, what are the causes of hypernatremia? Patient's got a sodium of 160.
DR. BINGHAM: So, anything that causes you to lose free water, so insensible losses, again, fevers, so insensible losses, again, fevers, you have your different types of diabetes insipidus. Probably the most common is iatrogenic through administration of normal saline, and then you can diurese somebody to be hypernatremic. [Dr. Martin] Good, and I think on the ABSITE, they're almost always going to give you, it's either DI or exogenous administration and you're trying to figure which of those two it is.
DR. BINGHAM: And the exogenous administration can usually be either by fluids, or oftentimes by tube feeds, especially the concentrated tube feeds, when you're not also giving them enough free water. So, let's say we have this 20-year-old guy, had a car crash, he's got a large subdural, and suddenly his urine output increases to 700 cc's an hour for three hours, and the sodium goes up to 165.
DR. BINGHAM: How do we figure out what's going on? So first off, just by the story, [chuckles] you should know the answer.
DR. MCCLELLAN: Yeah, a head injury--
DR. BINGHAM: Like a central diabetes insipidus. [Dr. Martin] That's DI. But how would we make the diagnosis?
DR. MCCLELLAN: We can calculate your free water deficit. [Dr. Martin] Well, that just tells you your free water deficit, doesn't tell you what caused it.
DR. BINGHAM: Well, you can give desmopressin and see how he responds. [Dr. Martin] Well, no, I mean, we just talked about the--
DR. MCCLELLAN: Oh, yeah, you can look at your urine output, your urine specific gravity--
DR. BINGHAM: Oh, right, okay.
DR. MCCLELLAN: Your serum sodium-- [Dr. Martin] So it's serum osm, serum osm, same thing for any sodium issue. Right? So here, your serum osms are going to be high, so what should your urine osms be?
DR. MCCLELLAN: Oh, they should be also.
DR. BINGHAM: Should be high.
DR. BINGHAM: [Dr. Martin] They should be high. And if they are, then you've given them excess sodium. And if your urine osms are low?
DR. BINGHAM: Well, that means that-- [Dr. Martin] What's your diagnosis?
DR. BINGHAM: Well, diabetes insipidus. [Dr. Martin] Yeah, so if they're in the opposite direction, that's DI, if they're in the same direction, they're both high, then that's exogenous sodium administration. Okay, so what do we want to do for this patient?
DR. BINGHAM: Give him back free water. [Dr. Martin] It's DI, you figure it out.
DR. BINGHAM: So typically you want to replace some free water. So you calculate the free water deficit, and then you give that to them, with giving the first half over the first eight hours. [Dr. Martin] Yeah, but if they're true DI, they're just going to keep pouring that out in their urine. And you need something to stop that.
DR. MCCLELLAN: You need DDAVP--
DR. BINGHAM: If it's central diabetes insipidus then you can treat with desmopressin, if it's a nephrogenic they're not going to respond to that. And you're just going to have to pump them full of a lot of free water to keep up with it. [Dr. Martin] So this patient, and when they give it you on the ABSITE, if it's DI, the answer's going to be DDAVP. You give them DDAVP. You did mention calculating the free water deficit, so, we'll say another patient who's been on tube feeds for days, and now their sodium has gone 150, 155, 159, and you check urine and serum osms and they're both high.
DR. BINGHAM: So, what are you going to do for that patient?
DR. BINGHAM: So that patient you just need to replace their free water, they're not getting an adequate replacement of their free water in their tube feeds. [Dr. Martin] And then how would we calculate their free water?
DR. MCCLELLAN: So actual sodium minus desired sodium, divided by your desired sodium, and then you take that times your total body water. [Dr. Martin] Good, and that will give you a volume in liters that you need to give them, so, how are you going to give that back to them? What are you going to write for?
DR. BINGHAM: D5W. [Dr. Martin] Okay, so it depends if you're going to give it IV or enteral, right? So yeah, if you're giving it IV then it's D5W, that's your free water. And if it's enteral, you can just give them free water, whatever that volume is that they need. Okay.
DR. MCCLELLAN: How fast do you give that back then? Actually, this is more of a clinical question than a ABSITE question. [Dr. Martin] So, I would go by about the same thing as hypo.
DR. MCCLELLAN: Okay. [Dr. Martin] You don't want to correct too quickly, so I think no more than one mEq per hour, and probably no more than 10 mEqs per day.
DR. MCCLELLAN: Okay. [Dr. Martin] But if they ask you a question about correcting too quickly, it's going to be hyponatremia, correcting too quickly.
DR. MCCLELLAN: Yeah, okay. [Dr. Martin] So, let's move on to potassium, and I think most people are familiar with this, so we we'll breeze through it. So, you've got a 55-year-old female with chronic renal failure, she's got a potassium of 7.5. What are we going to do?
DR. BINGHAM: So, we're going to get an EKG, and see if she has any changes on her EKG, and we're going to give her-- the first thing you're going to to do is give a cardiac stabilizing medications with calcium, and then there's several different things you can do to lower the potassium. [Dr. Martin] Good, and I think that's probably a question every year--
DR. MCCLELLAN: Yeah, I know, I see that every year. [Dr. Martin] You give them the calcium first to stabilize the myocardium. So, most common causes of hyperkalemia?
DR. BINGHAM: Renal failure in the hospitalized patient a lot of times, tumor lysis or cell lysis, we get asked a lot about succinylcholine and depolarizing neuromuscular blockers, especially in burn patients, things like diuretics, there can be endocrine dysfunction that can cause hyperkalemia. [Dr. Martin] Okay, those would be the big ones, and renal failure, definitely the most common, and the characteristic EKG finding?
DR. BINGHAM: Peaked T waves. [Dr. Martin] Okay. And then you can also get clots in your conduction system, and then finally ventricular arrhythmias, if it gets high enough. And so, what are our treatment options for hyperkalemia? And so, what are our treatment options for hyperkalemia? And again, this patient has a K of 7.5. You give him the calcium, then what else can we give him to start bringing the potassium down?
DR. BINGHAM: You can give them glucose with IV insulin, you can give them sodium bicarb, you can give them for a longer acting-- [Dr. Martin] Sodium bicarb would usually be the next thing I would reach for-- Oh, okay. [Dr. Martin] Because it'll be the next fastest acting agent, insulin and glucose. What else can we do?
DR. BINGHAM: Give him Lasix, it can diurese off some of the potassium. There are the resins that help them excrete potassium, Kayexalate. I guess you can do, if you need to, you can do emergent dialysis, to take the potassium out.
DR. MCCLELLAN: Don't forget about albuterol.
DR. BINGHAM: Oh, and of course albuterol, of course. [Dr. Martin chuckles] Which I get to see that--
DR. BINGHAM: That probably won't be the answer on the test. [Dr. Martin] Yeah, but good and dialysis would be your answer for this patient who's critically high. The thing with Kayexalate or the resins is, who would you give that to, and who wouldn't you give that to?
DR. BINGHAM: The resins? Oh, I guess you wouldn't give it to people who have a bowel obstruction. [Dr. Martin] Yeah, they have to have GI function for those to work, those also take a while. Okay, so now we have the patient who's got a K of 2.5. What do we think is the most common cause of that?
DR. MCCLELLAN: Usually it's from overdiuresis, or some iatrogenic reason. [Dr. Martin] Good, yeah, that's almost always iatrogenic. And when they go from 4, their K of 4 to a K of 3, what does that tell you about their total body potassium deficit?
DR. MCCLELLAN: Oh, that's actually nicely easy calculated. So their total body would be, if it's a drop of one mEq it's about 100 to 200 mEqs total body deficit. [Dr. Martin] Good. Yeah, remember K is your primary intracellular cation. So you're measuring the extracellular to try and get a picture. So you can have a pretty significant body store depletion with just a small drop.
DR. MCCLELLAN: And we always use the rule, so if you give them 40 mEqs of potassium, how much did that increase their potassium level?
DR. MCCLELLAN: That should go up four. [Dr. Martin] It should go up four.
DR. BINGHAM: By four.
DR. BINGHAM: [Dr. Martin] So about 0.1 for every 10 mEqs you give them. Okay, let's move on to calcium. So, patient had a total thyroidectomy and the calcium comes back at 6.2. So, what physical exam findings might be present?
DR. BINGHAM: So you could have weakness, perioral numbness, and tingling. You can have the Chvostek and Trousseau's sign-- You can have the Chvostek and Trousseau's sign-- [Dr. Martin] So Chvostek's-- Probably most likely would be the perioral numbness, it would be the most common. [Dr. Martin] Yeah, and then the Chvostek sign?
DR. BINGHAM: That's when you like tap on the face, and then they get the little twitches. [Dr. Martin] Yeah, so you're tapping on the facial nerve, and Trousseau's is the carpopedal spasm at blood pressure cuff. So how would you treat this patient?
DR. BINGHAM: So with IV calcium. [Dr. Martin] Good, and what else do we usually want to give them?
DR. MCCLELLAN: Same thing as hypokalemia, you give them magnesium as well. [Dr. Martin] Yeah, in general, you also want to give them vitamin D, to correct their calcium level.
DR. BINGHAM: Yeah, okay. [Dr. Martin] And what if this patient had a low serum albumin, how would we correct their serum calcium level?
DR. BINGHAM: So an easy way to do it is to think for every point-- so you take four as normal for albumin, and for every point below four, you add 0.8 to the calcium level, and that'll give you your corrected calcium. [Dr. Martin] Good, so if this patient's albumin was two, we would add 1.6 to that calcium level so it would probably be in the normal range. Okay, hypercalcemia.
DR. BINGHAM: What are the main causes of hypercalcemia?
DR. BINGHAM: So hyperparathyroidism would be a very common cause of hypercalcemia. [Dr. Martin] Yep, and what else?
DR. MCCLELLAN: Hyperparathyroidism, and also breast cancer, malignant causes. [Dr. Martin] So most common cause of hypercalcemia in an inpatient?
DR. BINGHAM: Iatrogenic? [Dr. Martin] Cancer. Cancer, okay. [Dr. Martin] Most common cause in an outpatient?
DR. BINGHAM: Oh, hyperparathyroidism, yeah, I remember that, the little most common, yeah. Inpatient - malignancy, outpatient - hyperparathyroidism. [Dr. Martin] Yeah, and the symptoms of hypercalcemia?
DR. BINGHAM: Those are the stones, bones, groans, and psychic overtones. So you have kidney stones, you have bone pain from osteolysis, you have abdominal pain for the groans, and then you can have some psychosis as well. [Dr. Martin] Okay, and what's your treatment going to be for this patient? Got a calcium of 10.5.
DR. BINGHAM: So usually you start with volume, so crystalloid, you run crystalloids to like two or 300 an hour, and then your loop diuretics. [Dr. Martin] Good, it's volume expansion and loop diuretic. And then what are some other medications we can use to treat this?
DR. MCCLELLAN: So if it's for cancer, you can also use your bisphosphonates. I've seen a question on that before. [Dr. Martin] Yep and those would be very good for hypercalcemia caused by what?
DR. MCCLELLAN: By like a cancer something-- [Dr. Martin] Yeah, if it's a question on cancer, hypercalcemia and a medication, that's going to be your answer, bisphosphonate. What else can we give them?
DR. MCCLELLAN: You can do calcitonin as well-- [Dr. Martin] Good. -as another option. And then glucocorticoids are always good too. [Dr. Martin] Yup and then finally, if they're critically high, and especially if they have renal dysfunction, you can dialyze them. Okay, let's move on to acid/base. What is the most important parts of the ABG, in terms of-- you're going to get some acid/base questions on the exam, I guarantee it.
DR. MCCLELLAN: And what are the most important parts of the blood gas to look at to sort those out?
DR. BINGHAM: So I guess the most important part, first you look at the pH to see whether they're acidotic or alkalotic. And then generally I'll look at the CO2, and at the bicarb, and generally in that order. [Dr. Martin] And I would actually argue the pH is less important. And I'll talk about how I like to do these, but PCO2 and then either the bicarb or base excess, base deficit, whichever one they give you, they'll usually give you one or the other.
DR. BINGHAM: So, how does elevated CO2 cause acidosis? Getting back to those chemistry formulas. What happens to that CO2 that forms an acid?
DR. BINGHAM: So it binds with water and forms carbonic acid. [Dr. Martin] Good. And then when we talk about compensation and we can compensate respiratory, and we can compensate metabolically, which of those happens rapidly?
DR. BINGHAM: So, respiratory compensation happens very rapidly, whereas the metabolic takes a little bit longer. [Dr. Martin] Good. So talk about normal values. What's your normal PCO2?
DR. MCCLELLAN: 35 to 45. [Dr. Martin] Yeah, so I would just use 40. Normal pH?
DR. MCCLELLAN: Guess you use like 7.4. [Dr. Martin] 7.4. Normal bicarb?
DR. MCCLELLAN: Ooh, 24, I think, 25? [Dr. Martin] Yep. Okay, so one of the things you need to know to sort out some of these questions about acid/base is whether the change in pH reflects the change in the PCO2, or whether there's something else affecting it. So, if we take a change in PCO2, what change in pH would we expect?
DR. MCCLELLAN: So you can take that times 0.08. So your delta pH equals your 0.08 times delta PCO2. [Dr. Martin] Good, and that would be for an acute change, for a chronic change, we usually use 0.03. Although they usually won't ask you a chronic question, it will usually be an acute change. The other way I remember it is, the pH will change by 0.1, for every 12 point change in CO2. So, if the CO2 goes from 40 to 52, then your pH should go from 7.4 to 7.3.
DR. BINGHAM: Okay. [Dr. Martin] But either way, the 0.08 or the 0.1 for every 12 will get you there, and that should be your expected change in pH. So, we'll talk about another quick and easy way to do it, to know if the change in CO2 is just normal respiratory compensation for a metabolic acidosis or alkalosis. And this is a real easy one, is your CO2 should equal the number that's after the decimal point in the pH.
DR. BINGHAM: So, for example, if you have a pH of 7.28 and a PCO2 of 28 and a bicarb of 16, because the CO2 of 28 and the 7.28, the 28-28 are equal, that tells you that your CO2 change is in response to some kind of metabolic. So, that would be a primary metabolic acidosis. And then that respiratory change was compensatory.
DR. MCCLELLAN: Yeah, okay.
DR. BINGHAM: Makes sense.
DR. BINGHAM: [Dr. Martin] So, we talked about the important factors and looking at the pH, and actually whenever I get a blood gas problem, I don't look at the pH first, because remember, you can have a complex disorder with a pH of 7.4 if they're balancing each other out. So actually, what I do is I look at the CO2, and then, remember start with a CO2 of 40, as normal, so first thing I look at is the CO2, and if it's high, I write down respiratory acidosis, and if it's low, I write down respiratory alkalosis.
DR. BINGHAM: Then I look at the bicarb, or base excess, whichever they give you, and same thing. If the bicarb is high, I write metabolic alkalosis, if the bicarb is low, I write metabolic acidosis. And then I look at the pH, and see which direction the pH is going. And then you look at the answer choices they give you. And in most of these they're asking you, to just identify what disorders are present.
DR. BINGHAM: So, right there, you've answered it. If you had a high CO2, and you wrote down respiratory acidosis, and a low bicarb metabolic acidosis, then you have a combined respiratory metabolic acidosis. And you look for that answer and that should be your answer. And then those calculations we just talked about. Those are really only required, if they're not looking for a simple "Which two disorders are present?" Sometimes they'll ask you, "What's the primary disorder?" Is the metabolic acidosis the primary, or is the respiratory alkalosis the primary, and those compensatory?
DR. BINGHAM: But usually you'll be able to tell that from the history they give you, because they'll give you a patient who is over-narcotized. And you know the primary then is going to be a respiratory acidosis. Right? Or a patient is hyperventilating, you know the primary is going to be a respiratory alkalosis. So, that's how I approach them. And we can look at a couple of those, when we will get to some examples.
DR. BINGHAM: So, let's talk about metabolic acidosis. And of course the first thing you have to do to sort out, what kind of metabolic acidosis is calculate what?
DR. MCCLELLAN: So you would calculate your anion gap in these patients. [Dr. Martin] Good, and the formula for that?
DR. MCCLELLAN: So, that's your sodium plus your potassium, and then you subtract your chloride plus your bicarb. [Dr. Martin] Good, and the normal should be 12 to 16, and so you're either going to have a gap acidosis, or a non-gap acidosis. So if you have a gap acidosis, what are the common causes going to be?
DR. MCCLELLAN: So that's your MUDPILES mnemonic, that we've learned back since the beginning of medical school. So, your methanol, uremia, DKA, your paraldehyde which I've never seen, your isoniazid, lactic acidosis, ethylene glycol, and then salicylates or aspirin. [Dr. Martin] Good. And the usual ones, almost always if they give you a gap, it's going to be a lactic acidosis, a diabetic ketoacidosis, a salicylate overdose.
DR. MCCLELLAN: Yeah, like you said, they rarely ask you about the other ones. So, what if you have a non-gap acidosis?
DR. MCCLELLAN: Yes, I think of ostomies, or small bowel fistulas, lactulose, or other things.
DR. BINGHAM: Iatrogenic from sodium chloride like hyperchloremic metabolic acidosis-- [Dr. Martin] Good.
DR. BINGHAM: Renal tubular acidosis. [Dr. Martin] Yup.
DR. MCCLELLAN: There is a mnemonic for that too. I can't remember what it is though. [Dr. Martin] Yeah, the big cause would be diarrhea.
DR. MCCLELLAN: Oh yeah. [Dr. Martin] So, the big ones for a non-gap acidosis are you gave them too much sodium chloride, they have a renal tubular acidosis, they're having diarrhea. The other ones are the ileal conduit is a classic one they'll ask you about, so a ureteral diversion of a non-gap acidosis, and then any medication?
DR. BINGHAM: Acetazolamide? [Dr. Martin] Yeah, carbonic anhydrase inhibitors.
DR. MCCLELLAN: And then mafenide acetate. [Dr. Martin] The mnemonic I use for that is HEARTCCU. So the H is hyperaldosterone or Addison's, and then E is expansion, so sodium chloride expansion, acid ingestion, renal tubular acidosis, the T is for trots or you can remember turds, diarrhea, and then CCU is carbonic anhydrase, chronic pyelo, and then U is the ureteral diversions. Okay, so let's talk about metabolic alkalosis.
DR. MCCLELLAN: And what's the most common cause, especially on the Boards?
DR. MCCLELLAN: That's almost always your NG suctions, so you get your hypokalemic metabolic alkalosis. [Dr. Martin] Good, so that's one and then there's one other one.
DR. MCCLELLAN: Oooh.
DR. BINGHAM: Contraction alkalosis? [Dr. Martin] Yeah. Which would be the CHF patient who is getting diuresed, and you just overdiurese them. So let's say you have a CHF patient who you're diuresing, and the bicarb is now 54, and the sodium is 130, the chloride is 90. So, obviously you have a metabolic alkalosis, and what's your treatment? A contraction alkalosis.
DR. BINGHAM: You could either replace some of their volume, or if you're still diuresing them, theoretically can do the Diamox, give them Diamox. But I think mostly they're probably going to be given some of their volume back. [Dr. Martin] Yeah, but what's the most important thing you have to give them back, or replace?
DR. BINGHAM: Sodium? [Dr. Martin] Chloride.
DR. BINGHAM: Chloride. [Dr. Martin] Yeah. So replacing their chloride for a contraction alkalosis is the most important thing you have to give them back. So, we'll give you the patient who's undergoing surgery, he has an NG tube and they suck out three liters from his NG tube during the case. And after the surgery, he gets a blood gas, and his pH is 7.55, his PCO2 is 52 and his bicarb is 40.
DR. BINGHAM: So what's the underlying acid/base disorder?
DR. BINGHAM: Okay, so his CO2 is 52, so that's high, so that's a respiratory acidosis. His bicarb is 40, so that's a metabolic alkalosis. And he's alkalotic, so it's a metabolic alkalosis with a respiratory compensation. [Dr. Martin] Excellent, excellent. And again, you started with the CO2. So, if they asked you what abnormalities are present, the answer would be a combined respiratory acidosis and metabolic alkalosis.
DR. BINGHAM: If they ask you which one's primary, then you pretty much know that from the history, you got acid sucked out of his stomach, so the metabolic component is going to be primary.
DR. BINGHAM: Correct me if I'm wrong, but you can always look there and you can look at the pH, because nobody's ever going to overcorrect with a compensation. [Dr. Martin] Yes.
DR. BINGHAM: So that would give you your primary. [Dr. Martin] Exactly. And you can also see it's pretty close to that decimal point rule. So, the pH is 7.55 and the PCO2 is 52, so, those are almost equal which tells you that the primary component is metabolic. Now, we have a patient who got admitted in a coma, and gets a blood gas, and the PCO2 is 16, the bicarb is 5 and the pH is 7.1.
DR. MCCLELLAN: So, you look at your CO2, you said it's 16, so that's a respiratory alkalosis. And then you have a bicarb, what was it you said, 5? [Dr. Martin] Yep.
DR. MCCLELLAN: So that would be considered a metabolic acidosis. [Dr. Martin] Yeah.
DR. MCCLELLAN: So it would be a metabolic acidosis with a respiratory compensation. [Dr. Martin] Yeah. It's a metabolic acidosis with an attempted respiratory compensation, but obviously, it's not very well compensated, because the pH is 7.1. So, the primary disorder's the metabolic acidosis, and there is a respiratory alkalosis present, but it's certainly not compensating because the pH is 7.1. All right, last question.
DR. MCCLELLAN: So, we got a climber climbing a mountain, and he reaches an altitude of 5,000 meters above sea level. What's going to happen to his arterial PCO2 and his pH?
DR. BINGHAM: So, his PCO2 it's going to go down, he's going to be hyperventilating, and his pH is going to go up. [Dr. Martin] Good, and what disorder is he going to have?
DR. BINGHAM: A respiratory alkalosis. [Dr. Martin] Good. Okay, let's do one more. You have a patient whose pH is 7.5, PCO2 is 50, and bicarb is 35.
DR. MCCLELLAN: So CO2 is high, so you have a respiratory acidosis. What was the bicarb again, I'm sorry? [Dr. Martin] 35.
DR. MCCLELLAN: 35, so that is a metabolic alkalosis. And your pH is 7.5, so, it's a metabolic alkalosis-- [Dr. Martin] With?
DR. MCCLELLAN: With respiratory compensation. [Dr. Martin] With a compensatory respiratory acidosis. Good, okay, great. Alright, you ready for some quick fire fluid, electrolytes, acid/base?
DR. MCCLELLAN: Let's do it.
DR. BINGHAM: Yep, let's do it. [Dr. Martin] Okay, remember there's no discussion. Just you should know the answer as the question is being read. Okay, the cation that determines serum osmolarity.
DR. BINGHAM: Sodium. [Dr. Martin] Primary intracellular cation?
DR. BINGHAM: Potassium. [Dr. Martin] So, sepsis resuscitation bolus amount in cc's per kg. This is the new sepsis guidelines.
DR. BINGHAM: 20 cc's per kg. [Dr. Martin] 30 cc's per kg.
DR. BINGHAM: 30 cc's per kg. [Dr. Martin] Pediatric patient who needs volume, what's the volume you bolus him with?
DR. MCCLELLAN: [& Dr. Bingham] 20 cc's per kg. [Dr. Martin] Good, and how about blood products?
DR. MCCLELLAN: [& Dr. Bingham] 10 cc's per kg. [Dr. Martin] Okay, you have an EKG with peaked T waves and a K of 6.5. What's your next therapy?
DR. MCCLELLAN: Your calcium gluconate. [Dr. Martin] Good, you have a patient on the liver transplant list, who was started on a water pill by his PCP, he doesn't know what it was, now he presents with a potassium of 2.5. What's the etiology?
DR. MCCLELLAN: Iatrogenic Lasix. [Dr. Martin] Good, what if his K was 5.5?
DR. BINGHAM: Spironolactone? [Dr. Martin] Excellent, great. Okay, so you're correcting a patient who came in severely hyponatremic with 3% normal saline, and they then develop spastic quadriplegia.
DR. BINGHAM: Central pontine myelinolysis. [Dr. Martin] Excellent. So, you have a patient who is hyponatremic, you put them on free water restriction, and that is not working. They are still hyponatremic.
DR. MCCLELLAN: So then you can do your 3% normal saline. [Dr. Martin] Okay, any drugs you can give them?
DR. BINGHAM: Vaptans.
DR. MCCLELLAN: Oh, the vaptans.
DR. MCCLELLAN: That's right, and then you also do diuresis. [Dr. Martin] Yeah, but that would be the patient you give a vaptan, or demeclocycline. And the vaptan works on what receptors in the kidney?
DR. BINGHAM: V2. [Dr. Martin] V2. You have a baby with pyloric stenosis who's been having emesis for one week.
DR. BINGHAM: Hypochloremic hypokalemic metabolic alkalosis. [Dr. Martin] And for a bonus what's the urinary abnormality?
DR. MCCLELLAN: Paradoxical--
DR. BINGHAM: Paradoxical aciduria.
DR. MCCLELLAN: Man, I was excited about that one. [Dr. Martin chuckles] You're a [unintelligible]. What would be the effect of acidosis on the oxyhemoglobin dissociation curve?
DR. MCCLELLAN: So that's a left shift. [Dr. Martin] So, you have a pH of 7.1. So, what's the effect going to be on oxygen unloading?
DR. MCCLELLAN: So, you're going to increase oxygen loading for these patients. [Dr. Martin] What's that?
DR. MCCLELLAN: You would increase oxygen loading. [Dr. Martin] You're going to increase oxygen unloading.
DR. MCCLELLAN: Yeah. [Dr. Martin] It will unload more readily in tissue. So it actually it will improve tissue oxygen delivery. Okay, so you gave a patient large volume resuscitation with Hextend, and they are now bleeding in the OR.
DR. BINGHAM: Platelet disfunction. [Dr. Martin] Excellent. Okay. So you have a patient with a marked metabolic alkalosis, and now they're having decreased respiratory drive. What drug could you give them to help their metabolic alkalosis?
DR. BINGHAM: Diamox. [Dr. Martin] Good.
DR. BINGHAM: Acetazolamide. [Dr. Martin] You have a patient with high NG tube output.
DR. MCCLELLAN: Incites your metabolic alkalosis. [Dr. Martin] Yep, how about prolonged emesis?
DR. BINGHAM: Same thing. [Dr. Martin] Okay, how about severe diarrhea?
DR. BINGHAM: Hypokalemia. [Dr. Martin] Okay, and what about acid/base?
DR. BINGHAM: Oh, so there'll be metabolic acidosis. [Dr. Martin] Good, how about the mountain climber?
DR. BINGHAM: Respiratory alkalosis. [Dr. Martin] Okay, now post-op day two after a Whipple, somnolent on the ward and pinpoint pupils. They're [unintelligible].
DR. MCCLELLAN: Oh yeah, I'd say respiratory alkalosis. [Dr. Martin] Respiratory acidosis. Patient who's got an ileal conduit and high output.
DR. BINGHAM: Metabolic acidosis.
DR. MCCLELLAN: Acidosis.
DR. MCCLELLAN: [Dr. Martin] What kind?
DR. BINGHAM: Non-gap. [Dr. Martin] Non-gap, excellent. All right.
DR. MCCLELLAN: All right, I love it. [Dr. Martin] I couldn't stump you again. So that's all the quick fires for this one. So, hopefully that covers most of what they might ask you on the Boards for the ever exciting fluids, electrolytes and acid/base. [narrator] Until next time, dominate the day. [upbeat outro music]