DIP Episode 470 - Numerical Acid-Base Problems on The USMLEs (a simplified, accurate approach)

Okay welcome my name is divine this is episode 470 of the Divine Intervention Podcasts. Into this podcast we're going to be describing a simplified approach to numerical acid-based problems on the USMLA exams. We're going to be describing a simplified approach to numerical acid-based problems on the USMLA exams. Now before I go into this the very first thing I'm going to say is most times if the USMLA is a testing compensation they will almost always discuss it from the perspective of metabolic acidosis about 95 plus percent of the time if the USMLA is a testing compensation they will almost always test it from the perspective of metabolic acidosis. The other kinds they are compensatory for those but those are things that are largely not necessary to know for purposes of the USMLA exams. So this podcast is going to be short but if you strictly diligently follow the rules I discussed here you're going to be in very good shape. So whenever you get any acid-based problem what is the very first thing you should always look at because sometimes people struggle with what do I look at first the very first thing you should look at is the pH. Ask yourself is the pH less than 7.35 or is the pH greater than 7.45 that's your first step. If it's less than 7.35 the person has an acidosis. If it's greater than 7.45 the person has an alkalosis that's the first rule you should keep at the back of your mind. Okay now let's go to rule number two.

Rule number two let's focus on the acidosis side of things. If your pH is less than 7.35 you know you have an acidosis. Now there are two ways you can have an acidosis. You can have an acidosis because you have excess CO2 that's a respiratory acidosis or you have an acidosis because you have depleted by carb. That's low by carb that's a metabolic acidosis. So literally in this step two if we're looking at things from the acidosis side of things literally look at what you see in the question because many people sometimes wonder, how do I know the primary disorder? The primary disorder is dictated by following these steps. If the pH is less than 7.35 your primary disorder is for sure an acidosis and then in step two just check what you get in the question is the PCO2 excessively high. If it's high that's a respiratory acidosis. But if the PCO2 is like normal or it's low but then you notice that man the by carb is pretty low then that person very likely has a metabolic acidosis. Well if you're looking at things from the alkalosis side of things so from step one you see that while the pH is over 7.45 it tells you that this person has an alkalosis. Well if you go to step two there are two is you can have an alkalosis. You can have an alkalosis because you have a depleted PCO2 or because you have elevated by carb. If your PCO2 is very low and you have an alkalosis then your primary problem is a respiratory alkalosis.

But if a person has a pH over 7.45 and the by carb is very elevated then that person certainly has a metabolic alkalosis. Certainly has a metabolic alkalosis. So now let's shift back to the acidosis side of things. So let's go to step three. Step three let's focus on metabolic acidosis for now. So if you determine from step one the pH is less than 7.35. Step two you notice that the by carb is low then you know that you're dealing with some kind of metabolic acidosis. So what is step three? Step three is the following. When you're dealing with a metabolic acidosis question do two things. One calculate winter's formula. Two calculate the anion gap. One calculate winter's formula. Two calculate the anion gap. The one that's probably the most important is going to be winter's formula. So what is winter's formula? Winter's formula is literally 1.5 multiplied by your by carb plus 8 plus or minus 2. That plus or minus 2 gives you a range. And what do you get from this winter's formula? The stuff you get from the winter's formula is what you expected PCO2 should be if you're compensating for the metabolic acidosis. It tells you what you expected PCO2 should be. Okay it tells you what you expected PCO2 should be. So let's say for example you get the number 20 for the person's PCO2. If you plug it into the formula it's going to be 1.5 times 20 that's 30 plus 8 that's 38 plus or minus 2. So if you add to that gives you 40 if you subtract to that gives you 36.

So basically your range is 36 to 40 that's your PCO2 range 36 to 40 again I just made up that number so we know the primary problem is the metabolic acidosis but then we see that wow okay we have a PCO2 given in the question and we haven't expected range. So how do you deal with this expected range? If you calculate what your PCO2 should look like right what your PCO and by the way I think I may have made a small mistake earlier on. Instead of PCO2 of 20 I should have said a by-carb of 20. The thing that goes into the formula is the by-carb not the PCO2 so my apologies there right. So 1.5 multiplied by your by-carb plus 8 plus or minus 2 okay so the thing that's plugging is the by-carb sorry I must have been spoken and say PCO2 my apologies okay so 1.5 multiplied by your by-carb so let's say your by-carb is 20 let's stick with that number so that tells us that oh the expected PCO2 should be between 36 and 40 should be between 36 and 40. So let me tell you this how do you handle the number because whatever you get from doing your math with the by-carb your job is to then compare it to the PCO2 that's literally giving to you in the question. If the PCO2 that's giving to you in the question is within the range you have if the PCO2 is 36, 37, 38, 39 or 40 then that person has perfect compensation. Since they have perfect compensation they do not have a concomitant disorder.

I'm gonna say that again if the person's number basically if the PCO2 giving in the question matches the number you get from your math then the person does not have a concomitant disorder so if they get a if you do your math right and we did the math with the numbers I made up and we got a range of 36 to 40.

If the PCO2 giving in the question is 36, 37, 38, 39 or 40 the compensation is perfect so if they're trying to ask you to pick an answer for the disorder they have in the question pick metabolic acidosis only that's the only thing they have that's the only thing they have that's the only thing they have because the USML is many people fall down with this kind of problem where they see you know the USML is many times they will put an answer that says metabolic acidosis only and then sometimes they'll put metabolic acidosis and respiratory acidosis sometimes you put metabolic acidosis and respiratory acidosis right and people are many times go with this 40 thinking that oh the fact that they compensated well means that they have a concomitant problem that's not how you should do it on the USML exams again this podcast is not for medical decision making is just for test prep that's literally all I'm teaching for an obviously you mileage me very but I'm telling you this trust me on this if you do your math with the bike car and get a range and the piece you to giving in the question falls in that range the person has only one disorder they only have metabolic acidosis that's it they do not have any concurrent problem okay now if you do that math you know we got a range of 36 to 40 and you get a number the number the piece you to giving in the question is less than that range then that person has a concomitant respiratory acidosis so let's say for example the piece you to giving in the question is 30 that means the person over compensated for the metabolic acidosis they blow off too much CO2 so if the number given in the question is lower than that range the 36 to 40 I just spat out then so let's say like 30 for example is the piece you to giving the question then the right answer is going to be metabolic acidosis plus respiratory alkalosis in the same vein if the piece yo

u to giving in the question if the piece you to giving in the question is more than the range so let's say the piece you to the question is like 45 that's obviously like not one of the five numbers between 36 and 40 then go ahead and pick that you have a metabolic acidosis and a concurrent respiratory acidosis okay that's a very very smart way to handle these states literally if you understand what I've talked about to this point you're probably going to be set for most of your of your questions now anion gap obviously is sodium minus your chloride plus bicarb so you basically add your chloride plus bicarb and you go ahead and subtract that from your sodium so if you calculate your anion gap and you get a number that is bigger than 12 you have an a high anion gap in a polycacidosis because that's the thing that you have to do is love to do they give you a bunch of answers and then because you'll be a metabolic acidosis question you give you a bunch of answers that caused metabolic acidosis but some of them will be high anion gap some of them will be normal anion gap right so obviously if it's a high anion gap metabolic acidosis then you should think about your mot paths no money that you probably remember you're probably remember you know learning at some point right there many things that cause you know high anion gap metabolic acidosis like methanol uranium gas so you put a HCKD people that are in decay or they have like an alcoholic you know acidosis paraldehyde isoniazide lactic acidosis you know you know let's say from metformin or from sepsis or septic shock right ethanol ethylene glycol right rubbed on my all this is renal failure causes a high anion gap metabolic acidosis and then also if you're taking salicylics right all those things can cause a high anion gap metabolic acidosis right no that anion gap metabolic acidosis the big ones they love to go after an

exams because they are many crazy nomonic people memorize but honestly the smartest classic ones to know are diarrhea renal tuberculosis acribonic anhydrous inhibitors and also if you're taking an aldosterone antagonist those are the classic causes so if you have adrenal insufficiency those are the classic causes of a normal anion gap metabolic acidosis there's some other lower yield ones that people love to memorize all the time like hyperalimitation having a ureteropelvic shunt and all those things and that's fine that's good to know but again those represent an extremely extremely rare is almost like trying to find like an endangered species on your USML exams those things are like endangered species so just be wise and know what is important and know what is high high yield so that's kind of like the simple way to handle those things so now some people may be like what divine can you please give us some comment because you've dealt with metabolic acidosis in great detail how about if the primary problem is respiratory acidosis or a metabolic alkalosis or respiratory alkalosis like what do I do if I do see those things on my exams well let me tell you what you should do here's the thing here's the smart way to go about this this stuff many times when they test those other disorders on exams they usually test it from the context of arrows or they'll give you a bunch of values so either test it from the context of arrows or they'll give you a bunch of values and then ask you to pick the right permutation or the right combination that makes sense for the disorder that is being dealt with let me tell you some general tips to keep in mind again always take it from the top step one and step two that I described today is very important if the pH giving in a question is more than 7.45 you have your primary problems and alkalosis is less than 7.35 your primary problems and

acidosis that's the first rule to keep in mind second rule again knowing that there are two ways you can have an acidosis or two ways you can have an alkalosis based on what I described a third rule I'll then bring in for these arrow questions or number of these questions is remembering this whenever you compensate you cannot compensate back to what a normal pH is okay you can never like if you're on USM at least for USM it purposes when you compensate for a primary disorder the pH should not compensate exactly back to normal or overshoot what normal is let me explain so let's say for example the primary disorder in a question is a metabolic alkalosis well obviously for pressing as a metabolic alkalosis then your pH if you're taking things from the top as I described should already be greater than 7.45 and obviously from reading the question the bicarb should be some really high number so the primary problem is metabolic alkalosis obviously the compensation for metabolic alkalosis is a respiratory acidosis now here's the deal here's the skinny if for example you have to pick a bunch of arrows and you're like hmm pick the right arrow or whatever that agrees with this question then you should not pick a series of arrows that agrees with the pH from compensation being 7.45 or being less than 7.45 you can never overshoot the primary problem in the process of compensation on USM in exams on USM in exams right so if you're because you have a metabolic alkalosis your pH is over 7.45 you cannot compensate so good that you get back to 7.45 no and you cannot compensate so good that you dip below 7.45 that makes no sense okay so that's something that's just a wise thing to keep in mind if you keep that rule in mind most times in my experience you can easily pick out the right set or series of arrows that answer the question and answer it correctly so that's what I'm going to sa

y if you're looking at it from an acidosis perspective so let's say wow pH is less than 7.35 and you notice that the PCO2 was high so obviously the primary problem is a respiratory acidosis so obviously the way you're gonna compensate for respiratory acidosis is with a metabolic alkalosis okay so your bicarb in compensation should be elevated so like for example if you're doing a bunch of arrow questions and you see wow this pressing the primary problem is respiratory acidosis you see a bunch of arrows and then you're picking an arrow that says that while the bicarb should go down you've already screwed up there because the way you compensate for respiratory acidosis is with a metabolic alkalosis your bicarb should go up and then if you're trying to ask you to pick a pH you should not pick any pH that is 7.35 or higher there is no way you'll be correct doing that because basically you again since the primary problem is respiratory acidosis regardless of how awesome your compensation is literally regardless of how awesome your compensation is your compensatory pH has to be some number that is less than 7.35 it cannot be 7.35 it cannot be over it cannot be 7.36 7.37 no it doesn't make any sense again I would really hope that for people that are carefully listening to this and not trying to multitask as they're doing this you will get this point if you understand this point honestly if you understand this point and what I've said so far in this podcast in the first 17 minutes and like nine seconds of this podcast I don't see how you you have an issue with correctly and answering any numerical acid based question notice I literally call this numerical acid based podcast simplified approach because the thing is obviously they're otherwise they can test acid based and I've addressed that in previous podcasts right like oh like I've had I think I have like a clutch metabolic

acidosis podcast for example right so there are many different ways they can go with these things and you know if there are topics I've not addressed I'll probably address them in future in future podcasts but literally just follow the rules I thought you here that's it that's all you need to know if you follow these rules you will confidently crush any numerical what do I mean by numerical it means they give you like numbers in the question numerical acid based question three questions that involve arrows or numbers so I'm gonna go ahead and stop here for those that are taking the step two step three or step one exam soon I do have some review course is starting tomorrow I have a test taking class tomorrow for step one to step three from five to seven thirty p.m.

Pacific time I have a biostatistics class it's a four hour class for step one to step three taking place on Friday from photo a p.m. Pacific time and then I have a five hour social sciences and ethics class is like a quality improvement healthcare systems communications class on Saturday five hours long for step one to step three from three to eight p.m.

Pacific time and the next week from Monday to Friday except Wednesday we have a 20 hours or a million five hours for four days we have a 20 hour step two step three review course many people have taken these courses they don't extremely well like literally the 20 hour course that was held last week also on the ticket and present just email me out like wow divine I have already bumped up to a 260's literally right after the course I took it my next practice test so again there are many people that do really well with these courses and again these courses are not lectures if you're looking for a lecture don't attend the course these courses are not lectures they're all clinical scenarios that's number one number two I tried to give context not just oh I give you the details but I give you the context behind those details because many us many questions another answer because you know one detail that you memorized from some flash card but is answered correctly because you understand the concept you know the context you know the story behind that context that that concept so if you want to truly understand pathophysiology and then see the clinical context which will make your knowledge way more powerful for the exam they should be an email and I can give you some more detailed information about these classes and then I also offer one or one children first step one to step three and medical school exams and then I have these podcasts on the major apps Apple Google and Spotify I have a You Tube channel divine intervention USM only podcasts and videos and then I have another website divine intervention life lessons calm it's a many well you know I'm a Christian so Bible based website and every week upload two podcasts that deal with our life lesson like just a common problem that is faced by humanity but from a biblical perspective and pretty sure I have like 200 podcasts on t

here at this at this point and there's an Apple podcast as well that will not call the divine intervention life lessons podcasts and then finally I help with era's applications personal statements recommendation letters and things like that especially with edits walk interviews I've done that for years many people have worked with our attendants residents and all that stuff so if you're interested just shoot me an email I'll give you some more information so thank you for listening to me today again follow this podcast this is a higher podcast and we show the very best on your exams God bless you bye for now thank you

Check the pH. Is it < 7.35 (acidosis) or > 7.45 (alkalosis)?

Acidosis.

Check PaCO2 first. High PaCO2 suggests Respiratory Acidosis; low HCO3 suggests Metabolic Acidosis.

The expected (compensated) PaCO2 range.

Respiratory alkalosis (the patient over-compensated).

AG = Na - (Cl + HCO3).

Methanol, Uremia, Lactic Acidosis (or MUDPILES mnemonic components).

7.35 to 7.45.

Normal Anion Gap Metabolic Acidosis (Diarrhea causes HCO3 loss, not an increase in unmeasured anions).

Winter's Formula: $1.5 \times \text{HCO}_3 + 8 \pm 2$.

Adrenal insufficiency (due to aldosterone deficiency).

Metabolic Alkalosis (elevated HCO3).

Concomitant respiratory acidosis.

7.35 to 7.45.

Normal Anion Gap Metabolic Acidosis (Diarrhea causes HCO3 loss, not an increase in unmeasured anions).

Winter's Formula: $1.5 \times \text{HCO}_3 + 8 \pm 2$.

Adrenal insufficiency (due to aldosterone deficiency).

Metabolic Alkalosis (elevated HCO3).

Concomitant respiratory acidosis.