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#1
12-02-2014
 USMLE Forums Veteran Steps History: 1+CK+CS Posts: 214 Threads: 29 Thanked 62 Times in 40 Posts Reputation: 72
Acid-Base Confusion

My main question is how do you tell the difference between
1* Metabolic acidosis and
1* Respiratory Acidosis
when PCO2 is Increased and HCO3 is Decreased and the pH is not within normal range.

(Side question - If pH was in Normal Range and PCO2 and HCO3 had abnormal values does that signify Mixed Disorder automatically? )

Do we use (i think i got this from DIT for step 1)
pH = HCO3/PCO2 ---> propotionality? --- i don't understand this concept anyway.

I had a question in Uworld with the following values
pH 7.15
PO2 90
PCO2 60
HCO3 18

Dx being 1* Metabolic acidosis with Respiratory Acidosis calculated with Winter's formula (which i understand).

So my question is how do we know that its 1*Metabolic Acidosis and NOT 1* Respiratory Acidosis
(Winters formula is applicable only once we understand it is a 1*Metabolic Acidosis and are checking to see Respiratory Compensation as i understand it)

So should we be looking for other clues in the question to point us to a 1*M.Acidosis over a 1*R.Acidosis like for in this question the Pt had ARF so that would fit into MUDPILES as Uremia telling us its 1*M.Acidosis.

And would it be 1*R.Acidosis if the Pt had COPD/Asthma or Opiate Use?

I Hope i was able to frame and explain my confusion out correctly.
I just want to know if my thought process about this concept is moving along in the right direction,thanks.
 The above post was thanked by: samba1200 (12-05-2014)

#2
12-02-2014
 Guest Steps History: 1 + CK Posts: 116 Threads: 4 Thanked 173 Times in 65 Posts Reputation: 183

Acid base I think is the toughest. However I think you may be overthinking this question a little bit.

pH 7.15
PO2 90
PCO2 60
HCO3 18

I am done with uworld, and mixed acid base is difficult stuff, I am by no means an expert but this is how I would approach this question.

Looking at pH I automatically know its an acidosis, now if I just look at bicarb without looking at CO2 I know its metabolic (bicarb is low). If I use winter's formula to see how CO2 would compensate and input the values (1.5(18)+8 +/- 2 = 33-37). So CO2 should be around 35 (lower than normal value).

In this case CO2 is way above 35 so there is no compensation happening, also remember that compensation will never OVERcompensate but will try its best to bring the value to normal, I can then only assume the patient also has a respiratory acidosis as well.

If you start by looking at CO2 first it is actually easier. You look at CO2 and it is elevated you assume it is respiratory, if you apply the rule that bicarb should increase by 1 for every 10 mmHg increase in CO2, by this rule bicarb should be 30, 18 is way under what bicarb should be so you know no compensation is happening, meaning that he also has a metabolic acidosis.

In the case of a person with COPD then you already know he will have respiratory acidosis, and if he then develops a lactic acidosis because of something else (exacerbation, hypoxia, ischemia or whatever) then you've got both of your acidosis right there. And you could during the test choose this answer without doing any calculations, you just see COPD, pH low, CO2 high, Bicarb low, most likely he has both a respiratory AND metabolic acidosis because no compensation is happening.

I think that calculations are more more helpful in cases where you have for example a resp. alkalosis with a met. acidosis likeor viceversa. I had a question like that, it was aspirin toxicity with a pH of 7.42 (normal) and low CO2 (25) and low bicarb (14), if you start with CO2 as resp alk. and calculate the compensation bicarb is lower than where it should be (bicarb goes down 2 for each 10 mmHg decrease in CO2, so bicarb should be 20 at most).

Hope this helps!

Last edited by Fibroblast; 12-02-2014 at 12:42 PM.
 The above post was thanked by: DoktorNo (05-01-2015)
#3
12-02-2014
 USMLE Forums Veteran Steps History: 1+CK+CS Posts: 214 Threads: 29 Thanked 62 Times in 40 Posts Reputation: 72

thats an awesome explanation man,just what i needed.

well im glad someone else finds it hard,i was thinking all this time im the stupid one for not being able to pick this stuff up quickly.
i think in the timed mode i just get flustered and pick some answer so i have to work on speed....but i guess understanding it well first is important.

okay so i knew the looking at HCO3 method,but looking at PCO2 method is new and frankly seems easier cause there's lesser calculation,but i don't quiet understand it yet

so is this the general rule : -
HCO3 Increases by 1 for CO2 Increase by 10, and
HCO3 Decreases by 2 for CO2 Decrease by 10.

just so i understand it clearly(bear with me) if PCO2 is 65 and we consider normal PCO2 to be 35.Then there is an increase of 30mmHg in PCO2, so that would increase HCO3 by 3 points(as per the rule) so that should make HCO3 31,since we consider normal HCO3 to be 28.....is it safe to say this is right?

#4
12-02-2014
 Guest Steps History: 1 + CK Posts: 116 Threads: 4 Thanked 173 Times in 65 Posts Reputation: 183

Quote:
 Originally Posted by samren17 thats an awesome explanation man,just what i needed. well im glad someone else finds it hard,i was thinking all this time im the stupid one for not being able to pick this stuff up quickly. i think in the timed mode i just get flustered and pick some answer so i have to work on speed....but i guess understanding it well first is important. okay so i knew the looking at HCO3 method,but looking at PCO2 method is new and frankly seems easier cause there's lesser calculation,but i don't quiet understand it yet so is this the general rule : - HCO3 Increases by 1 for CO2 Increase by 10, and HCO3 Decreases by 2 for CO2 Decrease by 10. just so i understand it clearly(bear with me) if PCO2 is 65 and we consider normal PCO2 to be 35.Then there is an increase of 30mmHg in PCO2, so that would increase HCO3 by 3 points(as per the rule) so that should make HCO3 31,since we consider normal HCO3 to be 28.....is it safe to say this is right?
I think you've got it man. Using your example if you had a pCO2 of 65 and a HCO3 of 31, then it would be a respiratory acidosis with an adequate metabolic compensation (bicarb is higher trying to get the pH back to normal, so pH could be in the normal range or still be acidotic).

Per the rules if your bicarb goes higher than 31 then you would have to call this a respiratory acidosis and a metabolic alkalosis (remember that the body never overcompensates), if it was lower than than normal (say 14), then it becomes a respiratory acidosis and a metabolic acidosis like the original question.

Just to review the general rules, this table is from uworld.

DISORDER-----------------------------------------COMPENSATION

Metabolic acidosis (bicarb low)--------------pCO2 = 1.5(HCO3) +8 + or -2 (Winter's)
Metabolic alkalosis (bicarb high)------------pCO2 increases by 0.7 for every 1 HCO3
Respiratory acidosis (CO2 up)---------------HCO3 increases by 1 for every 10 CO2
Respiratory alkalosis (CO2 down)-----------HCO3 decreases by 2 for every 10 CO2

This has helped me as well, you know whenever you feel you understand something try to explain it, then you will truly see if you understand it.

For the test I would not worry too much about this I think ,unless we've got enough time during the test then we may be able to apply the formulas quickly (or memorize them the day before and jot them down the day of the test). However questions like the COPD one should be doable if you don't look just at the numbers and think of the clinical context (COPD = resp. acidosis so you can basically eliminate all the answers that don't have that, then if bicarb is low BAM! metabolic acidosis as well and then you go on and become the best resident ever)
 The above post was thanked by: Laurentiu (12-03-2014)
#5
12-03-2014
 USMLE Forums Veteran Steps History: 1+CK+CS Posts: 214 Threads: 29 Thanked 62 Times in 40 Posts Reputation: 72

funny enough,i did a few more questions today and got the question with the table you typed out....but nice of you to type the whole thing out.

yeah even typing out your thoughts about a concept you aren't getting helps,cause you keep going over it i guess.

anyway,thanks again,and good luck.
 The above post was thanked by: Fibroblast (12-03-2014)
#6
12-03-2014
 Guest Steps History: 1 + CK Posts: 116 Threads: 4 Thanked 173 Times in 65 Posts Reputation: 183

Quote:
 Originally Posted by samren17 funny enough,i did a few more questions today and got the question with the table you typed out....but nice of you to type the whole thing out. yeah even typing out your thoughts about a concept you aren't getting helps,cause you keep going over it i guess. anyway,thanks again,and good luck.
Glad I could be of some help, trying to answer question helped me out a lot, I decided to type it out to see if I could finally memorize that table, I think it worked, lol. Best of luck on your exam!
#7
12-03-2014
 USMLE Forums Scout Steps History: 1 + CS Posts: 95 Threads: 7 Thanked 67 Times in 21 Posts Reputation: 87
acid base

First, I congratulate you for the assessment . Brilliant ! .

Second ,lets add to PCO2 calculation the following:

PCO2 increases by 10 when HCO3 increases buy 1 ( in acute setting ) and by 4( in chronic setting )

PCO2 decreases by 10 when HCO3 decreases by 2 ( in acute setting ) and by 5 (in chronic setting ).

Having said that, the stem question needs to be read carefully to figure out which setting we consider and not to lose time with calculations .

Quote:
 Originally Posted by Fibroblast I think you've got it man. Using your example if you had a pCO2 of 65 and a HCO3 of 31, then it would be a respiratory acidosis with an adequate metabolic compensation (bicarb is higher trying to get the pH back to normal, so pH could be in the normal range or still be acidotic). Per the rules if your bicarb goes higher than 31 then you would have to call this a respiratory acidosis and a metabolic alkalosis (remember that the body never overcompensates), if it was lower than than normal (say 14), then it becomes a respiratory acidosis and a metabolic acidosis like the original question. Just to review the general rules, this table is from uworld. DISORDER-----------------------------------------COMPENSATION Metabolic acidosis (bicarb low)--------------pCO2 = 1.5(HCO3) +8 + or -2 (Winter's) Metabolic alkalosis (bicarb high)------------pCO2 increases by 0.7 for every 1 HCO3 Respiratory acidosis (CO2 up)---------------HCO3 increases by 1 for every 10 CO2 Respiratory alkalosis (CO2 down)-----------HCO3 decreases by 2 for every 10 CO2 This has helped me as well, you know whenever you feel you understand something try to explain it, then you will truly see if you understand it. For the test I would not worry too much about this I think ,unless we've got enough time during the test then we may be able to apply the formulas quickly (or memorize them the day before and jot them down the day of the test). However questions like the COPD one should be doable if you don't look just at the numbers and think of the clinical context (COPD = resp. acidosis so you can basically eliminate all the answers that don't have that, then if bicarb is low BAM! metabolic acidosis as well and then you go on and become the best resident ever)
 The above post was thanked by: Fibroblast (12-04-2014)

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