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  #1  
Old 03-31-2010
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Question The cause of Acid Base Disturbance?

A previously healthy 6-month-old boy is brought to the physician because of a 12-hour history of vomiting and diarrhea. He vomits after all feedings, the vomitus does not contain blood or bile. His mother says that he has had fewer wet diapers than usual during this period. He appears dehydrated and is crying without tears. He is at the 50th percentile for length and 30th percentile for weight. He appears lethargic. His temperature is 38C (100.4F), pulse is 180/min, and blood pressure is 60/40 mm Hg. Examination shows sunken eyes, dry mucous membranes, and a sunken anterior fontanel. Arterial blood gas analysis on room air shows:
pH 7.2
PCO2 38 mm Hg
PO2 90 mm Hg

Which of the following is the most likely explanation for this patient's arterial blood gas findings?
A) Excessive metabolic acid formation
B) Impaired ventilation
C) Increased chloride loss
D) Increased CO2 concentration in the extracellular fluid
E) Increased metabolic acid produced by the gastrointestinal tract .
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Old 03-31-2010
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The cause here should be loss of bicarbonate but it's not in the options.
Then I choose B as the only possibly correct answer
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  #3  
Old 03-31-2010
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Choice B cannot be correct, because PaCO2 is in the normal-to-low range!

This is obviously a case of compensated metabolic acidosis (low pH, normal O2 & CO2), thus estimation of the anion gap is necessary to make a diagnosis. However, with a more critical look at the question stem, we can make out that this poor boy is dehydrated (shrunken fontanels, no tears, excessive vomiting, low BP, tachycardia, etc) probably due to an infection (fever is present).

Any kind of infection is a hypermetabolic state, where the aerobic backups of the body are pushed to their limits and thus anaerobic metabolism emerges, so as to satisfy energy needs. Lactic acidosis ensues, which is a well known cause of metabolic acidosis. Consequently, I think the correct answer is A.

B is wrong for the reasons I explained above.

C is wrong, because in this context one would expect alkalosis, not acidosis (as seen in hypochloremic hypokalemic alkalosis following persistent vomiting). This child's vomiting is a potential distractor by this point of view.

E is wrong, because if more acid was produced in the GIT, it would remain within the GIT; perhaps this would have an errosive effect on the lumen, but this doesn't mean that would get into the bloodstream! It would be buffered!

I am not yet convinced about choice D, I'm still working it out, but I have a strong feeling for A!!! I have been exhausted today, please show mercy!!!
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  #4  
Old 03-31-2010
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Quote:
Originally Posted by ath.pantelis View Post
Choice B cannot be correct, because PaCO2 is in the normal-to-low range!

This is obviously a case of compensated metabolic acidosis (low pH, normal O2 & CO2), thus estimation of the anion gap is necessary to make a diagnosis. However, with a more critical look at the question stem, we can make out that this poor boy is dehydrated (shrunken fontanels, no tears, excessive vomiting, low BP, tachycardia, etc) probably due to an infection (fever is present).

Any kind of infection is a hypermetabolic state, where the aerobic backups of the body are pushed to their limits and thus anaerobic metabolism emerges, so as to satisfy energy needs. Lactic acidosis ensues, which is a well known cause of metabolic acidosis. Consequently, I think the correct answer is A.
This is not a compensated acidosis because the PH is still 7.2!

Babies with sever vomiting and diarrhea develops normal anion gap acidosis. Which is the cause here, not the infection as you presume.

The PCO2 is in the normal range yah right but it's inadequate for the degree of acidosis. A patient with PH 7.2 should be hyperventilating and washing out CO2 to at least 20 or 15.

So this baby is having inadequate respiratory compensation for the acidosis may be because of the obtundation that he has from the dehydration
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  #5  
Old 03-31-2010
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and how is the fever explained?
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Old 03-31-2010
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Quote:
Originally Posted by ath.pantelis View Post
and how is the fever explained?
I am not saying that he has no infection, definitely he has like a severe rota virus infection for example.
Also remember the dehydration causes fever
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Ok, I know I shouldn't be posting after 17 consecutive hours without sleep! Thanx dr. Salwan85! I just realized that the question wants us to explain the paradoxically normal pCO2 levels, despite the acidosis. My apologies once again to the users of this forum...
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Quote:
Originally Posted by ath.pantelis View Post
Ok, I know I shouldn't be posting after 17 consecutive hours without sleep! Thanx dr. Salwan85! I just realized that the question wants us to explain the paradoxically elevated pCO2 levels, despite the acidosis. My apologies once again to the users of this forum...
You don't need to apologize, that's just a healthy discussion, your posts are great and very informative
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Old 04-01-2010
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my answer wud be E,
diarrhea causing loss of bicarb leading to acidosis which wud be metabolic.
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Old 04-01-2010
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Quote:
Originally Posted by syedaanaqvi View Post
my answer wud be E,
diarrhea causing loss of bicarb leading to acidosis which wud be metabolic.
There is no acid produced when you lose bicarbonate!
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Old 04-01-2010
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Quote:
Originally Posted by ath.pantelis View Post
Ok, I know I shouldn't be posting after 17 consecutive hours without sleep! Thanx dr. Salwan85! I just realized that the question wants us to explain the paradoxically normal pCO2 levels, despite the acidosis. My apologies once again to the users of this forum...
ACTUALLY I M WITH DOCTTOR SALWAN cuz its not a compansated metabolic acidosis at all ,its decompansated and that rate of hyper ventilation will definitly be high to washout for co2 as a response from the respiratory system for correction of acid base disturbance will take few hours to correct the change in the PH.
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Old 12-19-2011
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I wish to disagree wit Dr. Salwan.. I feel the option is A

The ABG shows acidosis, the HCO3- value is not mentioned , but since PCO2 is normal the diagnosis is Metabolic Acidosis. Severe dehydration is evident and hypotension is also seen ! There is a history of diarrhea which can cause Metabolic acidosis with a Normal Anion gap. That means it causes Hyperchloremic acidosis. Since the patient is in hypovolemic shock , and low BP is seen -> Lactic acidosis. Which is a high AG acidosis. So in this case it has to excess acid that is Lactic acid production !

If its a ventilatory disturbance the co2 should be either high or low.. SInce the symptoms have been there for 12 hours its probably being compensated
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  #13  
Old 12-20-2011
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It too go with A. question suggests its an uncompensated metabolic acidosis, as low Ph normal pco2 and po2. and cannot be impaired ventillation.
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Old 12-20-2011
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Cool I think it is impaired ventilation

Actually when I read the q the first time I went with A
But several things changed my mind
This patient clearly has metabolic acidosis the q is it anion gap or not??!!
Bcs the q is deficient of adequate information and there is clear hx of diarrhea,it is most likely non anion gap. Either ways there is decreased compensation ( should be wasting of co2), and if you calculate A-a difference it is > 10 so it extrapulmonary cause. So there is mixed metabolic acidosis with decreased ventilation or i think so
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Old 12-20-2011
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I wud go with A.

The reason for it is: Severe dehydration will produce Contraction Alkalosis so for it Respi Acidosis shud be the compensation.

but here we see no compensation and infact pH is acidotic which suggest somewhere excess acid is being produced and now body is trying now to remove it by taking PCO2 levels towards Respi ALkalosis.
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Old 09-11-2014
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Its a mixed acid base disorder with meatbolic acidosis plus respiratory acidosis so the pco2 is still high (didnt compensate the metabolic acidosis)thats why I think the answer is B
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Acid-Base-, Nephrology-, Pediatrics-, Step-2-Questions

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