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NBME 20 Answers

nbme20/Block 1/Question#37

A 1-month-old male newborn is brought to the ...

Na+: 132; K+: 3.2; Cl−: 90; HCO3−: 37

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 +2  upvote downvote
submitted by hayayah(244),

With chronic vomiting, you lose electrolytes and a lot of acid. It triggers metabolic alkalosis which is why all the serum values are low (or on the lower end of the normal range) except for bicarbonate.

ergogenic22  decreased K+ (from increased RAAS due to volume loss) and decreased Cl- (loss of HCl from the stomach), Alkalosis from loss of HCl and thus high bicarb. For this reason high to mid range K is wrong +1  
sbryant6  Wouldn't increased RAAS lead to increased Na+? The answer shows decreased Na+. +  
sbryant6  Also, remember Bulimia Nervosa is associated with hypokalemia. +  
sugaplum  so the range they gave for K is 3-6? so 3.2 is WNL then? or are we just operating on "it is on the lower end of normal in peds" +  

 +2  upvote downvote
submitted by masonkingcobra(36),

In metabolic alkalosis, potassium moves into the cells

The loss in volume through emesis triggers RAAS resulting in increased Aldosterone release and further potassium excretion

http://www.labpedia.net/test/116


 +0  upvote downvote
submitted by ark110(0),

But what is the difference between option A and option C (132; 4.9; 90; 35)

sympathetikey  K+ shouldn't increase. It's moving into cells due to metabolic alkalosis. +  
home_run_ball  In the parietal cell of the stomach Hydrogen ions are formed from the dissociation of carbonic acid. Water is a very minor source of hydrogen ions in comparison to carbonic acid. Carbonic acid is formed from carbon dioxide and water by carbonic anhydrase. The bicarbonate ion (HCO3−) is exchanged for a chloride ion (Cl−) on the basal side of the cell and the bicarbonate diffuses into the venous blood, leading to an alkaline tide phenomenon. +  
ergogenic22  RAAS increases from volume loss, and thus more aldosterone leads to low K+ +  
sinforslide  Three reasons for hypokalemia. First, some K+ is lost in gastric fluids. Second, H+ shifts out of cells and K+ shifts into cells in metabolic alkalosis. Third, ECF volume contraction has caused increased secretion of aldosterone. +2  

 +0  upvote downvote
submitted by apop(0),

These explanations arent great. It doesnt really have much to do with RAAS activation. It has almost everything to do with a phenomenon called the alkaline tide. In chronic vomitting, you would expect Cl- to be low and K= to be low (similar to lab results in someone with bullemia). To compensate for the low Cl-, the stomach has an antiporter which exchanges Cl- for HCO3-. Therefore, Cl- will be replenished in the stomach, while HCO3- will increase in the blood, causing an alkalosis. In alkalotic states, the H+/K+ antiporter will begin to activate, shifting K+ INTO cells (hypokalemia) and increasing pH. RAAS probably plays a role in making the hypokalemia worse but the alkaline tide is more important here.


 +0  upvote downvote
submitted by iamapotato(0),

No one here bothered to actually discuss what's going on. This is a 1-month old newborn.

He vomits after feedings, 5-day history.

At 1 month old, with vomiting after feedings, it is most likely a case of PYLORIC STENOSIS due to hypertrophy of the pyloric sphincter (usually occurs 6 weeks later).

Due to him vomiting stomach contents, he will LOSE Cl-, K+, and Na+.

He will ALSO lose H+. This is why you go into the metabolic alkalosis with a resultant increase in the HCO3-.