[Seetal]

An 18-year-old woman has gastroenteritis with nausea and vomiting and is able to ingest only small amounts of water. After 3 days, she develops light-headedness, especially when sitting or standing. Arterial blood gas analysis is most likely to show which of the following sets of values?

pH - PCO2(mm Hg) - HCO3-(mEq/L)
(A) 7.30 - 28 - 15
(B) 7.30 - 55 - 27
(C) 7.40 - 40 - 24
(D) 7.50 - 30 - 22
(E) 7.50 - 47 - 35

 

[doctorF]

E is the ryt ans in my opinion

 

[Seetal]

E is the ryt ans in my opinion

that was fast! and yes u are ryte.

 

[khanar]

Can you please explain why we should think of alkalosis?

 

[ath.pantelis]

If I am not that wrong again (doctorF should remember ), this is a typical case of hypochloremic hypokalemic metabolic alkalosis, due to persistent loss of fluid through the upper GI tract. Poor water intake only adds to volume contraction, which in turn aggravates this contraction alkalosis.

Activation of the renin-angiotensin-aldosterone axis increases Na+/H+ exchange & decreases HCO3- reabsorption at the PCT of the nephron, as well as increasing H+ excretion at the DCT (through the intercalated cells).

 

[Seetal]

well dr. ath's explanation was exactly correct (its like out of a book!!)

however i cant always remember explanations like this so a quick way i think when i come across these questions:

vomiting causes met alkalosis (because u lose acid from yr stomach)
diarrhea causes met acidosis (because u lose alkali from yr intestine)

now i see the HCO3- (met alkalosis would be >24) and as a compensation, the resp CO2 has increased a bit to compensate.

oh n the pH must follow vomiting (met. alkalosis)

 

[ath.pantelis]

After repeating high-yield concepts over & over again, I suppose I have learned a few things by heart (although I HATE parroting ). And of course you are right about the rest; vomiting=met. alkalosis vs. diarrhea=met. acidosis (this is the way I actually remember them, too!). But, as you have understood till now, I dare not to post a comment if I don't have a concise mechanism in hand .