AssessmentsMetabolic and respiratory alkalosis: Clinical practice
USMLE® Step 2 style questions USMLE
A 3-week-old infant is brought to the emergency department because of projectile vomiting after feeding. She has vomited like this for three days, but her mother says the infant has a good appetite despite losing weight. The mother remembers that there may have been a similar problem with the baby's paternal uncle, who needed surgery when he was an infant. Her temperature is 37.0°C (98.6°F), pulse is 140/min, respirations are 45/min, and blood pressure is 70/45 mm Hg. The abdomen is non-tender and non-distended, but a small mass can be palpated below the costal margin. Which of the following findings is most likely a characteristic sign of this condition?
Content Reviewers:Rishi Desai, MD, MPH
In metabolic alkalosis, the blood pH is above 7.45, and it’s due to a bicarbonate or HCO3 concentration in the blood over 27 mEq/L.
Associated symptoms are related to the underlying cause.
The diagnosis is usually based on an ABG, and in addition to a pH above 7.45, and HCO3 levels above 27 mEq/L, if there’s respiratory compensation, the pCO2 is usually above 45 mm Hg.
Generally, for every 1 mEq/L elevation in HCO3 above the normal level of 27 mEq/L, pCO2 increases by about 0.7 mm Hg above the normal level of 45 mm Hg, but pCO2 doesn’t usually rise above 55 mm Hg, regardless of HCO3 levels.
Let’s take an example and say that HCO3 level is 30 mEq/L - so it’s 3 mEq/L above the baseline.
In addition, electrolytes are also done to see if there’s any imbalances, like hypokalemia.
Now, if the cause of metabolic alkalosis isn’t obvious from the history, then a spot urine chloride is measured.
If the urine chloride is below 20 mEq/L, that suggests volume depletion from a variety of causes like vomiting which leads to loss of hydrochloric acid, so the treatment is really aimed at addressing the underlying cause of vomiting.
A related cause is aggressive nasogastric suction, so the treatment is stopping or slowing the removal of gastric secretions.
Another cause is loop or thiazide diuretics which block hydrogen ion and chloride ion reabsorption in the kidney.
Chloride is a negatively charged ion, so loss of chloride leads to increased reabsorption of bicarbonate to compensate for the loss.
The loss of chloride causes the urine chloride to go above 20 mEq/L, and a lot of hydrogen is lost-leading to metabolic alkalosis.
Treatment relies on stopping the use of loop or thiazide diuretics.
Finally, a rare cause of metabolic alkalosis is diarrhea.
These include diarrhea due to laxative abuse which causes chloride to be lost in the GI tract, a villous adenoma which secretes chloride into the lumen of the gut, or diarrhea due congenital chloridorrhea, which is a genetic condition that causes chronic, chloride-rich diarrhea.
In short, all of these types of diarrhea lead to chloride losses in the GI tract.
Again, treatment really depends on the underlying cause.
And in addition, individuals that are hypovolemic are given IV isotonic saline- which helps reverse metabolic alkalosis by correcting the chloride deficit .
Now, sometimes, individuals with heart failure or cirrhosis develop metabolic alkalosis, because they’re intravascularly volume depleted from diuretic therapy, but they have peripheral edema and ascites.
If this doesn’t correct the alkalosis, then acetazolamide is given because it’s a diuretic that specifically increases bicarbonate excretion.
When urine chloride is above 20 mEq/L, this means that too much chloride is being lost through the urine.
In this case IV isotonic saline won’t be effective in reversing the metabolic alkalosis, because chloride will still be excreted in the urine.
Treatment relies on surgically removing the adenoma. When there’s also hypokalemia, oral potassium chloride is given when potassium levels are between 3 and 3.5 mEq/L, and IV potassium chloride is given when potassium levels are below 3 mEq/L.
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