Metabolic and respiratory alkalosis: Clinical

To be retired ⓘ

00:00 / 00:00



Metabolic and respiratory alkalosis: Clinical

USMLE® Step 2 questions

0 / 6 complete


USMLE® Step 2 style questions USMLE

of complete

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?


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.

Individuals with metabolic alkalosis can be asymptomatic or can cause hypoventilation, due to respiratory compensation.

Associated symptoms are related to the underlying cause.

For example, if there’s a history of vomiting, nasogastric suction, laxative abuse or use of loop or thiazide diuretics, then there may be symptoms of dehydration.

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.

This means that our pCO2 should be 45- which is the baseline for pCO2- plus 3 times 0.7, which equals 47.1 mm Hg.

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.


  1. "Clinical manifestations and evaluation of metabolic alkalosis" undefined (undefined)
  2. "Simple and mixed acid-base disorders" undefined (undefined)
  3. "Hyperventilation syndrome in adults" undefined (undefined)

Copyright © 2023 Elsevier, its licensors, and contributors. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

Cookies are used by this site.

USMLE® is a joint program of the Federation of State Medical Boards (FSMB) and the National Board of Medical Examiners (NBME). COMLEX-USA® is a registered trademark of The National Board of Osteopathic Medical Examiners, Inc. NCLEX-RN® is a registered trademark of the National Council of State Boards of Nursing, Inc. Test names and other trademarks are the property of the respective trademark holders. None of the trademark holders are endorsed by nor affiliated with Osmosis or this website.