American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the expected decrease in the serum bicarbonate for chronic respiratory alkalosis with a decrease in PCO2 of 10 mm Hg?

1. 2 to 3 mEq/L

2. 4 to 5 mEq/L

3. 6 to 7 mEq/L

4. 8 to 9 mEq/L

The correct answer is: 4 to 5 mEq/L

In chronic respiratory alkalosis, the body responds to sustained lower levels of carbon dioxide (PCO2) by decreasing serum bicarbonate (HCO3-) concentrations. This adjustment typically occurs over a period of days to weeks as the kidneys excrete bicarbonate in response to the lower CO2 levels, which are associated with a higher pH in the blood. For every decrease of 10 mm Hg in PCO2, the expected drop in serum bicarbonate is generally around 4 to 5 mEq/L. This correlation is based on the body's buffering system and compensatory mechanisms that maintain acid-base homeostasis. Thus, when considering the expected bicarbonate decrease associated with the given change in PCO2 in chronic respiratory alkalosis, the choice reflecting a decrease of 4 to 5 mEq/L aligns with established physiological responses and is the correct answer.