American Board of Internal Medicine (ABIM) Certification Practice Exam

In chronic respiratory alkalosis, what is the decrease in HCO3 for each 10 mm Hg decrease in PCO2?

1. 2.0 mEq/L

2. 3.0 mEq/L

3. 4.0-5.0 mEq/L

4. 6.0 mEq/L

The correct answer is: 4.0-5.0 mEq/L

In chronic respiratory alkalosis, the compensatory response by the kidneys is to excrete bicarbonate (HCO3-) in order to help restore acid-base balance. This adaptation typically occurs over days to weeks, and the relationship between the decrease in arterial partial pressure of carbon dioxide (PCO2) and the decrease in bicarbonate concentration follows a specific pattern. For every 10 mm Hg decrease in PCO2, there is generally a corresponding decrease in HCO3- concentration of about 4 to 5 mEq/L. This compensatory change is a part of the body’s efforts to counteract the increased pH caused by reduced PCO2 levels. Therefore, understanding this relationship is essential when evaluating patients with chronic respiratory alkalosis, as it informs clinicians of the expected bicarbonate levels and helps in monitoring and managing such patients effectively. The other options do not accurately reflect the established compensatory mechanisms in chronic respiratory alkalosis.