American Board of Internal Medicine (ABIM) Certification Practice Exam

What is a significant consequence of a decrease in PCO2 by 10 mm Hg on serum bicarbonate levels?

1. A decrease of 1.0 mEq/L

2. A decrease of 2.0 mEq/L

3. An increase of 2.5 mEq/L

4. No significant change

The correct answer is: A decrease of 2.0 mEq/L

A decrease in PCO2 by 10 mm Hg typically leads to a specific change in serum bicarbonate levels due to the body's compensatory mechanisms in response to changes in carbon dioxide levels. In the context of respiratory mechanisms, when PCO2 decreases, there is a decrease in carbonic acid formation, which can lead to a drop in serum bicarbonate levels. The relationship between PCO2 and serum bicarbonate levels is particularly important in understanding acid-base balance. For every decrease of 10 mm Hg in PCO2, there is generally a decrease of about 2 mEq/L in serum bicarbonate. This correlates with the concept of respiratory compensation, where a respiratory alkalosis resulting from a drop in CO2 leads to a corresponding decrease in bicarbonate as the body works to restore pH balance. This principle helps medical professionals predict and understand changes in acid-base status in patients, particularly in those with conditions affecting respiration. Therefore, recognizing that a decrease in PCO2 of 10 mm Hg results in a decrease of approximately 2 mEq/L in bicarbonate provides valuable insight into acid-base homeostasis during clinical assessments.