American Board of Internal Medicine (ABIM) Certification Practice Exam

In respiratory alkalosis, what is the expected change in HCO3 in acute cases for each 10 mm Hg decrease in PCO2?

1. 1.0 mEq/L

2. 2.0 mEq/L

3. 3.0 mEq/L

4. 4.0 mEq/L

The correct answer is: 2.0 mEq/L

In acute respiratory alkalosis, there is a rapid decrease in arterial carbon dioxide tension (PCO2) due to hyperventilation, which leads to a drop in hydrogen ions (H+) in the blood, raising the blood pH. The compensatory response primarily involves a decrease in bicarbonate (HCO3−) levels as the kidneys take some time to respond to acidosis or alkalosis. For an acute change, it is established that for every 10 mm Hg decrease in PCO2, approximately 2 mEq/L decrease in bicarbonate (HCO3−) is expected. This is because the body attempts to compensate for the change in pH primarily through the respiratory system first, followed by renal compensation, which takes longer to manifest. Since the question specifies acute cases, this response reflects the immediate physiological changes without the influence of more gradual renal compensation that would occur in chronic conditions. Thus, the expected change in bicarbonate in acute respiratory alkalosis correlating with a 10 mm Hg decrease in PCO2 is accurately represented by the choice that indicates a 2.0 mEq/L decrease in HCO3−.