American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the expected acute decrease in serum bicarbonate 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

The relationship between carbon dioxide (PCO2) levels and serum bicarbonate is an important aspect of acid-base physiology, particularly in understanding how the body compensates for respiratory changes. In cases where there is an acute decrease in PCO2, typically due to hyperventilation, a compensatory decrease in serum bicarbonate occurs as the body attempts to maintain acid-base balance. For every 10 mm Hg decrease in PCO2, the expected acute decrease in serum bicarbonate is approximately 2.0 mEq/L. This relationship is derived from the physiological principles governing the bicarbonate buffering system and how bicarbonate and carbonic acid interact in the blood. When the level of carbon dioxide decreases, there is a corresponding decrease in carbonic acid, which leads to a reduction in bicarbonate as part of the body’s compensation mechanism. This understanding is essential for clinicians in managing patients with respiratory disorders where shifts in PCO2 can occur, ensuring that they can evaluate and interpret the associated changes in acid-base status effectively.