American Board of Internal Medicine (ABIM) Certification Practice Exam

For acute respiratory acidosis, what is the increase in HCO3 for each 10 mm Hg increase in PCO2?

1. 1.0 mEq/L

2. 2.0 mEq/L

3. 3.5 mEq/L

4. 4.0 mEq/L

The correct answer is: 1.0 mEq/L

In acute respiratory acidosis, for every 10 mm Hg increase in arterial PCO2, the bicarbonate ion (HCO3-) concentration typically increases by approximately 1 mEq/L. This relationship is well established in understanding the body's compensatory mechanisms in response to respiratory acidosis. When PCO2 rises due to respiratory failure or hypoventilation, the resulting increase in carbonic acid leads to a decrease in pH (indicating acidity). In response, the kidneys attempt to compensate for this acidosis primarily by increasing the reabsorption of bicarbonate in an effort to raise the pH back towards normal. However, this renal compensation takes time and is not immediate, reflecting the acute nature of the condition. In acute respiratory acidosis, the bicarbonate levels will show a modest increase, reflecting the body's initial response to rising CO2 levels. The incremental change of approximately 1 mEq/L per 10 mm Hg increase in PCO2 is critical for clinical practitioners to understand, as it helps interpret arterial blood gas results in the context of respiratory disturbances. This understanding aids in diagnosing the severity of acidosis and guiding appropriate management strategies. The options that suggest a higher increment of bicarbonate per 10 mm Hg increase in P