American Board of Internal Medicine (ABIM) Certification Practice Exam

What is the formula for calculating the expected PCO2 in a patient with acute metabolic acidosis?

1. PCO2 = [HCO3] + 8

2. PCO2 = (1.5) [HCO3] + 8

3. PCO2 = 0.8 [HCO3]

4. PCO2 = (1.5) [HCO3] - 2

The correct answer is: PCO2 = (1.5) [HCO3] + 8

In the context of acute metabolic acidosis, understanding the relationship between bicarbonate (HCO3) levels and carbon dioxide (PCO2) is essential for estimating the respiratory compensation that occurs in response to metabolic derangements. The correct formula, PCO2 = (1.5) [HCO3] + 8, accurately reflects how the body's respiratory system adjusts to a decrease in pH. In metabolic acidosis, when HCO3 levels drop, the body tries to compensate by increasing ventilation to lower PCO2, which subsequently raises the pH towards normal. This formula is derived from the principles of acid-base balance, indicating that for every decrease of 1 mmol/L in bicarbonate, the expected decrease in PCO2 should be approximately 1.5 mmHg plus an adjustment factor (in this case, +8) to account for the inherent variance in normal physiological responses. This allows for an estimation of PCO2 that helps clinicians evaluate the adequacy of respiratory compensation in relation to the degree of metabolic acidosis present. Understanding this relationship is crucial in clinical practice, as it provides insight into whether a patient's compensatory mechanisms are sufficient or if further clinical intervention may be needed.