American Board of Internal Medicine (ABIM) Certification Practice Exam

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What is the expected change in PCO2 for each 1.0 mEq/L increase in HCO3 in metabolic alkalosis?

0.5 mm Hg
0.7 mm Hg
1.0 mm Hg
1.3 mm Hg

The correct answer is: 0.7 mm Hg

In the context of metabolic alkalosis, the relationship between bicarbonate (HCO3) levels and carbon dioxide (PCO2) levels is rooted in the body's compensatory mechanisms. When metabolic alkalosis occurs, typically characterized by an increase in HCO3, the body attempts to maintain acid-base homeostasis. For each increase of 1.0 mEq/L in HCO3, the expected change in the partial pressure of carbon dioxide (PCO2) is approximately 0.7 mm Hg. This reflects the body's physiological response to the elevated bicarbonate. The respiratory compensation involves hypoventilation or a decrease in respiratory rate, which leads to CO2 retention. Thus, a rise in HCO3 concentration results in a corresponding increase in PCO2 as the respiratory system adjusts to maintain equilibrium, but this adjustment is not a 1:1 increase. The factor of 0.7 mm Hg is well-established in clinical practice, allowing healthcare professionals to predict expected changes in blood gases during metabolic alkalosis accurately. This understanding is crucial for diagnosing and managing conditions involving electrolyte imbalances and identifying appropriate interventions to correct the acid-base disturbance.