In which ABG scenario would PaCO2 be decreased as a compensatory response?

• A. Metabolic acidosis with respiratory compensation
• B. Primary respiratory acidosis with no compensation
• C. Metabolic alkalosis with respiratory compensation
• D. Primary respiratory alkalosis with metabolic compensation

Answer: (A)

When the body faces metabolic acidosis, the respiratory system responds by blowing off more CO2 to raise pH. Lowering PaCO2 shifts the carbonic acid balance toward fewer hydrogen ions, helping to counteract the acidic state. This respiratory compensation follows the primary metabolic problem of low bicarbonate.

So in metabolic acidosis with respiratory compensation, PaCO2 would be decreased as the body attempts to normalize pH by removing CO2 through increased ventilation. Winter’s formula can help estimate the expected PaCO2 in this situation: approximately 1.5 × [HCO3-] + 8 ± 2.

The other scenarios don’t produce a decreased PaCO2 as a compensatory response. Primary respiratory acidosis already elevates PaCO2. Metabolic alkalosis would drive ventilation down, increasing PaCO2. In primary respiratory alkalosis, metabolic compensation would raise bicarbonate to help normalize pH, not further reduce PaCO2.