In metabolic acidosis, what compensatory change occurs in ventilation?

• A. In metabolic acidosis, PaCO2 increases.
• B. In metabolic acidosis, PaCO2 decreases as compensation.
• C. There is no change in PaCO2.
• D. Both PaCO2 and HCO3- increase.

Answer: (B)

Metabolic acidosis lowers bicarbonate and pH, so the respiratory system responds by increasing ventilation to blow off CO2. This reduce in CO2 decreases the concentration of carbonic acid, helping raise the pH toward normal. In other words, the compensatory change is a decrease in PaCO2 due to hyperventilation.

If PaCO2 rose, or stayed the same, there would be no appropriate respiratory compensation, and increasing both PaCO2 and HCO3- wouldn’t fit metabolic acidosis (where HCO3- is low). A quick way to check is to remember that with metabolic acidosis, the body should hyperventilate to lower PaCO2, and you can estimate the expected PaCO2 using Winter’s formula: PaCO2 ≈ 1.5 × [HCO3-] + 8 ± 2, which typically results in a PaCO2 lower than normal.