An acute increase in PaCO2 of 10 mm Hg will result in a decrease in pH of

An acute increase in PaCO2 of 10 mm Hg will result in a decrease in pH of

• A. 0.01 pH units
• B. 0.02 pH units
• C. 0.04 pH units
• D. 0.08 pH units

Answer: (D)

In acute respiratory acidemia, a rise in PaCO2 lowers pH because more CO2 shifts the carbonic acid–bicarbonate system toward H+, and there isn’t time for bicarbonate to compensate yet. The relationship can be seen with Henderson-Hasselbalch: pH = 6.1 + log([HCO3-]/(0.03×PaCO2)). If bicarbonate stays the same, increasing PaCO2 reduces the ratio and drops pH. For a 10 mm Hg increase in PaCO2, the pH fall is about 0.08 units. For example, with PaCO2 around 40 and HCO3- about 24, pH is about 7.40; increasing PaCO2 to 50 with the same bicarbonate brings pH to roughly 7.30, a drop close to 0.08. Therefore, the best answer is a decrease of about 0.08 pH units.

In acute respiratory acidemia, a rise in PaCO2 lowers pH because more CO2 shifts the carbonic acid–bicarbonate system toward H+, and there isn’t time for bicarbonate to compensate yet. The relationship can be seen with Henderson-Hasselbalch: pH = 6.1 + log([HCO3-]/(0.03×PaCO2)). If bicarbonate stays the same, increasing PaCO2 reduces the ratio and drops pH. For a 10 mm Hg increase in PaCO2, the pH fall is about 0.08 units. For example, with PaCO2 around 40 and HCO3- about 24, pH is about 7.40; increasing PaCO2 to 50 with the same bicarbonate brings pH to roughly 7.30, a drop close to 0.08. Therefore, the best answer is a decrease of about 0.08 pH units.