A patient with diabetic ketoacidosis (pH 7.18, HCO3− 8 mEq/L, PaCO2 20 mmHg, anion gap 28) is intubated and ventilated at a rate that raises PaCO2 to 40 mmHg. What will happen to the pH immediately?
- A pH will improve towards 7.35 because CO2 is a weaker acid than the organic acids
- B pH will fall further (worsen), potentially to a lethal level ✓
- C pH will remain unchanged because Kussmaul breathing had equilibrated the system
- D pH will rise to 7.4 because the PaCO2 is now in the normal range
Correct answer: B. pH will fall further (worsen), potentially to a lethal level
Explanation
In DKA, the patient hyperventilates (Kussmaul breathing) to compensate for metabolic acidosis, lowering PaCO2 to maintain a higher pH. If ventilation abruptly normalizes PaCO2 to 40 mmHg (removing the respiratory compensation) while severe metabolic acidosis persists (HCO3− 8 mEq/L), pH will acutely fall further. Using Henderson-Hasselbalch: pH = 6.1 + log(8/[0.03×40]) = 6.1 + log(8/1.2) = 6.1 + log(6.67) = 6.1 + 0.82 = 6.92, which is severely acidotic and life-threatening.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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