An adolescent in the ED with fever and persistent right lower quadrant abdominal pain is anxious, fearful, and hyperventilating. The nurse anticipates which acid-base imbalance?

• A. Metabolic acidosis
• B. Metabolic alkalosis
• C. Respiratory acidosis
• D. Respiratory alkalosis

Answer: (D)

Hyperventilation from anxiety shifts the acid-base balance toward respiratory alkalosis. When someone breathes rapidly, they blow off CO2 faster than it’s produced. CO2 combines with water to form carbonic acid, which dissociates into hydrogen ions and bicarbonate. As CO2 is expelled, the concentration of hydrogen ions falls, pH rises, and the blood becomes alkalemic. The body can compensate by the kidneys later on by excreting bicarbonate, but that renal adjustment takes hours to days, so in the acute ED setting the hallmark is low CO2 with a higher-than-normal pH. The patient’s anxious, hyperventilating state fits this pattern.

Metabolic acidosis would involve a low bicarbonate with a low pH due to metabolic acids accumulating or bicarbonate loss. Metabolic alkalosis would have elevated bicarbonate with high pH, usually from loss of hydrogen or gain of bicarbonate. Respiratory acidosis would present with high CO2 and low pH due to hypoventilation. In this scenario, the rapid breathing from anxiety best explains respiratory alkalosis.