If a patient exhibits low PaCO2 and high pH, which acid-base disorder is most likely?

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

Answer: (C)

When the pH is high (alkalemia) and the PaCO2 is low, the primary issue is respiratory rather than metabolic. Carbon dioxide acts as an acid in the body; blowing off CO2 via rapid breathing reduces acidity and raises pH. So a low PaCO2 with an elevated pH points to respiratory alkalosis. The body may compensate over time by reducing bicarbonate reabsorption in the kidneys, but the key finding is the combination of low CO2 and alkalemia. The other patterns don’t fit: metabolic alkalosis would not hinge on a low CO2, respiratory acidosis requires high CO2 with a low pH, and metabolic acidosis shows low pH with typically low HCO3- rather than a high pH.