Which ABG interpretation indicates respiratory alkalosis?

• A. The pH is high and the PCO2 is low.
• B. The pH is low and the PCO2 is high.
• C. The pH is high and the PCO2 is high.
• D. The pH is low and the PCO2 is low.

Answer: (A)

Respiratory alkalosis shows an elevated pH with a lowered PCO2 on arterial blood gas. When you hyperventilate, you blow off CO2, which shifts the chemical equilibrium and reduces hydrogen ion concentration, raising the pH. So the pattern of high pH and low PCO2 is the hallmark of a respiratory origin.

The other patterns don’t fit this scenario: a high pH with high PCO2 would imply metabolic factors maintaining or raising pH despite CO2 build-up, not a primary respiratory alkalosis; a low pH with low PCO2 indicates a metabolic acidosis with compensatory changes; and a low pH with high PCO2 points to respiratory acidosis.