Which statement describes the hallmarks of respiratory alkalosis?

• A. Normal PaCO2 and normal pH.
• B. Low PaCO2, High pH; compensation: Low HCO3.
• C. High PaCO2, low pH; compensation: High HCO3.
• D. Low PaCO2, low pH; compensation: Low HCO3.

Answer: (C)

Respiratory alkalosis happens when ventilation increases and blows off CO2, lowering the PaCO2. Since CO2 is acidic, reducing it raises the blood pH, producing alkalemia. The body’s compensatory response is renal: it decreases bicarbonate (HCO3-) reabsorption to help bring the pH back toward normal. So the hallmark pattern is a low PaCO2 with a high pH, plus a compensatory decrease in bicarbonate that may be more pronounced in chronic cases.

In contrast, options describing normal values, or a high PaCO2 with low pH, reflect other conditions (no disturbance or respiratory acidosis, respectively), not respiratory alkalosis. The option mentioning high PaCO2 with low pH would be respiratory acidosis with metabolic compensation, which is the opposite scenario.