Which statement describes the hallmarks of respiratory acidosis?

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

Answer: (C)

Respiratory acidosis arises when ventilation is reduced, allowing carbon dioxide to accumulate in the blood. That excess CO2 drives the hydrogen ion concentration up, lowering the pH (acidemia). The body’s primary compensatory response is renal: the kidneys retain and generate bicarbonate (HCO3-) to buffer the acid and help raise the pH, with more noticeable compensation in chronic cases.

Thus, the hallmark is high PaCO2 with a low pH, accompanied by elevated HCO3- as compensation. The other patterns describe different scenarios: low PaCO2 with high pH indicates respiratory alkalosis; low HCO3- with low pH points to metabolic acidosis with respiratory compensation; and normal CO2 with normal pH shows no primary disturbance.