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

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

Answer: (D)

The main idea here is that elevated PaCO2 with a low pH points to a respiratory source of acidosis. CO2 buildup drives the formation of carbonic acid, which releases hydrogen ions and lowers blood pH. When ventilation is reduced, CO2 accumulates, so PaCO2 rises and pH falls, indicating respiratory acidosis.

Metabolic disorders involve changes in bicarbonate. In metabolic acidosis, HCO3- is low and respiration may compensate by lowering PaCO2; in metabolic alkalosis, HCO3- is high with compensatory increases in PaCO2. Respiratory alkalosis shows a low PaCO2 with a high pH. Given the combination of high PaCO2 and low pH, the most likely disorder is respiratory acidosis. If it persists, the kidneys may compensate by raising bicarbonate, which can partially restore pH over time.