What is the most accurate method to assess acid-base balance in a patient with suspected respiratory failure, and which ABG findings indicate acute respiratory acidosis?

• A. Arterial blood gas analysis showing PaCO2 >45 mmHg and pH decreased with minimal bicarbonate compensation.
• B. Venous blood gas analysis showing elevated PaCO2.
• C. Serum bicarbonate level alone indicating metabolic alkalosis.
• D. Capnography alone showing elevated CO2.

Answer: (A)

Interpreting acid-base balance in respiratory failure relies on arterial blood gas because it directly measures the variables that define the disturbance: PaCO2, pH, and bicarbonate. Venous gas is less reliable for assessing systemic acid-base status, capnography shows CO2 in the expired gas and reflects ventilation, not the blood's acid-base equation, and a serum bicarbonate value alone cannot determine the overall acid-base disorder.

In acute respiratory acidosis, ventilation fails and PaCO2 rises, leading to acidemia. On an arterial blood gas you would expect a high PaCO2 (above 45 mmHg) with a decreased pH. Because compensation begins only slowly via renal mechanisms, bicarbonate is not significantly elevated yet; you may see little to no bicarbonate compensation. Over time (chronic cases) bicarbonate increases as the kidneys compensate, but in the acute setting the primary clues are elevated CO2 with acidemia and minimal compensatory rise in bicarbonate.

So the most accurate method is arterial blood gas, and the pattern indicating acute respiratory acidosis is high PaCO2 with low pH and little bicarbonate compensation.