What ABG pattern is typical in an acute COPD exacerbation, and how does metabolic compensation distinguish acute from chronic disease?

• A. Acute: respiratory alkalosis with elevated pH and decreased PaCO2; metabolic compensation would be minimal.
• B. Chronic COPD shows elevated HCO3- from renal compensation.
• C. Acute COPD presents with normal ABG values.
• D. Acute: respiratory acidosis with elevated PaCO2 and decreased pH; minimal metabolic compensation; chronic COPD shows elevated HCO3- from renal compensation.

Answer: (D)

In an acute COPD exacerbation the lungs can’t effectively eliminate CO2, so the main ABG pattern is respiratory acidosis from CO2 retention. This means the pH is decreased and PaCO2 is elevated. Because this is an abrupt change, metabolic compensation by the kidneys hasn’t had time to develop, so bicarbonate (HCO3-) remains normal or only minimally elevated.

If COPD is chronic, the body adapts to chronically high CO2 by the kidneys retaining more bicarbonate, so the ABG shows an elevated HCO3- as metabolic compensation. The pH may be closer to normal or only mildly acidemic despite high PaCO2.

So the best description is acute respiratory acidosis with high PaCO2 and low pH, with minimal metabolic compensation; chronic COPD, in contrast, shows elevated HCO3- from renal compensation.