In a patient with COPD and chronic respiratory acidosis on ABG, what pattern would you expect?

• A. Low pH with elevated PaCO2; possible elevated bicarbonate if chronic compensation
• B. Low PaO2 only
• C. High pH with decreased PaCO2
• D. Normal pH with normal PaCO2

Answer: (A)

In COPD, chronic CO2 retention causes a primary respiratory acidosis. The immediate ABG pattern you’d expect is a low pH with an elevated PaCO2, reflecting the retained carbon dioxide and acidemia from impaired ventilation. Because the condition is chronic, the kidneys compensate by increasing bicarbonate reabsorption, so the bicarbonate level often rises as well. This metabolic compensation blunts the acidosis somewhat, so you may see a near-normal or only mildly acidotic pH with a high PaCO2 and a higher bicarbonate.

This differs from patterns like respiratory alkalosis (high pH with low PaCO2), hypoxemia without CO2 retention (low PaO2 with normal or near-normal pH and CO2), or a normal ABG (which wouldn’t reflect the COPD-related acid-base disturbance).