ABG interpretation: pH 7.42; pCO2 50 mm Hg; HCO3- 34 mmol/L. Which pattern best fits this result?

ABG interpretation: pH 7.42; pCO2 50 mm Hg; HCO3- 34 mmol/L. Which pattern best fits this result?

• A. Compensated respiratory acidosis
• B. Compensated metabolic acidosis
• C. Uncompensated respiratory alkalosis
• D. Uncompensated metabolic alkalosis

Answer: (A)

The key idea is distinguishing the primary disorder from the body's compensation by looking at pCO2, HCO3-, and pH together. Here, the pCO2 is elevated (50 mm Hg), indicating a respiratory problem causing CO2 retention. The bicarbonate is also high (34 mmol/L), which is the kidney’s metabolic response to chronic CO2 retention. The pH is essentially normal (7.42) because the buffering by the raised bicarbonate has compensated for the elevated CO2. This combination—elevated CO2 with a strong metabolic (bicarbonate) compensation and a normal pH—fits a compensated respiratory acidosis, most consistent with a chronic process. Why this fits best: the primary disturbance is respiratory (high CO2), and the elevated bicarbonate shows the body has had time to compensate. If it were uncompensated, the pH would be clearly abnormal (acidic). If it were metabolic in origin, the bicarbonate would be low or the pattern wouldn’t show the same CO2 elevation with compensation.

The key idea is distinguishing the primary disorder from the body's compensation by looking at pCO2, HCO3-, and pH together. Here, the pCO2 is elevated (50 mm Hg), indicating a respiratory problem causing CO2 retention. The bicarbonate is also high (34 mmol/L), which is the kidney’s metabolic response to chronic CO2 retention. The pH is essentially normal (7.42) because the buffering by the raised bicarbonate has compensated for the elevated CO2. This combination—elevated CO2 with a strong metabolic (bicarbonate) compensation and a normal pH—fits a compensated respiratory acidosis, most consistent with a chronic process.

Why this fits best: the primary disturbance is respiratory (high CO2), and the elevated bicarbonate shows the body has had time to compensate. If it were uncompensated, the pH would be clearly abnormal (acidic). If it were metabolic in origin, the bicarbonate would be low or the pattern wouldn’t show the same CO2 elevation with compensation.