An ABG from a patient with diabetic ketoacidosis shows pH 6.95, PaCO2 30 mm Hg, HCO3- 6 mEq/L. This pattern represents what acid-base disorder?

An ABG from a patient with diabetic ketoacidosis shows pH 6.95, PaCO2 30 mm Hg, HCO3- 6 mEq/L. This pattern represents what acid-base disorder?

• A. Primary metabolic acidosis with appropriate respiratory compensation
• B. Primary metabolic alkalosis with respiratory compensation
• C. Primary respiratory acidosis with metabolic compensation
• D. Mixed metabolic and respiratory disorder

Answer: (A)

Interpreting acid-base status relies on identifying the primary disturbance and then checking whether the respiratory response matches what’s expected. In diabetic ketoacidosis you expect a primary metabolic acidosis (low bicarbonate) with respiratory compensation (hyperventilation lowering the CO2). Here, the pH is 6.95, which shows severe acidemia, and the bicarbonate is very low at 6 mEq/L, indicating a strong metabolic acidosis. If this were a pure metabolic acidosis with appropriate compensation, you’d use Winter’s formula: predicted PaCO2 ≈ 1.5 × [HCO3−] + 8 ≈ 1.5 × 6 + 8 ≈ 17 mm Hg (within a small ±2 range). The measured PaCO2, however, is 30 mm Hg, which is not at all “appropriately compensated” by metabolic acidosis; it’s actually higher than expected, indicating a second process causing CO2 retention. Therefore, the pattern reflects a mixed disorder: metabolic acidosis from the ketoacidosis plus a concurrent respiratory acidosis or hypoventilation component. In other words, there’s metabolic acidosis with an inadequate respiratory response (or additional respiratory dysfunction) rather than a simple metabolic acidosis with proper compensation.

Interpreting acid-base status relies on identifying the primary disturbance and then checking whether the respiratory response matches what’s expected. In diabetic ketoacidosis you expect a primary metabolic acidosis (low bicarbonate) with respiratory compensation (hyperventilation lowering the CO2).

Here, the pH is 6.95, which shows severe acidemia, and the bicarbonate is very low at 6 mEq/L, indicating a strong metabolic acidosis. If this were a pure metabolic acidosis with appropriate compensation, you’d use Winter’s formula: predicted PaCO2 ≈ 1.5 × [HCO3−] + 8 ≈ 1.5 × 6 + 8 ≈ 17 mm Hg (within a small ±2 range). The measured PaCO2, however, is 30 mm Hg, which is not at all “appropriately compensated” by metabolic acidosis; it’s actually higher than expected, indicating a second process causing CO2 retention.

Therefore, the pattern reflects a mixed disorder: metabolic acidosis from the ketoacidosis plus a concurrent respiratory acidosis or hypoventilation component. In other words, there’s metabolic acidosis with an inadequate respiratory response (or additional respiratory dysfunction) rather than a simple metabolic acidosis with proper compensation.