A 72-year-old woman with diabetic ketoacidosis presents with: pH 7.18, PaCO₂ 22 mmHg, HCO₃⁻ 8 mEq/L, anion gap 18 mEq/L. Which of the following is NOT a correct interpretation of her acid-base status?

Question

Accepted Answer

C. A concurrent respiratory acidosis is present because PaCO₂ remains elevated at 22 mmHg. ## Analysis of Diabetic Ketoacidosis Acid-Base Status

**Key Point:** This patient has **primary metabolic acidosis with appropriate respiratory compensation** — NOT a concurrent respiratory acidosis. The PaCO₂ of 22 mmHg is **appropriately low**, not inappropriately elevated.

### Step-by-Step Interpretation

1. **Primary disorder:** pH 7.18 is acidemic; HCO₃⁻ 8 mEq/L is low → **metabolic acidosis**.
2. **Anion gap:** 18 mEq/L (normal ~8–12) → **high-anion-gap metabolic acidosis** (from ketoacids in DKA).
3. **Expected respiratory response:** Winter's formula predicts expected PaCO₂:
   $$	ext{Expected PaCO₂} = 1.5 	imes [HCO₃^-] + 8 \pm 2$$
   $$= 1.5 	imes 8 + 8 \pm 2 = 12 + 8 \pm 2 = 20 \pm 2 	ext{ mmHg (range: 18–22 mmHg)}$$
4. **Observed PaCO₂:** 22 mmHg falls **within the expected range** → respiratory compensation is **appropriate**.

### Interpretation Table

| Parameter | Value | Interpretation |
| --- | --- | --- |
| pH | 7.18 | Acidemia |
| HCO₃⁻ | 8 mEq/L | Low (metabolic acidosis) |
| PaCO₂ | 22 mmHg | Low (respiratory compensation) |
| Anion gap | 18 mEq/L | High (ketoacidosis) |
| Expected PaCO₂ (Winter's) | 18–22 mmHg | Observed 22 is at upper limit — appropriate |

**High-Yield:** Winter's formula is the gold standard for assessing whether respiratory compensation is appropriate in metabolic acidosis.

### Why Option 3 Is Incorrect

Option 3 claims "a concurrent respiratory acidosis is present because PaCO₂ remains elevated at 22 mmHg."

- **This is FALSE.** A PaCO₂ of 22 mmHg is **low**, not elevated.
- **Respiratory acidosis** requires PaCO₂ > 45 mmHg.
- PaCO₂ = 22 mmHg represents **appropriate hyperventilation** (respiratory alkalosis *response*), not respiratory acidosis.
- The statement misinterprets the direction and meaning of the PaCO₂ value.

**Clinical Pearl:** In DKA, the respiratory system is working *correctly* — hyperventilation (Kussmaul respiration) lowers PaCO₂ to compensate for metabolic acidosis. If PaCO₂ were elevated (e.g., 45–50 mmHg) despite severe metabolic acidosis, *that* would signal concurrent respiratory acidosis (a dangerous mixed disorder).

Answer

A. The anion gap of 18 indicates a high-anion-gap metabolic acidosis from ketoacids

Answer

B. The expected PaCO₂ for this degree of metabolic acidosis is approximately 20–24 mmHg

Answer

D. The primary disorder is metabolic acidosis with appropriate respiratory compensation

A 72-year-old woman with diabetic ketoacidosis presents with: pH 7.18, PaCO₂ 22 mmHg, HCO₃⁻ 8 mEq/L, anion gap 18 mEq/L. Which of the following is NOT a correct interpretation of her acid-base status?

Explanation

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