Clinical Scenario: Type 1 Diabetes | Shock | Acid-base imbalance
A 12-year-old boy with known type 1 diabetes mellitus presents to the emergency department with vomiting, abdominal pain, and increasing drowsiness for 8 hours. He missed his insulin doses for the last 2 days because of poor oral intake. On examination, he is dehydrated with deep rapid breathing. His blood pressure is 90/56 mmHg, pulse rate is 124/min, and capillary refill time is 4 seconds. Investigations show serum sodium 136 mmol/L, chloride 102 mmol/L, bicarbonate 8 mmol/L, blood glucose 520 mg/dL, and strongly positive serum ketones. Arterial blood gas reveals pH 7.18, PaCO₂ 12 mmHg, and bicarbonate 8 mmol/L.
Which of the following best describes his acid–base disorder?
A. High anion gap metabolic acidosis with appropriate respiratory compensation
B. High anion gap metabolic acidosis with additional respiratory alkalosis
C. High anion gap metabolic acidosis with additional respiratory acidosis
D. Normal anion gap metabolic acidosis with respiratory alkalosis
E. Mixed high and normal anion gap metabolic acidosis
Correct answer & Explanation:
Correct Answer
B. High anion gap metabolic acidosis with additional respiratory alkalosis
Detailed Explanation
This child has diabetic ketoacidosis (DKA), but the question tests recognition of a mixed acid–base disorder.
Step 1: Identify the primary disturbance
pH = 7.18
The blood is acidemic.
Bicarbonate = 8 mmol/L
Therefore:
✅ Primary metabolic acidosis
Step 2: Calculate anion gap
Anion gap:
Normal anion gap ≈ 12
Therefore:
✅ High anion gap metabolic acidosis
Cause:
Accumulation of ketoacids:
- β-hydroxybutyrate
- Acetoacetate
Step 3: Assess respiratory compensation
Use Winter’s formula:
Expected PaCO₂:
18–22 mmHg
Measured PaCO₂:
12 mmHg
The PaCO₂ is significantly lower than expected.
Therefore, this is not compensation alone.
There is an additional:
✅ Respiratory alkalosis
Why can DKA cause respiratory alkalosis?
Possible causes include:
- Associated sepsis
- Pain and anxiety
- Excess respiratory drive
- Early systemic inflammatory response
The child is ventilating more than required to compensate.
Why the other options are incorrect
A. High anion gap metabolic acidosis with appropriate respiratory compensation
Incorrect.
The expected PaCO₂ is 18–22 mmHg.
Actual PaCO₂ = 12 mmHg.
The fall is excessive, indicating another respiratory process.
B. High anion gap metabolic acidosis with additional respiratory alkalosis
Correct.
The child has:
- Ketoacidosis → high AG metabolic acidosis
- Excessive CO₂ elimination → respiratory alkalosis
C. High anion gap metabolic acidosis with additional respiratory acidosis
Incorrect.
Respiratory acidosis requires an elevated PaCO₂.
D. Normal anion gap metabolic acidosis with respiratory alkalosis
Incorrect.
The anion gap is clearly elevated.
E. Mixed high and normal anion gap metabolic acidosis
Incorrect.
There is no evidence of additional bicarbonate loss (e.g., diarrhea).
CPSP Pearls ⭐
1. Always use Winter’s formula in metabolic acidosis
Expected PaCO₂:
1.5 × HCO₃⁻ + 8 ± 2
Do not assume low PaCO₂ is always compensation.
2. DKA acid–base pattern
Usually:
High anion gap metabolic acidosis + appropriate respiratory compensation
But mixed disorders occur.
3. How to recognize respiratory alkalosis in metabolic acidosis:
If:
Expected PaCO₂ = 18–22
Actual PaCO₂ = 12
→ the lungs are removing more CO₂ than required.
4. CPSP examination trap
A common mistake:
“DKA + low PaCO₂ = compensation”
Not always.
The question is testing whether the PaCO₂ compensation is appropriate or excessive.
