Clinical Scenario: Diarrhea | Electrolytes Imbalance
A 6-year-old boy is admitted with 3 days of profuse watery diarrhea and repeated vomiting. He is lethargic, tachypneic, and moderately dehydrated. His blood pressure is 86/52 mmHg and capillary refill time is 4 seconds. Laboratory investigations show serum sodium 140 mmol/L, chloride 112 mmol/L, bicarbonate 10 mmol/L, glucose 96 mg/dL, urea 18 mmol/L, and creatinine 0.7 mg/dL. Arterial blood gas shows pH 7.25, PaCO₂ 24 mmHg, and HCO₃⁻ 10 mmol/L. Serum lactate is 4.5 mmol/L. Which of the following best describes his acid–base disturbance?
A. High anion gap metabolic acidosis with appropriate respiratory compensation
B. High anion gap metabolic acidosis with respiratory alkalosis
C. High anion gap metabolic acidosis with respiratory acidosis
D. Normal anion gap metabolic acidosis with respiratory alkalosis
E. Mixed high and normal anion gap metabolic acidosis with appropriate respiratory compensation
Correct answer & Explanation:
Correct answer: E. Mixed high and normal anion gap metabolic acidosis with appropriate respiratory compensation
Detailed explanation:
This question requires three separate steps:
- Identify the primary disturbance.
- Assess whether respiratory compensation is appropriate.
- Determine whether there is more than one metabolic process.
Step 1: Calculate the anion gap
Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
= 140 − (112 + 10)
= 18 mmol/L
The usual anion gap is approximately 12 mmol/L when potassium is excluded.
Therefore, this child has an elevated anion gap metabolic acidosis.
The elevated lactate provides an additional explanation: dehydration and hypoperfusion have produced lactic acidosis.
However, that is not the whole story.
Step 2: Assess respiratory compensation
For metabolic acidosis, use Winter’s formula:
Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2
= 1.5 × 10 + 8
= 23 ± 2 mmHg
Expected PaCO₂ is therefore approximately 21–25 mmHg.
Measured PaCO₂ = 24 mmHg.
Therefore, respiratory compensation is appropriate.
There is no significant additional respiratory acidosis or respiratory alkalosis.
Step 3: Look for a second metabolic process
The important clue is the combination of:
- Anion gap = 18
- Bicarbonate = 10
- Chloride = 112
The increase in anion gap is:
18 − 12 = 6 mmol/L
If this were a pure high-anion-gap metabolic acidosis, the fall in bicarbonate should be roughly similar to the rise in anion gap.
But bicarbonate has fallen from approximately 24 to 10:
24 − 10 = 14 mmol/L
So:
ΔHCO₃⁻ = 14
while:
ΔAG = 6
The bicarbonate has fallen considerably more than expected from the rise in anion gap.
This indicates an additional normal-anion-gap metabolic acidosis.
The likely cause is bicarbonate loss from severe diarrhea.
Thus the child has:
High-anion-gap metabolic acidosis
+
Normal-anion-gap metabolic acidosis
with appropriate respiratory compensation.
Therefore, the best answer is E.
Why the other options are wrong
A. High anion gap metabolic acidosis with appropriate respiratory compensation
Partly correct, but incomplete.
There is indeed a high-anion-gap metabolic acidosis and respiratory compensation is appropriate. However, the disproportionate fall in bicarbonate indicates an additional normal-anion-gap metabolic acidosis.
B. High anion gap metabolic acidosis with respiratory alkalosis
Incorrect.
A respiratory alkalosis would produce a PaCO₂ lower than the expected compensatory range.
Expected PaCO₂ is approximately 21–25 mmHg, while the measured value is 24 mmHg.
Therefore compensation is appropriate.
C. High anion gap metabolic acidosis with respiratory acidosis
Incorrect.
Respiratory acidosis would mean inadequate CO₂ elimination, producing a PaCO₂ higher than expected.
The PaCO₂ of 24 mmHg is appropriate for the degree of metabolic acidosis.
D. Normal anion gap metabolic acidosis with respiratory alkalosis
Incorrect.
The anion gap is clearly elevated at 18 mmol/L.
Although diarrhea produces normal-anion-gap acidosis, this child also has lactic acidosis from hypoperfusion, producing an elevated anion gap.
E. Mixed high and normal anion gap metabolic acidosis with appropriate respiratory compensation
Correct.
The child has:
- High AG acidosis: lactic acidosis from hypoperfusion
- Normal AG acidosis: bicarbonate loss from diarrhea
- Appropriate respiratory compensation: PaCO₂ 24 mmHg
Pearls ⭐
1. Don’t stop after calculating the anion gap
An elevated anion gap tells you that a high-AG metabolic acidosis exists, but it does not tell you whether another metabolic disorder is present.
Always consider the delta gap/delta bicarbonate relationship when the clinical situation warrants it.
2. Winter’s formula
For metabolic acidosis:
Expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2
- Actual PaCO₂ within expected range → appropriate compensation
- Higher than expected → additional respiratory acidosis
- Lower than expected → additional respiratory alkalosis
3. Diarrhea classically causes
Normal-anion-gap metabolic acidosis
because of bicarbonate loss and relative chloride retention.
4. Shock/hypoperfusion causes
Lactic acidosis → high-anion-gap metabolic acidosis.
Therefore, severe diarrhea complicated by hypovolemic shock can produce both types of metabolic acidosis simultaneously.
5. High-yield exam trap
A candidate sees:
Diarrhea + low bicarbonate
and immediately chooses:
Normal-anion-gap metabolic acidosis.
But the elevated lactate and anion gap tell you that this child has more than one metabolic process.
