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Difficulty: MediumNephrology

A 26-year-old man is brought to the emergency department due to intractable, non-bilious vomiting for the past 4 days. He has type 1 diabetes mellitus and reports running out of his insulin prescription three days ago. He has been unable to tolerate any oral intake. He appears lethargic with dry mucous membranes. Vital signs show a heart rate of 118/min and blood pressure of 98/60 mm Hg.

Serum chemistry results are as follows:
Na+Na^+: 142 mEq/L
K+K^+: 3.3 mEq/L
Cl−Cl^-: 90 mEq/L
HCO3−HCO_3^-: 24 mEq/L
Blood urea nitrogen: 35 mg/dL
Creatinine: 1.4 mg/dL
Glucose: 520 mg/dL

Serum ketones are positive. Arterial blood gas (on room air) shows:
pH: 7.42
pCO2pCO_2: 38 mm Hg

Which of the following is the most accurate interpretation of this patient's acid-base status?

  1. Mixed high anion gap metabolic acidosis and metabolic alkalosisAnswer
  2. B
    Isolated high anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Normal acid-base status with an artifactual anion gap elevation
  4. D
    Primary respiratory alkalosis with appropriate metabolic compensation
  5. E
    Mixed normal anion gap metabolic acidosis and metabolic alkalosis

Answer

Mixed high anion gap metabolic acidosis and metabolic alkalosis
The correct interpretation involves identifying two independent metabolic processes that are neutralizing each other's effect on the overall pH. The patient's lack of insulin led to Diabetic Ketoacidosis (creating a High Anion Gap Metabolic Acidosis), which would normally drastically lower the HCO3−HCO_3^- and pH. Simultaneously, his intractable vomiting caused a severe loss of gastric hydrochloric acid (creating a Metabolic Alkalosis), which raises the HCO3−HCO_3^- and pH. The anion gap is unequivocally elevated at 28 mEq/L. By calculating the delta gap (28 - 12 = 16), we find that the expected bicarbonate should be around 8 mEq/L. Because the actual bicarbonate is 24 mEq/L, there is a large concurrent metabolic alkalosis present.

Step-by-Step Solution

1
Calculate the serum anion gap (AG) to screen for hidden acid-base disorders despite the normal pH.
AG=Na+−(Cl−+HCO3−)=142−(90+24)=28AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (90 + 24) = 28 mEq/L.
An elevated AG (normal is ~12 mEq/L) definitively identifies the presence of a High Anion Gap Metabolic Acidosis (HAGMA), which in this case is diabetic ketoacidosis.
2
Calculate the delta gap (ΔAG\Delta AG) to determine if there are additional metabolic derangements.
ΔAG=Patient’s AG−Normal AG=28−12=16\Delta AG = \text{Patient's AG} - \text{Normal AG} = 28 - 12 = 16 mEq/L.
For every 1 mEq/L rise in the anion gap, the serum HCO3−HCO_3^- should drop by approximately 1 mEq/L.
3
Calculate the expected serum bicarbonate if HAGMA were the only disturbance.
Expected HCO3−=24−ΔAG=24−16=8HCO_3^- = 24 - \Delta AG = 24 - 16 = 8 mEq/L.
Comparing the expected bicarbonate to the actual bicarbonate reveals hidden concurrent metabolic disorders.
4
Compare expected bicarbonate to actual bicarbonate.
The patient's actual HCO3−HCO_3^- is 24 mEq/L, which is 16 mEq/L higher than the expected 8 mEq/L.
Because the actual bicarbonate is significantly higher than what is expected from the ketoacidosis alone, there must be an independent primary process generating bicarbonate. This is a severe Metabolic Alkalosis secondary to the patient's protracted vomiting.

Key Concept

Delta Gap Interpretation in Mixed Acid-Base Disorders
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Topics

Nephrology

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