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Difficulty: MediumNephrology

A 52-year-old woman presents to the emergency department with a 2-day history of diffuse abdominal pain, polyuria, and severe, intractable vomiting. She has a history of type 1 diabetes mellitus and states she ran out of her insulin three days ago. Her temperature is 37.2°C (99.0°F), blood pressure is 95/60 mmHg, pulse is 118/min, and respirations are 24/min. Physical examination reveals dry mucous membranes and diffuse, mild abdominal tenderness without rebound or guarding. Laboratory studies show:

TestValue
Sodium140 mEq/L140 \text{ mEq/L}
Potassium3.4 mEq/L3.4 \text{ mEq/L}
Chloride90 mEq/L90 \text{ mEq/L}
Bicarbonate16 mEq/L16 \text{ mEq/L}
Blood urea nitrogen42 mg/dL42 \text{ mg/dL}
Creatinine1.8 mg/dL1.8 \text{ mg/dL}
Glucose510 mg/dL510 \text{ mg/dL}

Arterial blood gas on room air reveals:

ParameterValue
pH7.327.32
pCO2_232 mmHg32 \text{ mmHg}
pO2_296 mmHg96 \text{ mmHg}

Which of the following best describes this patient's acid-base status?

  1. High anion gap metabolic acidosis and metabolic alkalosisAnswer
  2. B
    High anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    High anion gap metabolic acidosis and respiratory alkalosis
  4. D
    High anion gap metabolic acidosis and respiratory acidosis
  5. E
    Non-anion gap metabolic acidosis and metabolic alkalosis

Answer

High anion gap metabolic acidosis and metabolic alkalosis
The correct answer identifies a mixed metabolic disorder. The patient has acidemia (pH 7.32) and a low bicarbonate (16 mEq/L), indicating metabolic acidosis. The anion gap is 140 - (90 + 16) = 34 mEq/L, confirming a High Anion Gap Metabolic Acidosis (HAGMA) due to DKA. Winter's formula (1.5 × 16 + 8 = 32) perfectly matches the measured pCO2, ruling out a primary respiratory disorder. However, a delta ratio must be performed for any HAGMA. The change in anion gap (34 - 12 = 22) compared to the change in bicarbonate (24 - 16 = 8) gives a delta ratio of 2.75. A ratio >2.0 means the bicarbonate has not fallen as much as expected given the accumulation of acids, which reveals a concurrent primary metabolic alkalosis caused by the patient's severe vomiting.

Step-by-Step Solution

1
Determine the primary disorder based on pH and bicarbonate.
pH is 7.32 (acidemia) and bicarbonate is low (16 mEq/L), indicating a primary metabolic acidosis.
Establishing the primary direction of the pH derangement dictates the subsequent calculation steps.
2
Calculate the serum anion gap.
Anion Gap = Na - (Cl + HCO3) = 140 - (90 + 16) = 34 mEq/L.
An elevated anion gap (>12 mEq/L) confirms the presence of a High Anion Gap Metabolic Acidosis (HAGMA), highly consistent with Diabetic Ketoacidosis in this patient.
3
Assess for appropriate respiratory compensation using Winter's formula.
Expected pCO2 = 1.5 × HCO3 + 8 ± 2 = 1.5 × 16 + 8 = 32 ± 2 mmHg. The measured pCO2 is 32 mmHg, meaning respiratory compensation is appropriate and there is no concurrent primary respiratory disorder.
A pCO2 outside the expected range would indicate an additional mixed respiratory disorder, but this patient's ventilation appropriately matches the metabolic acidosis.
4
Calculate the delta ratio to check for additional hidden metabolic disorders.
Delta Gap = 34 - 12 = 22. Delta Bicarbonate = 24 - 16 = 8. Delta Ratio = 22 / 8 = 2.75.
Because the delta gap is significantly larger than the drop in bicarbonate, the ratio is >2.0. This indicates an independent process is adding bicarbonate to the blood, signifying a concurrent metabolic alkalosis (caused by the patient's severe vomiting).

Key Concept

In patients with a high anion gap metabolic acidosis, the delta ratio (ΔAG / ΔHCO3) must be calculated to identify hidden, concurrent metabolic disorders; a ratio >2.0 indicates a superimposed metabolic alkalosis.
Estimated Time:2m 0s
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Topics

Nephrology

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