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Difficulty: HardNephrology

An unresponsive 26-year-old woman is admitted to the intensive care unit after being found lethargic in her apartment by a roommate. The roommate notes that the patient has a history of type 1 diabetes mellitus and has been suffering from a severe gastrointestinal illness with copious vomiting for the past four days. The patient stopped all insulin therapy during this time because she could not tolerate oral intake. At presentation, her blood pressure is 92/58 mm Hg, pulse is 122/min, and respiratory rate is 28/min. Physical examination reveals deep, rapid breathing and profoundly dry mucous membranes.

Initial laboratory evaluation reveals the following:
Serum sodium: 142 mEq/L
Serum potassium: 3.2 mEq/L
Serum chloride: 96 mEq/L
Serum bicarbonate: 16 mEq/L
Blood urea nitrogen: 52 mg/dL
Serum creatinine: 1.8 mg/dL
Plasma glucose: 610 mg/dL

Arterial blood gas analysis on room air:
pH: 7.32
pCO2: 32 mm Hg
pO2: 98 mm Hg

Based on these findings, 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 and respiratory alkalosis
  3. C
    High anion gap metabolic acidosis only
  4. D
    Normal anion gap metabolic acidosis and metabolic alkalosis
  5. E
    High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis

Answer

High anion gap metabolic acidosis and metabolic alkalosis
The patient's laboratory values reveal a mixed acid-base disorder. First, the anion gap (AG) is calculated as Na+−(Cl−+HCO3−)=142−(96+16)=30Na^+ - (Cl^- + HCO_3^-) = 142 - (96 + 16) = 30 mEq/L. An AG greater than 12 indicates a high anion gap metabolic acidosis (HAGMA), which in this case is diabetic ketoacidosis. Next, to check for a second metabolic disorder, the delta-delta gap (corrected bicarbonate) must be evaluated. The delta AG is 30−12=1830 - 12 = 18 mEq/L. Adding this to the measured bicarbonate (16 mEq/L) gives a corrected bicarbonate of 34 mEq/L. Because the corrected bicarbonate is greater than the normal expected value of 24 mEq/L, there is a concomitant metabolic alkalosis, likely secondary to her intractable vomiting (loss of gastric acid and volume contraction). Finally, Winter's formula [Expected pCO2=(1.5×HCO3−)+8±2pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2] is used to assess respiratory compensation. Expected pCO2=(1.5×16)+8=32pCO_2 = (1.5 \times 16) + 8 = 32 mm Hg. The patient's actual pCO2pCO_2 is exactly 32 mm Hg, indicating appropriate respiratory compensation without a primary respiratory disorder. Therefore, the patient has a mixed HAGMA and metabolic alkalosis.

Step-by-Step Solution

1
Calculate the serum anion gap to identify the primary metabolic disturbance.
The anion gap is calculated as Na+−(Cl−+HCO3−)=142−(96+16)=30Na^+ - (Cl^- + HCO_3^-) = 142 - (96 + 16) = 30 mEq/L.
An elevated anion gap (> 12 mEq/L) establishes the presence of a high anion gap metabolic acidosis (HAGMA), which in this patient is due to diabetic ketoacidosis.
2
Calculate the delta-delta gap (corrected bicarbonate) to uncover any hidden metabolic disorders.
The Δ\Delta gap is 30−12=1830 - 12 = 18 mEq/L. The corrected HCO3−=ΔHCO_3^- = \Delta gap + measured HCO3−=18+16=34HCO_3^- = 18 + 16 = 34 mEq/L.
In a pure HAGMA, the drop in bicarbonate should equal the rise in the anion gap. A corrected bicarbonate >24> 24 mEq/L reveals a superimposed metabolic alkalosis, driven here by the patient's severe vomiting.
3
Apply Winter's formula to determine if respiratory compensation is appropriate.
Expected pCO2=(1.5×HCO3−)+8±2=(1.5×16)+8=32±2pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 16) + 8 = 32 \pm 2 mm Hg. The measured pCO2pCO_2 is exactly 32 mm Hg.
Since the actual pCO2pCO_2 matches the expected pCO2pCO_2, the low pCO2pCO_2 is purely a compensatory response (Kussmaul respirations) rather than a primary respiratory alkalosis.

Key Concept

Interpretation of complex mixed acid-base disorders requires a systematic three-step approach: calculating the anion gap, calculating the delta-delta gap (corrected bicarbonate) to check for concurrent metabolic disorders, and applying Winter's formula to assess for respiratory compensation.
Estimated Time:2m 0s
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Nephrology

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