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Difficulty: MediumNephrology

A 28-year-old individual with a history of type 1 diabetes mellitus presents to the acute care clinic with 48 hours of relentless nausea and forceful emesis. The patient reports being unable to retain oral fluids and consequently withheld their scheduled insulin doses. On physical assessment, the patient is lethargic with dry mucous membranes and delayed capillary refill. Vital signs show a temperature of 37.1°C (98.8°F), blood pressure of 96/58 mm Hg, heart rate of 115/min, and respiratory rate of 24/min.

Laboratory analysis reveals:
Serum sodium: 140 mEq/L
Serum potassium: 3.4 mEq/L
Serum chloride: 94 mEq/L
Serum bicarbonate: 16 mEq/L
Serum creatinine: 1.8 mg/dL
Blood glucose: 625 mg/dL

Arterial blood gas obtained on room air shows:
pH: 7.32
pCO₂: 32 mm Hg
pO₂: 94 mm Hg

Based on these findings, which of the following represents the most accurate characterization of this patient's acid-base status?

  1. Elevated anion gap metabolic acidosis and concurrent metabolic alkalosisAnswer
  2. B
    Isolated elevated anion gap metabolic acidosis
  3. C
    Elevated anion gap metabolic acidosis and concurrent respiratory acidosis
  4. D
    Normal anion gap metabolic acidosis and concurrent metabolic alkalosis
  5. E
    Elevated anion gap metabolic acidosis and concurrent normal anion gap metabolic acidosis

Answer

Elevated anion gap metabolic acidosis and concurrent metabolic alkalosis
The correct answer accurately identifies the presence of two distinct metabolic disorders. The patient's initial laboratory values demonstrate acidemia (pH 7.32) and a low bicarbonate (16 mEq/L), indicating a primary metabolic acidosis. Calculating the anion gap yields 140−(94+16)=30140 - (94 + 16) = 30 mEq/L, confirming an elevated anion gap metabolic acidosis, which is expected in diabetic ketoacidosis. To determine if respiratory compensation is adequate, Winter's formula is utilized (Expected pCO2=1.5×HCO3−+8±2Expected \ pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2). This calculation yields 32±232 \pm 2 mm Hg; the measured pCO2pCO_2 is 32 mm Hg, confirming appropriate respiratory compensation without a primary respiratory disorder. Finally, the delta-delta gap is evaluated to uncover any hidden metabolic derangements. The excess anion gap is 30−12=1830 - 12 = 18. Adding this back to the measured bicarbonate yields an adjusted bicarbonate of 16+18=3416 + 18 = 34 mEq/L. Because the adjusted bicarbonate is substantially greater than the normal value of 24 mEq/L, a concurrent metabolic alkalosis is diagnosed. This secondary alkalosis is clinically consistent with the patient's history of intractable vomiting and resultant hydrochloric acid loss.

Step-by-Step Solution

1
Identify the primary acid-base disturbance by evaluating the pH and serum bicarbonate.
The pHpH is 7.32 (acidemia) and HCO3−HCO_3^- is 16 mEq/L (low), indicating a primary metabolic acidosis.
Determining the primary disorder is the essential first step in acid-base analysis.
2
Calculate the anion gap to determine the etiology of the metabolic acidosis.
Anion gap = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 140−(94+16)=30140 - (94 + 16) = 30 mEq/L, which is elevated.
An elevated anion gap (> 12 mEq/L) points to the presence of unmeasured anions, such as ketones in diabetic ketoacidosis.
3
Apply Winter's formula to assess for a concurrent respiratory disorder.
Expected pCO2=(1.5×16)+8=32±2pCO_2 = (1.5 \times 16) + 8 = 32 \pm 2 mm Hg. The measured pCO2pCO_2 is exactly 32 mm Hg, meaning respiratory compensation is appropriate.
To determine if there is an independent respiratory acidosis or alkalosis occurring simultaneously.
4
Calculate the delta gap (Δ-Δ) to check for additional underlying metabolic disorders.
Delta gap = Calculated AG (30) - Normal AG (12) = 18. Adjusted HCO3−HCO_3^- = Measured HCO3−HCO_3^- (16) + Delta gap (18) = 34 mEq/L.
The delta gap reveals if there is a hidden normal anion gap metabolic acidosis or a concurrent metabolic alkalosis.
5
Interpret the adjusted bicarbonate level.
Since the adjusted bicarbonate (34 mEq/L) is significantly greater than the normal baseline of 24 mEq/L, a concurrent metabolic alkalosis is diagnosed.
The excess bicarbonate unmasked by the calculation confirms a secondary alkalotic process, consistent with the patient's severe vomiting.

Key Concept

Identification of a mixed metabolic disorder using the anion gap, Winter's formula, and the delta-delta gap calculations.
Estimated Time:1m 30s
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Topics

Nephrology

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