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Difficulty: MediumNephrology

A 45-year-old man is evaluated in the emergency department for a 3-day history of intractable nausea, nonbilious vomiting, and diffuse abdominal pain. He has a known history of severe alcohol use disorder and reports his last alcoholic beverage was approximately 48 hours ago. On physical examination, he appears agitated, diaphoretic, and tremulous.

Vital signs are:
Temperature: 37.2∘C37.2^\circ\text{C} (99.0∘F99.0^\circ\text{F})
Blood pressure: 105/65 mm Hg105/65\text{ mm Hg}
Heart rate: 115/min115/\text{min}
Respiratory rate: 26/min26/\text{min}

Laboratory studies show:
TestResultReference Range
Sodium140 mEq/L140\text{ mEq/L}136−145 mEq/L136-145\text{ mEq/L}
Potassium3.1 mEq/L3.1\text{ mEq/L}3.5−5.0 mEq/L3.5-5.0\text{ mEq/L}
Chloride90 mEq/L90\text{ mEq/L}98−106 mEq/L98-106\text{ mEq/L}
Bicarbonate16 mEq/L16\text{ mEq/L}22−28 mEq/L22-28\text{ mEq/L}
Blood urea nitrogen38 mg/dL38\text{ mg/dL}8−20 mg/dL8-20\text{ mg/dL}
Creatinine1.7 mg/dL1.7\text{ mg/dL}0.7−1.3 mg/dL0.7-1.3\text{ mg/dL}
Glucose85 mg/dL85\text{ mg/dL}70−110 mg/dL70-110\text{ mg/dL}
Arterial blood gas analysis on room air reveals:
TestResultReference Range
pH7.437.437.35−7.457.35-7.45
pCO2\text{pCO}_225 mm Hg25\text{ mm Hg}35−45 mm Hg35-45\text{ mm Hg}
pO2\text{pO}_292 mm Hg92\text{ mm Hg}75−105 mm Hg75-105\text{ mm Hg}

Urinalysis is strongly positive for ketones.

Based on these findings, which of the following best describes this patient's acid-base disorder?

  1. High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosisAnswer
  2. B
    High anion gap metabolic acidosis and respiratory alkalosis
  3. C
    High anion gap metabolic acidosis and metabolic alkalosis with appropriate respiratory compensation
  4. D
    Primary respiratory alkalosis with compensatory metabolic acidosis
  5. E
    Normal anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis

Answer

High anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis
This patient's clinical presentation points to three distinct acid-base insults: alcoholic ketoacidosis, vomiting-induced volume depletion, and alcohol withdrawal-mediated hyperventilation. The diagnosis of a triple acid-base disorder is mathematically confirmed in three steps. First, the Anion Gap is 140−(90+16)=34 mEq/L140 - (90 + 16) = 34\text{ mEq/L}, confirming a High Anion Gap Metabolic Acidosis. Second, Winter's formula dictates the expected pCO2\text{pCO}_2 should be (1.5×16)+8=32 mm Hg(1.5 \times 16) + 8 = 32\text{ mm Hg}. The actual pCO2\text{pCO}_2 is 25 mm Hg25\text{ mm Hg}, indicating a primary Respiratory Alkalosis is driving the CO2\text{CO}_2 down further than compensation alone. Third, the Delta Gap (ΔAG\Delta\text{AG}) is 34−12=2234 - 12 = 22. Adding this back to the measured bicarbonate yields a corrected bicarbonate of 16+22=38 mEq/L16 + 22 = 38\text{ mEq/L}. Because this is significantly greater than 24 mEq/L24\text{ mEq/L}, a concurrent Metabolic Alkalosis is confirmed.

Step-by-Step Solution

1
Calculate the Anion Gap (AG) to identify the first disorder.
AG=Na+−(Cl−+HCO3−)=140−(90+16)=34 mEq/L\text{AG} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (90 + 16) = 34\text{ mEq/L}.
Since the normal AG is roughly 12, a value of 34 confirms a High Anion Gap Metabolic Acidosis (HAGMA), likely alcoholic ketoacidosis given the history and positive ketones.
2
Apply Winter's formula to determine if respiratory compensation is appropriate.
Expected pCO2=(1.5×HCO3−)+8±2=(1.5×16)+8±2=32±2\text{Expected pCO}_2 = (1.5 \times \text{HCO}_3^-) + 8 \pm 2 = (1.5 \times 16) + 8 \pm 2 = 32 \pm 2 (range 30−34 mm Hg30-34\text{ mm Hg}). The measured pCO2\text{pCO}_2 is 25 mm Hg25\text{ mm Hg}.
Because the measured pCO2\text{pCO}_2 (25) is lower than the expected compensatory range (30-34), the patient is hyperventilating more than expected, confirming a concurrent primary Respiratory Alkalosis (likely from alcohol withdrawal and agitation).
3
Calculate the Delta-Delta Gap (Δ/Δ\Delta/\Delta) to uncover hidden metabolic disorders.
ΔAG=Calculated AG−Normal AG=34−12=22\Delta\text{AG} = \text{Calculated AG} - \text{Normal AG} = 34 - 12 = 22.
Corrected HCO3−=Measured HCO3−+ΔAG=16+22=38 mEq/L\text{Corrected HCO}_3^- = \text{Measured HCO}_3^- + \Delta\text{AG} = 16 + 22 = 38\text{ mEq/L}.
The corrected HCO3−\text{HCO}_3^- estimates what the bicarbonate would be if the HAGMA were not present. Since 38 mEq/L38\text{ mEq/L} is much higher than a normal baseline of 24 mEq/L24\text{ mEq/L}, a concurrent primary Metabolic Alkalosis is also present, secondary to the patient's severe vomiting.

Key Concept

The systematic diagnosis of mixed acid-base disorders requires calculating the anion gap to identify HAGMA, applying Winter's formula to assess respiratory compensation, and utilizing the delta-delta gap to unmask hidden concurrent metabolic disorders.
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Nephrology

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