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Difficulty: MediumNephrology

A 19-year-old college student is evaluated in the emergency department for confusion, nausea, and bilateral ringing in her ears. Her roommate reports that the patient ingested a large, unknown quantity of over-the-counter pain-relieving pills earlier in the day following a stressful exam. Temperature is 37.8°C (100.0°F), blood pressure is 112/68 mm Hg, pulse is 115/min, and respirations are 28/min. Physical examination shows a diaphoretic young woman with deep, rapid breathing.

Laboratory studies show:
Serum sodium: 140 mEq/L
Serum chloride: 104 mEq/L
Serum bicarbonate: 14 mEq/L
Blood urea nitrogen: 18 mg/dL
Serum creatinine: 0.9 mg/dL

Arterial blood gas (room air):
pH: 7.45
pCO2: 21 mm Hg
pO2: 98 mm Hg

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

  1. Mixed high anion gap metabolic acidosis and respiratory alkalosisAnswer
  2. B
    Primary metabolic acidosis with appropriate respiratory compensation
  3. C
    Primary respiratory alkalosis with appropriate metabolic compensation
  4. D
    Mixed normal anion gap metabolic acidosis and respiratory alkalosis
  5. E
    Mixed high anion gap metabolic acidosis and respiratory acidosis

Answer

Mixed high anion gap metabolic acidosis and respiratory alkalosis
The correct answer identifies that this patient has two concurrent primary acid-base disorders, which is classic for salicylate (aspirin) toxicity. First, calculate the anion gap: 140−(104+14)=22140 - (104 + 14) = 22 mEq/L. Because this is greater than 12 mEq/L, a primary high anion gap metabolic acidosis (HAGMA) is present. Next, use Winter's formula to determine if the respiratory compensation is adequate: ExpectedpCO2=(1.5×14)+8=29±2Expected pCO_2 = (1.5 \times 14) + 8 = 29 \pm 2 mm Hg. The patient's actual pCO2 is 21 mm Hg. Because the actual pCO2 is lower than the expected range, the patient is hyperventilating beyond what is required for compensation, indicating a concurrent primary respiratory alkalosis. This dual disorder creates a relatively normal or slightly alkalemic pH (7.45 in this case) despite severe metabolic derangements.

Step-by-Step Solution

1
Calculate the anion gap to evaluate the metabolic component.
AnionGap=Na+−(Cl−+HCO3−)=140−(104+14)=22Anion Gap = Na^+ - (Cl^- + HCO_3^-) = 140 - (104 + 14) = 22 mEq/L.
An elevated anion gap (> 12 mEq/L) confirms the presence of a primary high anion gap metabolic acidosis (HAGMA), regardless of the serum pH.
2
Calculate the expected pCO2 using Winter's formula to check for appropriate compensation.
ExpectedpCO2=1.5×[HCO3−]+8±2=1.5×14+8=29±2Expected pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 14 + 8 = 29 \pm 2 (range 27-31 mm Hg).
Winter's formula determines if the respiratory response to a metabolic acidosis is appropriate. If the actual pCO2 falls outside the expected range, a secondary mixed disorder is present.
3
Compare the actual pCO2 to the expected pCO2 to identify the mixed disorder.
The actual pCO2 is 21 mm Hg, which is lower than the expected 27-31 mm Hg.
Because the actual pCO2 is lower than what would be expected from compensatory hyperventilation alone, the patient has an independent, concurrent primary respiratory alkalosis.

Key Concept

Patients with salicylate toxicity classically present with a mixed primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation) and a primary respiratory alkalosis (due to direct central medullary stimulation). Winter's formula is essential for identifying these concurrent primary disorders.
Estimated Time:1m 30s
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Topics

Nephrology

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