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Difficulty: MediumNephrology

A 19-year-old college student is brought to the emergency department by her roommate due to agitation, nausea, and ringing in her ears for the past several hours. The roommate states she found several empty bottles of over-the-counter pain relievers on the patient's desk. The patient is breathing rapidly.

Her temperature is 38.1∘C38.1^\circ\text{C} (100.6∘F100.6^\circ\text{F}), blood pressure is 112/70112/70 mm Hg, pulse is 118118/min, and respirations are 2828/min. Physical examination shows a diaphoretic, anxious young woman. Her lungs are clear to auscultation.

Laboratory studies show:

Laboratory TestResult
Serum sodium142142 mEq/L
Serum potassium3.83.8 mEq/L
Serum chloride105105 mEq/L
Serum bicarbonate1414 mEq/L
Arterial pH7.427.42
Arterial PaO2_29898 mm Hg
Arterial PaCO2_22121 mm Hg

Which of the following best characterizes this patient's current acid-base disturbance?

  1. Mixed increased anion gap metabolic acidosis and primary respiratory alkalosisAnswer
  2. B
    Primary increased anion gap metabolic acidosis with adequate respiratory compensation
  3. C
    Primary respiratory alkalosis with adequate metabolic compensation
  4. D
    Mixed increased anion gap metabolic acidosis and primary metabolic alkalosis
  5. E
    Primary non-anion gap metabolic acidosis with adequate respiratory compensation

Answer

Mixed increased anion gap metabolic acidosis and primary respiratory alkalosis
The patient's laboratory results demonstrate an elevated anion gap of 2323 mEq/L (142−(105+14)142 - (105 + 14)), which confirms the presence of a primary increased anion gap metabolic acidosis. By applying Winter's formula (1.5×[HCO3−]+8±21.5 \times [HCO_3^-] + 8 \pm 2), the expected PaCO 2_2 for maximal respiratory compensation is 29±229 \pm 2 mm Hg. The patient's actual PaCO 2_2 is 2121 mm Hg. Because the measured PaCO 2_2 is lower than the expected compensatory value, there is a concurrent primary respiratory alkalosis. This mixed acid-base disorder is the classic presentation of salicylate (aspirin) toxicity, which both directly stimulates the medullary respiratory center (causing respiratory alkalosis) and uncouples oxidative phosphorylation (causing an accumulation of organic acids and a subsequent metabolic acidosis).

Step-by-Step Solution

1
Calculate the anion gap to check for the presence of a metabolic acidosis.
Anion Gap = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 142−(105+14)=23142 - (105 + 14) = 23 mEq/L.
An elevated anion gap (>12>12 mEq/L) definitively establishes the presence of an increased anion gap metabolic acidosis, regardless of the overall pH.
2
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO 2_2 = 1.5×14+8=29±21.5 \times 14 + 8 = 29 \pm 2 mm Hg.
This determines if the respiratory response is appropriate or if there is a superimposed primary respiratory disorder.
3
Compare the expected PaCO 2_2 to the measured PaCO 2_2.
The measured PaCO 2_2 (2121 mm Hg) is significantly lower than the expected range (27−3127-31 mm Hg).
A measured PaCO 2_2 that is lower than expected indicates an additional primary respiratory alkalosis.
4
Calculate the delta gap to rule out a third acid-base disorder.
Delta AG = 23−12=1123 - 12 = 11. Expected HCO 3−_3^- = 24−11=1324 - 11 = 13 mEq/L. This is very close to the measured 1414 mEq/L.
Because the expected and measured bicarbonate values are similar, there is no concurrent metabolic alkalosis or non-anion gap metabolic acidosis.

Key Concept

Salicylate toxicity typically presents with a mixed increased anion gap metabolic acidosis and primary respiratory alkalosis.
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Topics

Nephrology

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