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Difficulty: MediumNephrology

A 19-year-old woman is brought to the emergency department by her roommate due to nausea, vomiting, and "ringing" in her ears. The roommate reports that the patient was distraught after a severe argument and ingested an unknown quantity of an over-the-counter pain medication approximately 5 hours ago. On physical examination, her temperature is 37.8°C (100.0°F), blood pressure is 112/68 mm Hg, pulse is 115/min, and respirations are 32/min. She appears agitated and is breathing deeply and rapidly. Her lungs are clear to auscultation bilaterally.

Laboratory studies show:
* Serum sodium: 139139 mEq/L
* Serum potassium: 3.73.7 mEq/L
* Serum chloride: 103103 mEq/L
* Serum bicarbonate: 1414 mEq/L
* Blood urea nitrogen: 1616 mg/dL
* Serum creatinine: 0.90.9 mg/dL

Arterial blood gas analysis on room air reveals:
* pH: 7.467.46
* PaCO2PaCO_2: 2020 mm Hg
* PaO2PaO_2: 105105 mm Hg
* HCO3−HCO_3^-: 1414 mEq/L

Based on the laboratory findings, which of the following best describes this patient's current acid-base disturbance?

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

Answer

Concurrent primary high anion-gap metabolic acidosis and primary respiratory alkalosis
This patient presents with classic features of salicylate toxicity (aspirin overdose), including tinnitus, nausea, tachypnea, and agitation. Salicylates directly stimulate the medullary respiratory center, leading to hyperventilation and a primary respiratory alkalosis. Shortly after, the accumulation of organic acids causes a primary high anion-gap metabolic acidosis.

To diagnose this mixed disorder systematically:
1. Calculate the anion gap (AG): AG=Na+−(Cl−+HCO3−)=139−(103+14)=22AG = Na^+ - (Cl^- + HCO_3^-) = 139 - (103 + 14) = 22 mEq/L. An AG >12> 12 mEq/L confirms a primary high anion-gap metabolic acidosis.
2. Determine the expected respiratory compensation using Winter's formula: Expected PaCO2=(1.5×HCO3−)+8±2=(1.5×14)+8±2=29±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 mm Hg.
3. Compare actual vs expected PaCO2PaCO_2: The actual PaCO2PaCO_2 is 2020 mm Hg, which is significantly lower than the expected range. This means the patient is blowing off more CO2CO_2 than can be explained by simple compensation, confirming an independent, concurrent primary respiratory alkalosis. Furthermore, simple compensation will never result in an overcorrection to an alkalemic pH (7.467.46).

Step-by-Step Solution

1
Calculate the serum anion gap to evaluate for metabolic acidosis.
Anion Gap = 139−(103+14)=22139 - (103 + 14) = 22 mEq/L.
An anion gap greater than 1212 mEq/L confirms the presence of a primary high anion-gap metabolic acidosis (HAGMA), regardless of the overall pH.
2
Assess the arterial pH to identify the dominating acid-base disorder.
The pH is 7.467.46 (alkalemia).
A pH >7.45> 7.45 indicates that the net or dominating disturbance is an alkalosis. Since the patient has a confirmed metabolic acidosis (from the high AG), there MUST be a concurrent, stronger alkalosis driving the pH up.
3
Apply Winter's formula to determine the expected PaCO2PaCO_2 and identify the specific respiratory disturbance.
Expected PaCO2=(1.5×14)+8=29±2PaCO_2 = (1.5 \times 14) + 8 = 29 \pm 2 mm Hg. The actual PaCO2PaCO_2 (2020 mm Hg) is much lower than the expected range (27−3127-31 mm Hg).
Because the actual PaCO2PaCO_2 is significantly lower than the predicted compensatory range, there is a distinct, concurrent primary respiratory alkalosis rather than just appropriate compensation for the metabolic acidosis.

Key Concept

Salicylate toxicity classically causes a mixed acid-base disturbance: an early primary respiratory alkalosis (due to direct medullary respiratory center stimulation) and a concurrent primary high anion-gap metabolic acidosis (due to the accumulation of unmeasured organic acids like lactate, ketones, and salicylates).
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Nephrology

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