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Difficulty: Very hardNephrology

A 21-year-old woman is brought to the emergency department by her roommates due to confusion, rapid breathing, and excessive sweating. Her roommates report that she was studying for final exams and seemed highly stressed over the past few days. Upon arrival, the patient complains of a persistent ringing in her ears and nausea. Her temperature is 38.3°C (100.9°F), blood pressure is 110/70 mm Hg, pulse is 118/min, and respirations are 28/min. Physical examination reveals a diaphoretic, agitated patient with clear lungs and a regular heart rhythm.

Laboratory studies show the following:

Serum ChemistryResult
Sodium142 mEq/L
Potassium3.8 mEq/L
Chloride106 mEq/L
Bicarbonate16 mEq/L
Blood Urea Nitrogen (BUN)14 mg/dL
Creatinine0.9 mg/dL
Glucose98 mg/dL
Arterial Blood Gas (Room Air)Result
pH7.47
PaCO2PaCO_223 mm Hg
PaO2PaO_298 mm Hg

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

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

Answer

Mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis
This patient's clinical presentation of tinnitus, hyperventilation, fever, and altered mental status is highly suggestive of salicylate toxicity. The acid-base analysis confirms this. Step 1: The pH of 7.47 indicates alkalemia. Step 2: The low PaCO2PaCO_2 of 23 mm Hg indicates a primary respiratory alkalosis. Step 3: Calculation of the anion gap [142 - (106 + 16) = 20 mEq/L] reveals a concurrent high anion gap metabolic acidosis. Step 4: Applying Winter's formula [1.5×16+8=32±21.5 \times 16 + 8 = 32 \pm 2 mm Hg] shows that the actual PaCO2PaCO_2 (23 mm Hg) is much lower than the expected compensatory response (30-34 mm Hg). Because the actual PaCO2PaCO_2 is lower than expected, an independent primary respiratory alkalosis is confirmed alongside the primary high anion gap metabolic acidosis.

Step-by-Step Solution

1
Evaluate the patient's primary acid-base status using pH.
The pH is 7.47, indicating alkalemia.
Determining the direction of the pH shift identifies the predominant primary acid-base disorder (in this case, an alkalosis).
2
Identify the primary driver of the alkalemia.
The PaCO2PaCO_2 is low (23 mm Hg), which aligns with the alkalemia. Therefore, a primary respiratory alkalosis is present.
A primary respiratory disorder is characterized by a PaCO2PaCO_2 shift that correlates with the pH shift.
3
Calculate the serum Anion Gap (AG) to screen for hidden metabolic disorders.
AG=Na+−(Cl−+HCO3−)=142−(106+16)=20AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (106 + 16) = 20 mEq/L.
The normal AG is 8-12 mEq/L. An AG of 20 mEq/L definitively indicates the presence of a primary high anion gap metabolic acidosis, regardless of the overall pH.
4
Apply Winter's formula to determine if the PaCO2PaCO_2 is appropriately compensated for the metabolic acidosis.
Expected PaCO2=(1.5×HCO3−)+8±2Expected\ PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2
Expected PaCO2=(1.5×16)+8=32±2 mm HgExpected\ PaCO_2 = (1.5 \times 16) + 8 = 32 \pm 2\ mm\ Hg
Comparing the expected PaCO2PaCO_2 (30-34 mm Hg) to the actual PaCO2PaCO_2 (23 mm Hg) shows that the actual value is significantly lower. This proves the respiratory alkalosis is an independent primary disorder, not just compensation.
5
Synthesize the findings with the clinical presentation.
The patient has a mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis, classic for salicylate (aspirin) toxicity.
Salicylates directly stimulate the medullary respiratory center (causing respiratory alkalosis) and uncouple oxidative phosphorylation (causing lactic acidosis and ketoacidosis, resulting in HAGMA).

Key Concept

Salicylate toxicity classically presents with a mixed primary high anion gap metabolic acidosis and primary respiratory alkalosis, requiring calculation of both the anion gap and Winter's formula for accurate diagnosis.
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Nephrology

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