A 20-year-old woman is brought to the emergency department by her roommate due to confusion, nausea, and ringing in her ears for the past 6 hours. The patient has a history of major depressive disorder and recently went through a difficult breakup. On physical examination, she is diaphoretic and mildly disoriented. Temperature is 38.2°C (100.8°F), blood pressure is 115/75 mm Hg, pulse is 116/min, and respirations are 30/min. Lungs are clear to auscultation.
Laboratory studies show:
| Parameter | Result |
|---|---|
| Serum sodium | 140 mEq/L |
| Serum potassium | 3.6 mEq/L |
| Serum chloride | 103 mEq/L |
| Serum bicarbonate | 13 mEq/L |
| Blood urea nitrogen | 16 mg/dL |
| Serum creatinine | 0.9 mg/dL |
Arterial blood gas on room air:
| Parameter | Result |
|---|---|
| pH | 7.42 |
| pCO2 | 20 mm Hg |
| pO2 | 96 mm Hg |
Which of the following best describes this patient's acid-base disturbance?
- APrimary anion gap metabolic acidosis with appropriate respiratory compensation
- BPrimary respiratory alkalosis with appropriate metabolic compensation
- Mixed primary anion gap metabolic acidosis and primary respiratory alkalosisAnswer
- DMixed primary non-anion gap metabolic acidosis and primary respiratory alkalosis
- EMixed primary anion gap metabolic acidosis and primary respiratory acidosis
Answer
Mixed primary anion gap metabolic acidosis and primary respiratory alkalosis
The patient's presentation with tinnitus, tachypnea, fever, and confusion following a difficult breakup is classic for acute salicylate (aspirin) overdose. This toxicity classically causes a mixed acid-base disorder. First, calculate the anion gap: = mEq/L. Because this is greater than the normal range of 8-12 mEq/L, there is a primary anion gap metabolic acidosis (driven by uncoupled oxidative phosphorylation and accumulation of organic acids). Next, determine if the respiratory compensation is appropriate by using Winter's formula: Expected (25.5 to 29.5 mm Hg). The patient's actual is 20 mm Hg. Because the actual is lower than the expected compensatory range, she has an excessive respiratory drive, indicating a concomitant primary respiratory alkalosis (caused by salicylates directly stimulating the medullary respiratory center). Therefore, this is a mixed primary anion gap metabolic acidosis and primary respiratory alkalosis.
Step-by-Step Solution
Key Concept
Salicylate toxicity classically presents with a mixed primary anion gap metabolic acidosis (due to uncoupled oxidative phosphorylation) and primary respiratory alkalosis (due to direct stimulation of the medullary respiratory center). This is diagnosed by identifying an elevated anion gap and noting that the actual is lower than the predicted range using Winter's formula.