Respiratory · Year 1 · from Respiratory
Case 3: Salicylate Toxicity with Mixed Disorder
Case Presentation
A 19-year-old woman is brought to the emergency department after being found confused at home with multiple empty aspirin bottles. Her mother estimates she may have ingested 100-150 aspirin tablets (325 mg each) approximately 4 hours ago. On examination, she is agitated, diaphoretic, and hyperventilating. Vital signs show heart rate 125 bpm, blood pressure 100/65 mmHg, respiratory rate 36/min, temperature 38.5C, and oxygen saturation 99% on room air. Tinnitus is present.
Laboratory results show serum sodium 138 mEq/L, potassium 3.2 mEq/L, chloride 98 mEq/L, bicarbonate 12 mEq/L, glucose 85 mg/dL, BUN 22 mg/dL, and creatinine 1.1 mg/dL. Serum salicylate level is 85 mg/dL (therapeutic <20 mg/dL, toxic >40 mg/dL). Arterial blood gas shows pH 7.42, PaCO2 20 mmHg, PaO2 105 mmHg.
Analysis reveals a complex mixed acid-base disorder characteristic of salicylate toxicity:
- pH: 7.42 - near normal, but this does not exclude a disorder
- Primary disorders identified: Low PaCO2 (20) indicates respiratory alkalosis; low HCO3 (12) indicates metabolic acidosis
- Anion gap: 138 - (98 + 12) = 28 mEq/L (elevated)
This is a mixed anion gap metabolic acidosis AND primary respiratory alkalosis. Salicylate toxicity classically produces this dual disorder through two mechanisms: (1) direct stimulation of the medullary respiratory center causes primary hyperventilation (respiratory alkalosis), and (2) salicylate uncouples oxidative phosphorylation, causing lactic acid production, while salicylic acid itself is an organic acid (metabolic acidosis).
The near-normal pH is deceptive - it results from the opposing effects of respiratory alkalosis and metabolic acidosis, not from absence of disease. Neither disorder is compensating for the other; both are primary processes.
Treatment includes aggressive IV fluid resuscitation, IV sodium bicarbonate to alkalinize blood and urine (enhancing salicylate excretion), correction of hypokalemia, and emergent hemodialysis given severely elevated salicylate level with altered mental status. The patient undergoes hemodialysis with subsequent improvement in salicylate level and mental status.
Key Learning Points
- Salicylate toxicity causes mixed respiratory alkalosis AND anion gap metabolic acidosis
- Near-normal pH can mask severe underlying dual disorders
- Early: respiratory alkalosis predominates; late: metabolic acidosis worsens
- Treatment includes IV bicarbonate (alkalinizes urine for excretion) and hemodialysis for severe toxicity
- The two processes are both primary, not compensation for each other