A 45-year-old woman ingests an unknown quantity of tablets. On presentation she has nausea, abdominal pain, tinnitus, and hyperventilation. ABG shows: pH 7.49, PaCO₂ 22 mmHg, HCO₃ 17 mEq/L. Later, the repeat ABG shows pH 7.28 with elevated lactate. This pattern is MOST consistent with poisoning by:
- A Paracetamol (acetaminophen)
- B Tricyclic antidepressants
- C Metformin
- D Salicylates (aspirin) ✓
Explanation
Salicylate toxicity produces a characteristic biphasic acid-base pattern: early direct stimulation of the medullary respiratory centre causes respiratory alkalosis (low PaCO₂, raised pH), followed by progressive accumulation of organic acids (salicylic acid metabolites) and uncoupling of oxidative phosphorylation producing high-anion-gap metabolic acidosis with elevated lactate. This mixed picture (respiratory alkalosis superimposed on metabolic acidosis) is pathognomonic of salicylate poisoning. Paracetamol overdose causes hepatotoxicity/liver failure without this early respiratory alkalosis pattern. TCA overdose causes QRS widening and anticholinergic features with metabolic acidosis.
Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.
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Written and medically reviewed by the StethoPrep medical team.