salicylates (aspirin)

sources

• OTC: Aspirin, Pepto-Bismol (bismuth subsalicylate), Alka-Seltzer, Excedrin, Kaopectate.
• Rx: Aggrenox, Asacol (mesalamine).
• Topical/Herbal: Oil of Wintergreen (methyl salicylate - potent), Tiger Balm/Red Flower Oil, acne cleansers, wart removers.

pathophysiology

• Uncoupling of Oxidative Phosphorylation: Leads to metabolic acidosis and hyperthermia.
• CNS Neuroglycopenia: Salicylates uncouple neuronal oxidative phosphorylation. CSF glucose may be low despite normal serum glucose.
  • Pearl: Maintain serum glucose in the high-normal range to ensure adequate CNS delivery.

presentation

• Early (1–2h): Tinnitus, vertigo, nausea/vomiting, hyperventilation.
• Late: Hyperthermia (fever), non-cardiogenic pulmonary oedema, coma/seizures, arrhythmia, thrombocytopenia, AKI.
• Classic Gas: Respiratory Alkalosis (direct medullary stimulation) + Anion Gap Metabolic Acidosis.
  • Note: Concurrent Respiratory Acidosis implies acute lung injury, CNS depression, or mixed overdose and is a grave sign.

investigations

• Salicylate Levels:
  • Toxicity possible > 3.0 mmol/L.
  • Dialysis range > 7.2 mmol/L.
  • Measure q2–4h until declining.
• Labs: VBG/ABG (mixed disorder), lytes (gap, K+), glucose (gap), coags (hepatic compromise).

management

1. decontamination

• Activated Charcoal: 1 g/kg (up to 50 g) PO/NG within 2 hours.
  • Consider Multi-dose AC or Whole Bowel Irrigation for enteric-coated preparations or massive ingestions (bezoar formation).

2. enhanced elimination (urinary alkalinisation)

• Goal: Trap weak acid (salicylate) in urine (ion trapping).
• Targets:
  • Urine pH: 7.5–8.0.
  • Serum pH: 7.40–7.55 (Do not exceed 7.55).
• Dosing:
  • Bolus: 1–2 mmol/kg Sodium Bicarbonate IV.
  • Infusion: 150 mL (3 amps) Sodium Bicarbonate in 1 L D5W at 250 mL/hr (approx. 2x maintenance).
• Key Constraint: Must maintain Serum K > 4.0 mmol/L.
  • Hypokalaemia leads to renal $H^{+}/K^{+}$ exchange (resorption of acid), rendering alkalinisation ineffective.

3. haemodialysis

Indications:

• Levels: > 7.2 mmol/L (Acute) or > 3.6 mmol/L (Chronic).
• CNS: Altered mental status, coma, seizures.
• Respiratory: Hypoxaemia requiring supplemental $O_2$, non-cardiogenic pulmonary oedema.
• Renal/Metabolic: Renal failure (esp. if level > 6.5 mmol/L), severe acidosis (pH < 7.20), refractory electrolyte disturbances.
• Other: Hepatic compromise with coagulopathy, volume overload preventing bicarbonate administration.

airway pearl

Intubation is dangerous. Patients rely on massive respiratory compensation ($RR>30$) to maintain pH. Paralysis/sedation can cause a precipitous drop in pH (respiratory acidosis), driving un-ionised salicylate into the brain.

Maintain spontaneous respiration if possible. If intubation is necessary, match the pre-intubation minute ventilation (high RR).