Salicylate Poisoning Acid-Base Disturbance and Emergency Care
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Direct answer
Salicylate poisoning produces a mixed acid-base disorder, with a primary respiratory alkalosis from direct stimulation of the respiratory centre and a high anion gap metabolic acidosis from accumulated salicylic acid. Tinnitus, hyperventilation and altered mental status are the characteristic early clues. Treatment centres on sodium bicarbonate to alkalinise the urine, which keeps the drug ionised and trapped, and haemodialysis is reserved for severe toxicity or impaired kidney function.
What you must remember
- Acid-base pattern: Mixed respiratory alkalosis and high anion gap metabolic acidosis, so a near-normal arterial pH can hide severe toxicity.
- Early features: Tinnitus, vomiting, sweating and hyperventilation, with deafness as a later complication.
- Urinary alkalinisation: Sodium bicarbonate raises urine pH to about 7.5 to 8, keeping salicylate ionised so it is not reabsorbed.
- Potassium: Hypokalaemia is corrected so that urinary alkalinisation works effectively.
- Dialysis indications: Level above about 100 mg/dL in acute toxicity, or earlier with renal failure, pulmonary oedema, altered consciousness or refractory acidosis.
- Avoid acetazolamide: Its systemic acidosis drives salicylate into the brain and worsens toxicity.
A young adult with tinnitus and a mixed blood gas
A 22-year-old student is brought in after taking an unknown number of aspirin tablets about five hours earlier. She has tinnitus, sweating, a respiratory rate of 30 per minute and vomiting.
- Read the blood gas: Arterial pH is near normal, pCO2 is low and bicarbonate is reduced, which together describe a respiratory alkalosis with a high anion gap metabolic acidosis.
- Calculate the anion gap: The clinician calculates the anion gap and checks potassium and glucose.
- Time the level: A serum salicylate level is sent with the time of ingestion recorded, because a single level is interpreted against time since ingestion.
- Alkalinise the urine: Sodium bicarbonate is infused to alkalinise the urine, with urine pH and potassium monitored.
- Decide on dialysis: Haemodialysis is arranged if the level exceeds the dialysis threshold, or if confusion, pulmonary oedema or kidney failure develops.
- Refuse false reassurance: A near-normal pH does not mean a stable patient.
High-yield traps in salicylate questions
Salicylate questions often hinge on a reassuring blood gas or the wrong antidote.
- Trap: a near-normal pH means a mild case. Correct: in a mixed disorder compensation partly masks the acidosis, so a severe case can look deceptively calm.
- Trap: acetazolamide to alkalinise the urine. Correct: it causes systemic metabolic acidosis that drives salicylate into the central nervous system, so sodium bicarbonate is preferred.
- Trap: missing chronic salicylism in the elderly. Correct: it presents as delirium, pneumonia or heart failure, and serum levels can be lower than the severity suggests.
Older texts plot a single acute level on the Done nomogram against hours since ingestion, but it is unreliable, so treatment follows clinical features, repeated levels and the acid-base state.
Frequently asked questions
Why is acetazolamide avoided in salicylate poisoning?
Acetazolamide is avoided because it produces a systemic metabolic acidosis, which drives salicylate into the brain and worsens toxicity. Sodium bicarbonate is preferred for urinary alkalinisation because it achieves the goal without the acidosis.
What is the classic acid-base disorder in salicylate poisoning?
Salicylate poisoning typically produces a mixed respiratory alkalosis and high anion gap metabolic acidosis. The respiratory alkalosis results from direct stimulation of the respiratory centre, and the acidosis from accumulated salicylic acid and its metabolites.
What dialysis threshold applies in acute salicylate toxicity?
Haemodialysis is indicated in acute salicylate toxicity when the level is above about 100 mg/dL, or earlier when renal failure, pulmonary oedema, altered consciousness or refractory acidosis is present. Dialysis clears the drug more rapidly than urinary alkalinisation alone in these severe cases.
Why does salicylate poisoning cause noncardiogenic pulmonary oedema?
Salicylate-induced pulmonary oedema results from increased alveolar capillary permeability in severe toxicity, not from raised cardiac filling pressure. It signals severe poisoning and is an indication for urgent dialysis.