Expansion
Respiratory alkalosis from central stimulation plus a high anion gap metabolic acidosis
Expansion
Mechanism
- Direct stimulation of the respiratory centre causing hyperventilation and a respiratory alkalosis
- Uncoupling of oxidative phosphorylation, producing heat and lactate
- Accumulation of salicylate and ketoacids, giving a high anion gap metabolic acidosis
Features: tinnitus and deafness (often the earliest), nausea and vomiting, hyperventilation, sweating, hyperthermia, agitation then coma, and non-cardiogenic pulmonary oedema.
Children may present with acidosis alone, since the respiratory alkalosis phase is brief.
Management
- Activated charcoal within an hour, and consider repeated doses
- Fluid resuscitation and correction of hypokalaemia
- Urinary alkalinisation with sodium bicarbonate: raising urine pH traps ionised salicylate and greatly increases excretion, a direct application of ion trapping
- Haemodialysis for levels above about 700 mg/l, renal failure, pulmonary oedema, seizures or coma
Two practical cautions: potassium must be replaced before alkalinisation can succeed, and intubation is hazardous because any fall in minute ventilation removes the respiratory compensation and causes a precipitous fall in pH.