Expansion
A mixed respiratory alkalosis and metabolic acidosis, with tinnitus
Expansion
Features
- Tinnitus and deafness, often the earliest symptom
- Hyperventilation, from direct stimulation of the respiratory centre
- Nausea, vomiting, sweating, flushing
- Hyperthermia, from uncoupling of oxidative phosphorylation
- Confusion, seizures, coma and non-cardiogenic pulmonary oedema in severe poisoning
The acid-base picture is diagnostic: an early respiratory alkalosis from hyperventilation, followed by a high anion gap metabolic acidosis from uncoupled oxidative phosphorylation and lactate. Both are often present together.
“Ringing ears, blowing off carbon dioxide, and an acidosis at the same time.”
Treatment
- Activated charcoal if within 1 hour, and consider repeat doses, since salicylates form concretions and absorption is prolonged
- Fluids, and correct hypokalaemia, which must be done before alkalinisation will work
- Urinary alkalinisation with sodium bicarbonate, aiming for a urine pH above 7.5, which traps the ionised drug in the tubule
- Haemodialysis for levels over about 700 mg/l, renal failure, refractory acidosis, pulmonary oedema, seizures or coma
Avoid intubating if possible: these patients depend on a high minute volume, and ventilating them at a normal rate causes a catastrophic fall in pH.
Take levels at 4 hours and repeat, since absorption is erratic and levels can continue to rise.