Aspirin (acetylsalicylic acid) found commonly in OTC medications and used both as an anti-platelet agent and an anti-inflammatory analgesic. Its use in under 16s is very limited because of the association with Reye’s syndrome.
In overdose it causes a variety of symptoms – nausea,vomiting, sweating and tinnitus. Dizziness tachycardia and hyperventilation also occur. More severe poisoning can lead to delirium, agitation and convulsions.
Salicylates stimulate the respiratory centre of the brain and uncouple oxidative phosphorylation
Aspirin is rapidly hydrolysed to salicylate in the GI tract and blood. Systemic acidosis increases transfer to the brain. Salicylate is mainly excreted in the urine – alkalinisation of the urine increases renal secretion
Phase 1 of toxicity – hyperventilation resulting from direct respiratory centre stimulation. This leads to a respiratory alkalosis. K and HCO3 are excreted in the urine
Phase 2 of toxicity – paradoxic aciduria occurs as the respiratory alkalosis continues because so much K+ has been lost into the urine.
Phase 3 of toxicity – dehydration, hypokalaemia and increasing metabolic acidosis occurs
Common acid/base abnormalities include a mixed respiratory alkalosis/metabolic acidosis with a normal or raised pH. There usually will be a raised anion gap (beware falsely high chloride that can make the anion gap normal)