Aspirin

Learning objectives

Check with cardiologists before stopping early post stenting.

About : Always Aspirin check the BNF link here

• Also called Acetylsalicylic acid, a cheap and effective antiplatelet agent and antinflammatory
• Potent inhibitor of cyclooxygenase (COX) which produce prostaglandins
• Main medical effect is inhibiting platelet cyclooxygenase

Properties

• Has a pKa of 3.5 and is rapidly absorbed from the stomach
• Main metabolite is salicylate. Renal excretion increased in alkaline urine

Activity

• Anti inflammatory : effects are based on its inhibition of prostaglandin synthesis
• Antiplatelet : Irreversibly inhibits Thromboxane synthesis in platelets
• Analgesic and antipyretic

Mode of action

• Aspirin acetylates and deactivates cyclooxygenase which makes thromboxane A2
• This reaction is irreversible and if no further Aspirin is given platelet function is impaired for 10 days until a new population appears
• Aspirin also blocks endothelial COX activity but this can regenerate and so the predominant effect is on platelets

Indications/Dose

• Vascular antithrombotic dose : 75 mg OD, 81 mg OD, 300, 325 mg OD are commonly used doses. Aspirin 300 mg loading dose usually given. Higher doses not necessarily more effective.
• Ischaemic stroke: Aspirin 300 mg for 14 days then 75 mg.
• ACS/stent/IHD: Aspirin 75-300 mg OD.
• Analgesic: Aspirin 300-600 mg PO every 4 hours up to 3 g per day
• Rheumatic fever : Aspirin 300-600 mg PO every 4 hours up to 3 g per day
• Rheumatic diseases e.g. ankylosing spondylitis, osteoarthritis, rheumatoid arthritis, and SLE-associated arthritis and pleurisy: : Aspirin 3 g per day in divided doses

Dose range:You must check with BNF or drug datasheet

Contraindications

• Peptic ulcer, GI bleed, Severe renal or hepatic disease, Known allergy
• Bleeding disorders e.g. haemophilia

Side effects

• Gastric erosions, dyspepsia, peptic ulcer disease and upper GI Haemorrhage. Often treated with PPI or H2 blocker in those at increased risk of gastric side effects.
• Allergic reactions - angioedema, urticaria, Asthma, Renal toxicity
• Low dose (less than 2 g per day) increases serum uric acid
• High dose (more than 2 g per day) reduces serum uric acid
• High doses - tinnitus, vomiting, vertigo "salicylism"
• Toxicity causes hyperventilation and a respiratory alkalosis and then metabolic acidosis, respiratory depression and cardiac toxicity

References

• Aspirin.html">Aspirin in BNF