Low dose aspirin, commonly 75mg, daily has been shown to be beneficial
in the treatment of a number of vascular disorders, including myocardial
ischaemia. It is an anti-platelet agent that helps to prevent thrombus formation
and thrombus propagation following atheromatous plaque rupture.
Aspirin acetylates and inactivates cyclooxygenase (COX), the enzyme
required for production of the pro-aggregatory prostaglandin thromboxine A2.
This effect is permanent such that the effects of aspirin last for 10 days
until sufficient new platelets have been synthesised.
These include gastric erosions and GI bleeding, hypersensitivity
reactions, and the precipitation of asthma. The use of aspirin is
contraindicated in those with peptic ulceration or GI haemorrhage, bleeding
disorders, renal or hepatic impairment, and those with known hypersensitivity.
Aspirin is rapidly absorbed from the upper GI tract. It undergoes presystemic hydrolysis to salicylate, such that platelet COX may be inhibited as platelets pass through the portal circulation without greatly affecting the synthesis of blood vessel prostacyclin, also catalysed by COX