Low Dose Aspirin in Cardiovascular Disease

Aspirin is a traditional non-steroidal anti-inflammatory drug (NSAID) that preferentially inhibits cyclooxygenase-1 (COX-1) over cyclooxygenase-2 (COX-2).
Aspirin exhibits approximately 170-fold selectivity for COX-1.
At low doses, aspirin selectively targets platelets, inhibiting COX-1 enzyme in platelets over other cells like vascular endothelial cells or those in the gastrointestinal tract.

Arachidonic acid binds to the COX enzyme's hydrophobic pocket or channel at the catalytic site.
Serine 529 is a crucial amino acid residue acetylated by NSAIDs.
COX enzymes have bifunctional activity and produce prostaglandin $H_2$ .
Prostaglandin $H_2$ is converted to thromboxane A2, promoting platelet activation, aggregation, and vasoconstriction, which are essential for hemostasis by preventing excessive bleeding.

Aspirin acetylates and binds irreversibly to COX-1.
Platelets, being anucleated, cannot transcribe and translate more COX-1 enzymes.
Other cells regenerate COX-1 within 8 to 10 hours, but platelets cannot; thus, COX-1 activity is permanently inhibited for the platelet's lifespan (8-10 days).
Approximately 10% of platelets are replenished daily.
After a single aspirin dose, complete renewal of platelets takes about 1.5 weeks.
Aspirin also blocks prostacyclins, affecting both platelets and vascular endothelial cells.

Low dose aspirin is considered an antiplatelet dose, typically 75-100 mg per day orally.
This contrasts with anti-inflammatory or analgesic doses, which range from 300-900 mg four times daily.
Used in acute coronary syndrome, acute ischemic stroke, and intracoronary stent placement.
Primarily used for secondary prevention of vascular clotting events (strokes and myocardial infarctions).
Low dose aspirin reduces the risk of mortality from these events by 25-30%.

Following an atherosclerotic plaque rupture, the body attempts to plug the rupture.
Platelets adhere, activate, and aggregate with fibrin clotting factors to form a thrombus.
Inflammation or atherosclerosis increases the risk of thrombosis, including DVTs and pulmonary embolisms.

Antiplatelet agents (e.g., aspirin) disrupt platelet function.
Anticoagulants disrupt clotting pathways, reducing coagulation.
Fibrinolytics reduce the formation or break down fibrin.
Anti-fibrinolytic agents prevent bleeding events and are used in highly vascular surgeries.

COX-1 inhibition depends on dose and duration.
Aspirin is a full antagonist at COX-1 but only a partial agonist at COX-2.
After a single oral dose, the time for maximal concentrations (Tmax) is about 40 minutes.
A high dose of aspirin (approximately 300 mg) inhibits 95% of COX activity within 30 minutes.
Low dose aspirin requires accumulation over 4-5 days to effectively block platelets.
Patients experiencing myocardial infarction or stroke are given a high loading dose (300 mg) initially, followed by a low dose for maintenance.
Low dose aspirin does not significantly inhibit COX-2.
COX-2 inhibitors are not as effective at reducing thromboxane levels in platelets because platelets do not contain COX-2.

Toxicities are similar to those of high dose aspirin, but less frequent.
Less than 1% of patients experience serious bleeding events, such as intracranial hemorrhage.
Gastrointestinal irritation may occur in 5-10% of patients.
Regular full blood counts can monitor for asymptomatic blood loss.
Asthma-related bronchospasms, rashes, and rhinitis may occur; cross-reactivity with other NSAIDs should be considered.

History of aspirin allergy or aspirin-induced asthma.
Bleeding disorders.
Co-prescription with medications affecting coagulation.
Renal impairment requires caution and close monitoring.
Consideration of continuing or stopping aspirin before surgery based on guidelines.
Elderly patients are at higher risk.
Low dose aspirin is safe during breastfeeding.