Monday, October 10, 2011
How does aspirin reduce fever? Anti-pyretic action of aspirin:
Aspirin is an NSAID (Non-steroidal anti-inflammatory drug). All NSAIDs inhibits the cycloxygenase (COX-1 and COX-2) enzyme (by irreversible non-competetive inhibition) which is required for the synthesis of prostaglandin E2.
So, since no prostaglandin, particularly prostaglandin E2 in this case, is produced the temperature point in hypothalamus goes back to normal and thus the body temperature becomes normal again.
How does aspirin prevent thromboembolism / heart attack? Antiplatelete action of aspirin:
A thrombus is formed due to platelete aggregation. And platelete aggragation is mediated by a prostaglandin called thromboxane A2 (TX A2). As aspirin inhibits prostaglandin systhesis by inhibiting the COX enzyme, no thromboxane is produced.
So since there is no TX A2, there is no platelete aggragation and no thrombus or embolus formation.
This is particularly important in heart patients suffering from myocardial infarction. They have to take 75 mg of aspirin for life long to prevent myocardial infarction.
Aspirin is a non-selective, non-competetive irrversible cycloxygenase (COX) enzyme inhibitor. Even so it is about 160 times more specific to COX 1 than COX 2.
The COX enzymes are responsible for the systhesis of prostaglandins (PG) which are the mediators of inflammation, pain, fever, platelet aggregation.
So aspirin acts by inhibiting the prostaglandin synthesis.
Introduction to aspirin – an essential medicine:
Aspirin is a weak organic acid – acetylsalicylic acid. It is obtained naturally from the Willow bark which contains Salicin. It can also be produced synthetically.
It is mainly a non-opioid analgesic and non steroidal anti-inflammatory drug and is the initial choice of drug in the treatment of articular (arthritis) and musculoskeletal disorders.
Chief pharmacological actions of aspirin:
1. Anti-inflammatory action (Peripheral nervous system effect)
2. Analgesic action (pain-killer) (Both Central and Peripheral nervous system effect)
3. Anti-pyretic action (reduces fever) (Both Central and Peripheral nervous system effect)
4. Anti-platelet action (prevents platelet aggregation) ( Peripheral nervous system effect)
Other effects of aspirin:
1. Stimulates respiratory center so increases the repiration rate
2. Uncoupling of oxidative phosphorylation
1. Mild reversible hepatic damage
2. Increase SGPT and SGOT level
3. Reye’s syndrome
1. Increase steroid synthesis
2. Thyroid suppression by decreasing iodine uptake
3. Prolongs the labor period
Large dose (>5g/day) cause inhibition of prothrombin synthesis in the liver.
1. Gastric irritation
Bleeding leading to iron deficiency anaemia
2. Nausea and vomiting by stimulation of chemoreceptor trigger zone (CTZ)
3. Reduces the cytoprotective role of PG PG E2 and PG I2 ie decreases mucous production and increases the hydrochloric acid secretion
Decreases the renal blood flow leading to renal salt and water retention leading to renal damage.