Explain aspirin related to bvsc
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aspirin pharmacology BVSc veterinary uses doses cats dogs species sensitivity
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I have all the information needed. Here is the full BVSc-oriented explanation of Aspirin.
Aspirin (Acetylsalicylic Acid) - BVSc Pharmacology
Drug Classification
Aspirin belongs to the Salicylate group of NSAIDs (Non-Steroidal Anti-Inflammatory Drugs). It is the prototype and most studied member of this class.
- Chemical name: Acetylsalicylic acid (ASA)
- Drug class: NSAID - Salicylate derivative
- Nature: Weak organic acid
Mechanism of Action
Aspirin works by irreversibly inhibiting cyclooxygenase (COX) enzymes through acetylation.
The Arachidonic Acid Pathway:
- Cell membrane phospholipids are cleaved by phospholipase A2 to release arachidonic acid
- COX-1 converts arachidonic acid to Prostaglandin H2 (PGH2)
- PGH2 is further converted to:
- Thromboxane A2 (TXA2) - promotes platelet aggregation and vasoconstriction
- Prostacyclin (PGI2) - inhibits platelet aggregation, causes vasodilation
- Prostaglandins (PGE2, PGF2) - mediate inflammation, pain, fever
How Aspirin Acts:
Aspirin acetylates a serine residue on the active site of COX-1 (and to a lesser extent COX-2), thereby irreversibly inactivating the enzyme. This is a key distinction - while other NSAIDs are reversible, competitive inhibitors, aspirin's inhibition is permanent and irreversible.
Figure: Aspirin transfers its acetyl group to COX, inactivating it, and is itself converted to salicylic acid - Lippincott Illustrated Reviews: Pharmacology
Pharmacological Actions (The "3 A's")
| Action | Mechanism | Dose Required |
|---|---|---|
| Analgesic | Reduces PGE2, which sensitizes nerve endings to bradykinin/histamine | Low-moderate dose |
| Antipyretic | Inhibits PGE2 synthesis in hypothalamus, resets thermostat | Low-moderate dose |
| Anti-inflammatory | Reduces prostaglandin and thromboxane production at inflamed sites | High dose only |
| Antiplatelet | Irreversibly inhibits TXA2 synthesis in platelets | Very low dose (75-325 mg) |
Key BVSc point: Anti-inflammatory activity requires high doses (equivalent to 12-20 tablets/day in humans), while analgesia and antipyresis occur at lower doses. - Lippincott Illustrated Reviews: Pharmacology
Pharmacokinetics (ADME)
| Parameter | Detail |
|---|---|
| Absorption | Oral; passive diffusion; well absorbed from GI tract |
| Distribution | Widely distributed; crosses placenta and blood-brain barrier |
| Metabolism | Rapidly hydrolyzed to salicylic acid in liver and plasma by esterases |
| Excretion | Urine; rate depends on urinary pH (alkaline urine increases excretion) |
| Half-life (aspirin) | 15-20 minutes (very short - quickly converted to salicylate) |
| Half-life (salicylate) | 3-12 hours (dose-dependent, increases at high doses due to enzyme saturation) |
Veterinary Uses (BVSc Focus)
In Dogs:
- Treatment of mild to moderate pain (arthritis, musculoskeletal pain)
- Anti-inflammatory use in chronic joint disease
- Antithrombotic therapy in conditions like heartworm disease, DIC
- Dose: typically 10-25 mg/kg every 12 hours (with food)
In Cats - SPECIAL SPECIES SENSITIVITY:
Cats are highly sensitive to aspirin toxicity due to a deficiency in glucuronyl transferase enzyme, which impairs hepatic metabolism of salicylates. Cats cannot metabolize aspirin efficiently.
- The dosing interval in cats is every 48-72 hours (compared to every 12 hours in dogs)
- Use is very restricted; safer alternatives are preferred
- Cats can develop severe toxicity even at "therapeutic" doses
Other Species:
- Horses: Used for anti-inflammatory and antithrombotic purposes
- Cattle: Less common; GI irritation risk
Adverse Effects
Common:
- GI irritation - dyspepsia, gastritis, peptic ulceration (due to inhibition of cytoprotective prostaglandins - PGE2 and PGI2 in the gastric mucosa)
- Prolonged bleeding time - due to irreversible platelet COX inhibition; effects last for the entire platelet lifespan (~7-10 days)
- Renal effects - at high doses; reduces renal prostaglandins important for blood flow
Serious/Toxic Effects:
Acute Salicylate Toxicity (Overdose):
- Initial: Respiratory alkalosis - direct stimulation of the respiratory center in the medulla causes hyperventilation
- Later: Metabolic acidosis - uncoupling of oxidative phosphorylation, inhibition of Krebs cycle, accumulation of pyruvate and lactate
- Fatal doses: as low as 3 g in children (relevant to small animals proportionally)
Chronic Salicylism (at doses ~100 mg/kg/day):
- Tinnitus (ringing in ears)
- Headache, dizziness
- Mental confusion, drowsiness
- Nausea, vomiting, diarrhea
- Can progress to convulsions and coma
Reye's Syndrome: In young animals/children given aspirin during viral infections - rare but serious hepatic encephalopathy. Aspirin should be avoided in young animals with suspected viral illness.
Analgesic Nephropathy: Long-term use with acetaminophen can cause tubulointerstitial nephritis with renal papillary necrosis. - Robbins & Kumar Basic Pathology
Drug Interactions
| Interacting Drug | Effect |
|---|---|
| Anticoagulants (heparin, warfarin) | Increased bleeding risk |
| Other NSAIDs (ibuprofen) | Ibuprofen taken 2 hours before aspirin can block aspirin's antiplatelet effect by competing at the COX active site |
| Methotrexate | Increased methotrexate toxicity |
| Probenecid | Aspirin reduces uricosuric effect |
| Corticosteroids | Increased GI ulceration risk |
Contraindications
- Known aspirin hypersensitivity (can cause bronchospasm)
- Active GI ulceration or bleeding
- Bleeding disorders / coagulopathies
- Cats - with extreme caution only
- Young animals with viral infections (Reye's syndrome risk)
- Renal or hepatic insufficiency
- Third trimester pregnancy (premature closure of ductus arteriosus)
Aspirin Resistance
All forms of treatment failure with aspirin are collectively called "aspirin resistance," but true pharmacological resistance is rare. Pseudo-resistance - reflecting delayed and reduced drug absorption - is more common (e.g., with enteric-coated formulations). - Goodman & Gilman's Pharmacological Basis of Therapeutics
Summary - Key BVSc Exam Points
- Aspirin = irreversible COX inhibitor (unique among NSAIDs)
- Acts by acetylating serine on COX active site
- Triple action: analgesic, antipyretic, anti-inflammatory (+ antiplatelet at low dose)
- Anti-inflammatory requires high doses; analgesia/antipyresis at lower doses
- Cats are highly sensitive - deficient glucuronyl transferase = slow metabolism = narrow safety margin
- Antiplatelet effect lasts 7-10 days (platelet lifespan) - irreversible effect
- Toxicity sequence: respiratory alkalosis first, then metabolic acidosis
- Chronic toxicity = salicylism (tinnitus, confusion, GI bleeding)
- Avoid in viral infections in young animals (Reye's syndrome)
- Ibuprofen can antagonize aspirin's antiplatelet effect if given 2 hours before aspirin
Sources: Lippincott Illustrated Reviews: Pharmacology; Robbins & Kumar Basic Pathology; Goodman & Gilman's The Pharmacological Basis of Therapeutics; Katzung's Basic and Clinical Pharmacology
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