Injectable Anaesthetics

Classification

• Barbiturate Anaesthetics
  • Synthesized by Adolf Von Baeyer by condensation of urea with malonic acid.
  • Derivatives of barbituric acid (malonyl urea).
  • Do not cause CNS depression alone.
• Non-Barbiturates (Non-Dissociative Anaesthetics)
  • Phenol Derivatives: Propofol
  • Imidazole Derivatives: Etomidate, Metomidate
  • Steroidal Anaesthetics: Alfaxalone, Alfadolone
  • Benzodiazepines: Midazolam, Diazepam
  • Neuroleptanalgesics: Fentanyl-droperidol
  • Miscellaneous: Chloral hydrate, guaifenesin
• Dissociative Anaesthetics
  • Ketamine, Phencyclidine, Tiletamine

Barbiturates

• Synthesis and Chemistry
  • Essential for CNS depressant effect: substitution at C-5.
  • Optimal therapeutic effect: alkyl side chain of 4-9 carbons.
  • Longer chains = short-acting due to metabolism.
  • Addition of phenyl group at C-5 enhances anticonvulsant activity.

Mechanism of Action

• Dose-dependent CNS depression.
• GABA facilitatory at normal doses, mimetic at higher doses.
• Inhibit excitatory AMPA receptor and some ligand-gated ion channels.
• High doses increase Cl- conductance and inhibit neurotransmitter release.

Pharmacological Actions

• CNS: Sedative, anticonvulsant in anaesthetic doses.
• CVS: Slight decrease in BP and heart rate.
• Respiratory Centre: Dose-dependent depression.
• Kidney: High doses may cause oliguria/anuria.
• Digestive System: Initial depression then increased GI motility.
• Muscles: Weak muscle relaxant.
• Uterus & Foetus: Depresses uterine contraction and can cross placenta.

Clinical Uses

• Sedative: Pentobarbitone for large animals.
• General Anaesthetic: Without premedication: Dogs/cats, swine, sheep/goat.
• Anticonvulsant: For status epilepticus.
• Euthanasia: Pentobarbitone for dogs.

Propofol

• Short-acting IV anaesthetic, milky emulsion, given IV.
• Acts on GABA A receptor.
• Effects:
  • CNS: Smooth induction, rapid recovery.
  • CVS: Myocardial depression, peripheral vasodilation.
  • Respiratory: Higher incidence of apnoea on induction.
• Dose: Varies by species and presence of premedication.

Saffan

• Contains alfaxalone and alfadolone, used in veterinary medicine.

General Anaesthetics

Stages of General Anaesthesia

• Stage I (Analgesia): Loss of consciousness.
• Stage II (Delirium): Loss of consciousness, reflex actions.
• Stage III (Surgical Anaesthesia): Regular respiration, divided into planes.
• Stage IV (Medullary Paralysis): Cessation of breathing, cardiovascular collapse.

Induction and Maintenance

• Combination of drugs often used for balanced anaesthesia.
• Inhalation Anaesthetics: Ether, halothane, methoxyflurane, etc.
• Gaseous Agents: Nitrous oxide, cyclopropane.

Dissociative Anaesthetics and Sedatives

Dissociative Anaesthetics

• Cause marked sensory loss without apparent loss of consciousness.
• Ketamine: Binds NMDA receptor, increases cardiac output.

Sedatives & Hypnotics

• Benzodiazepines: Diazepam, Midazolam.
• Alpha 2 Adrenoceptor Agonists: Xylazine, Medetomidine.
• Barbiturates: Amobarbital, Phenobarbital.

Euthanasia

Methods

• Chemical: Inhalant agents, barbiturates, miscellaneous agents.
• Physical: Captive bolt, gunshot, cervical dislocation.

Mechanism

• Induces hypoxia or depresses CNS leading to death.

Local Anaesthetics

Mechanism of Action

• Prevent depolarization by blocking Na+ ion channels.

Classification

• Based on Duration: Ultra-short, short, intermediate, long-acting.
• Onset and Administration: Varies by chemical structure and use case.

Clinical Applications

• Topical Anaesthesia: Eyes, urethra, trachea.
• Infiltration and Nerve Block: Various local anaesthetics like procaine and lidocaine.

These notes provide an overview of injectable anaesthetics, their classification, mechanism, effects, and clinical uses, as well as general anaesthesia stages, dissociative anaesthetics, sedatives, euthanasia methods, and local anaesthetics.