Anesthesia and Inhalants in Veterinary Medicine

Anesthetic Agents

• Suppress synaptic transmission by inhibiting Na+ and Ca2+ channels and opening K+ channels
• Side effects: Depression of respiratory and Cardiovascular system, painful tissue irritation, slow recovery, and increase liver metabolism of other drugs

Barbiturates

• Ultrashort-acting barbiturates: Thiopental, Methohexital
  • Lipid soluble, cross the blood-brain barrier, rapid onset, and short duration of action (10-30 minutes)
  • Therapeutic uses: Induction of general anesthesia, short duration anesthesia in dogs and cats
• Short-acting barbiturates: Pentobarbital
  • Less lipid soluble, longer duration of action (30-60 minutes)
  • Therapeutic uses: Euthanasia, seizure control (anticonvulsant)
• Long-acting barbiturates: Phenobarbital
  • Least lipid soluble, slowest onset, and long duration of action (6-12 hours)
  • Therapeutic uses: Anticonvulsant

Cyclohexylamines (Ketamine)

• Produce dissociative anesthesia (analgesia and superficial sleep)

Inhalant Anesthetics

• Provide little to no analgesia, respiratory and cardiovascular depressants, and cause hypotension
• Agents with low solubility in blood: quick saturation, fast induction, and recovery
• Agents with high blood solubility: slow saturation, slow induction, and recovery
• Examples:
  • Halothane
    • Widely used in Veterinary Medicine
    • Used to maintain anesthesia following induction with an injectable agent
    • Licensed for use in most companion animal species
    • Pharmacokinetics: moderate blood:gas partition coefficient, moderate speed of induction and recovery
    • High oil:gas partition coefficient, low MAC, and high potency
    • Adverse effects: CNS depression, cerebral vasodilation, and reduced cardiac output

Tranquilizers

• Provide preoperative sedation
• Examples: Acepromazine, Benzodiazepines (midazolam and Diazepam), a2 agonist (Xylazine)

Anticholinergic Agents

• Advantages: prevent salivation, bradycardia, and cause bronchodilation
• Disadvantages: inhibit Intestinal motility, cause colic in horses, and bloat in ruminants
• Examples: Atropine and glycopyrrolate

Neuromuscular Blocking Agents

• Facilitate intubation and relaxation
• Examples: Succinylcholine

Pharmacokinetics of Inhalant Anesthetics

• Lipid solubility: determines potency
• Oil:gas partition coefficient: determines potency
• Minimum alveolar concentration (MAC): measures potency
• Relationship of MAC to lipid solubility: higher lipid solubility, lower MAC, and higher potency
• Solubility in blood (Blood:gas Partition coefficient): determines speed of induction and recovery