TOPIC 16-17: ANESTHETICS

Atropine, Glycopyrrolate

• Anticholinergic preanesthetic.

• Increases heart rate.

• Prevents or treats vagally mediated bradycardia.

• Decreases:

  • Salivary + Bronchial Secretions

  • GI motility

• Reversed by Physostigmine.

Physostigmine

Reversal agent for Atropine and Glycopyrrolate

Acepromazine

• Phenothiazine tranquilizer.

• Decreases anxiety and motor activity.

• Causes:

  • Peripheral Vasodilation.

  • Antiemetic effect

  • Tranquilization

• No specific reversal agent.

Xylazine, Medetomidine, Dexmedetomidine

• α₂-adrenoceptor agonist.

• Produces:

  • Profound Sedation

  • Analgesia

  • Muscle Relaxation

• Marked anesthetic-sparing effect.

• Reversed by Atipamezole or Yohimbine.

Atipamezole, Yohimbine

Reversal agent for Xylazine, Medetomidine, and Dexmedetomidine

Diazepam, Midazolam

• Benzodiazepine.

• Produces:

  • Sedation

  • Anxiolysis

  • Muscle Relaxation

  • Anticonvulsant Effect

• Reversed by Flumazenil.

Flumazenil

Reversal agent for Diazepam and Midazolam

Droperidol, Azaperone

• Butyrophenone tranquilizer.

• Produces:

  • Tranquilization

  • Antiemetic Effects

• Decreases motor activity.

Morphine, Methadone, Hydromorphone, Oxymorphone, Fentanyl

• Opioid analgesic preanesthetic.

• Provides:

  • Analgesia

  • Sedation

• Decreases anesthetic requirement.

• Improves perioperative comfort.

• Reversed by Naloxone.

Naloxone

Reversal agent for opioid analgesics including Morphine, Methadone, Hydromorphone, Oxymorphone, and Fentanyl

Halothane

• Potent, nonflammable, nonirritating, sweet-smelling volatile anesthetic.

• Sensitizes myocardium to catecholamines causing increased arrhythmia risk.

• 20% metabolized by liver, 80% exhaled unchanged.

Enflurane

• Clear, nonflammable, mildly sweet, relatively nonirritating volatile anesthetic.

• Lower incidence of arrhythmias.

• 2–3% metabolized.

Isoflurane

• Nonflammable, stable, pungent or musty odor.

• Less myocardial depression than halothane or enflurane.

• 0.25% metabolized.

• Preferred when hepatic or renal function is impaired.

Desflurane

• Nonflammable, very volatile, pungent.

• Requires temperature-controlled pressurized vaporizer.

• Irritates airway causing:

  • Coughing

  • Breath-Holding

  • Laryngospasm

  • Increased Tracheobronchial Secretions

• 0.02% metabolized.

• Minimal hepatic and renal effects.

Sevoflurane

• Nonflammable, nonpungent, pleasant-smelling.

• Ideal for mask induction.

• 3–5% metabolized.

• Minimal hepatic and renal adverse effects.

Diethyl Ether

• Pungent, highly irritating, highly flammable, explosive.

• Obsolete in veterinary anesthesia.

• Causes:

  • Struggling

  • Delirium

  • Excitement

  • Nausea

  • Vomiting

  • Little cardiac depression

  • Good analgesia

• 10% metabolized.

Methoxyflurane

• Nonflammable, fruity-smelling, highly potent.

• Produces strong analgesia

• 50–70% metabolized.

• Forms Inorganic Fluoride + Oxalate = Nephrotoxicity + Prolonged Recovery

Chloroform

• Clear, sweet-smelling, nonflammable.

• Not routinely used because of:

  • Severe Cardiac

  • Hepatic + Renal Toxicity

• Exposure to heat, air, or light = Phosgene Gas

Nitrous Oxide

• Colorless, nonirritating, slightly sweet, nonflammable but supports combustion.

• Low blood solubility = Rapid Onset + Recovery

• Blocks NMDA receptors and releases endogenous opioid peptides.

• Weak anesthetic with good analgesia.

• Minimal cardiovascular + respiratory depression.

• Prolonged exposure inhibits Vitamin B12 = Megaloblastic Anemia + Neuropathy

• Non-halogenated, no malignant hyperthermia.

Xenon

• Colorless, odorless, chemically inert, nonflammable.

• Very Low Blood Solubility = Rapid Onset + Recovery

• Inhibits NMDA receptors and activates potassium channels.

• Minimal cardiovascular, respiratory, and organ toxicity.

• Not metabolized, exhaled unchanged.

• Extremely expensive with limited availability.

Cyclopropane

• Colorless, nonirritating, sweet odor.

• Heavier than air.

• Mixtures with Air/Oxygen = Highly Flammable + Explosive

Thiopental, Thiamylal, Thialbarbital, Methohexital, Hexobarbital

• Ultra-short acting barbiturate

• Thiooental is highly fat-soluble, prolonged recovery in Greyhounds, Whippets, and Salukis.

• Methohexital is preferred in sighthounds.

Pentobarbital, Secobarbital

Short-acting barbiturate

Propofol

• Short-acting IV anesthetic.

• Potentiates GABA-A receptors and activates glycine receptors.

• Smooth induction and rapid recovery.

• Adverse effects include apnea with rapid bolus.

• Reduced by injection over 20–30 seconds.

• Arterial hypotension, no analgesia.

Fospropofol

Water-soluble prodrug of propofol

Etomidate

• Potentiates GABA-A receptors.

• No analgesia.

• Minimal cardiovascular + respiratory depression.

• Inhibits 11β-hydroxylase = Decreased Cortisol + aldosterone

Metomidate

Commonly combined with azaperone in pigs.

Alfaxalone + Alfadolone (Saffan)

• Potentiates GABA-A receptors.

• Original formulation contained Cremophor EL = Histamine

• Newer formulation uses cyclodextrin.

Ketamine

• Blocks NMDA receptors.

• Produces dissociative anesthesia.

• Retains:

  • Pharyngeal

  • Laryngeal

  • Palpebral

  • Corneal Reflexes

• Recovery may cause:

  • Delirium

  • Vocalization

  • thrashing

  • tonic-clonic spasms.

• Increases:

  • Heart Rate

  • Cardiac Output

  • Arterial Blood Pressure

  • Salivary

  • Lacrimal Secretions

• Commonly combined with other anesthetics.

Phencyclidine

No longer freely available dissociative anesthetic.

Tiletamine

• Blocks NMDA receptors.

• Commonly combined with zolazepam for:

  • Muscle Relaxation

  • Reduced Excitation

Procaine

Ester local anesthetic.

Benzocaine

• Ester local anesthetic.

• Unsuitable for injection because of very low water solubility, may cause methemoglobinemia.

Tetracaine

Long-acting ester local anesthetic.

Proxymetacaine

Topical ophthalmic ester local anesthetic.

Cocaine

• Ester local anesthetic producing vasoconstriction instead of vasodilation.

• Does not produce PABA.

Lidocaine

• Amide local anesthetic.

• Also a Class IB antiarrhythmic.

• Combined with epinephrine to prolong anesthesia.

Prilocaine

• Amide local anesthetic.

• Metabolite o-toluidine may cause methemoglobinemia.

Bupivacaine

Long-acting amide local anesthetic.

Levobupivacaine

• Long-acting amide local anesthetic.

• Lower CNS and cardiovascular toxicity than bupivacaine.

Mepivacaine

Amide local anesthetic.

Ropivacaine

• Long-acting amide local anesthetic.

• Less cardiotoxic than bupivacaine.