Veterinary Anesthesia Quiz

Premedication & Sedation

• Premedication involves administering medication(s) before anesthesia.
• Anxiolysis is a state of mental calm and relaxation, reducing locomotor activity, anxiety, and concern for the environment. Response to stimuli is lessened.
• Sedation is a state of mental calm, sleepiness, and disinterest in the environment, with poorer responsiveness to stimuli compared to anxiolysis.
• Narcosis is sedation produced by opioids.
• Neuroleptoanalgesia is the joint administration of a sedative drug and an opioid analgesic, often used for pain relief during surgery.

Learning Objectives

• Outline the reasons for premedication before general anesthesia.
• Explain the rationale and patient safety considerations for sedation versus general anesthesia.
• Describe the pharmacology of commonly used sedative drugs and opioids in veterinary anesthesia.
• Explain the principles of neuroleptoanalgesia in veterinary premedication and sedation.

Sedation vs General Anesthesia (GA)

• Sedation: Airway control (regurgitation/aspiration) may be problematic, monitoring and ventilation can be more difficult. Local blocks administration presents a challenge. Can take longer. Recovery might be slower to completely return to a functional state.
• GA: Airway control is good (regurgitation/aspiration is less likely), there is good monitoring and ventilation, and local blocks can be easily administered. Faster and recovers quicker to a functional state.

Why Premedicate?

• Decrease anxiety, stress, and catecholamine release.
• Ensure patient and staff safety and well-being.
• Facilitate handling.
• Facilitate IV catheter placement.
• Pre-emptive analgesia and balanced anesthesia.
• Promote smooth induction, maintenance, and recovery.
• Decrease autonomic nervous system (ANS) activation during surgery.

Premedication Options

• Acepromazine
• Alpha-2 agonists (e.g. medetomidine, dexmedetomidine)
• Benzodiazepines (e.g. diazepam, midazolam)
• Opioids (e.g. fentanyl, morphine, butorphanol, methadone, buprenorphine, pethidine/meperidine)
• Ketamine (discussed in another lecture).

Acepromazine

• Phenothiazine class
• Used in dogs, cats, and horses (SC, IM, IV, PO).
• Unpredictably sedative, especially in combination with other drugs.
• Slow onset of action (30-40 minutes IM).
• Long duration of action (4-8 hours).
• No reversal agent.
• High protein binding.
• Metabolized by the liver, excreted in urine and faeces.
• Crosses blood-brain barrier and placenta.
• Potential for side effects like "locked in syndrome".

Acepromazine (continued)

• Anti-adrenergic (alpha-1 receptor blockade), leading to vasodiation, hypotension, and hypothermia.
• Anti-dopaminergic (D1 & D2 receptor blockade).
• Weak antihistaminic (H1 receptor blockade).
• Potential for side effects like sedation extrapyramidal effects at high doses, including excitation, tremors, rigidity.
• Potential for inducing syncope in hypovolemic animals.
• Potential for anti-arrhythmic effect. Prolonged profound sedation can be a factor for breeds with MDR-1 gene mutations (Collies and Shepherds).
• Potential proconvulsant effect in dogs with epilepsy.
• Extremely low risk of priapism in stallions in contemporary use.

Alpha-2 Adrenergic Receptor Agonists

• Medetomidine, Dexmedetomidine, Xylazine, Detomidine, Romifidine, Zenalpha
• Selectively bind to alpha-2 adrenergic receptors.
• Activation of central and peripheral pre & postsynaptic alpha-2 receptors.
• Effects also on alpha1 & imidazoline receptors.
• Effects on the central nervous system (CNS) (cerebral cortex, locus coeruleus, rostroventral lateral medulla, noradrenaline release, and CNS neurotransmission).
• Effects on spinal cord, peripheral nerves, and organs (analgesia, muscle relaxation).
• Different a2:a1 selectivity ratios can be used for clinical treatment.
• Administered as SC, IM, IV, oral, transmucosal, nasal, intrarectal, intravaginal.
• Short onset (1-2minutes IV, 15-20 minutes IM).
• Duration of action depends on the drug (30-120 minutes)
• Synergism with opioids and benzodiazepines.
• Decrease dose of induction/inhalational agents (MAC sparing effect).
• Dose-dependent effects on sedation, analgesia, cardiovascular effects, etc.
• Potential side effects: emesis, decreased gut motility, ruminants (bronchoconstriction, pulmonary and vasoconstriction), pulmonary edema, hypoxemia, muscle relaxation, hyperglycemia, diuresis, reflex bradycardia, and peripheral vasoconstriction. Also bradycardia can persist.

Antagonists of Alpha-2 Adrenergic Receptor Agonists

• Atipamezole, Yohimbine, and Tolazoline
• Reversal agents for alpha-2 agonists.
• Atipamezole can be used with xylazine and detomidine.
• Yohimbine and Tolazoline (MK-467) mostly used to reverse cardiovascular effects.

Benzodiazepines

• Diazepam, Midazolam
• GABA A receptor agonists.
• Schedule IV drugs
• IV and IM (midazolam only)
• Dose range from 0.1-0.4mg/kg IV
• Hepatic metabolism (active metabolites). Duration of action varies (1-4 h (midazolam), 4-12 h (diazepam)).
• Potential for hepatic necrosis in cats.
• Renal and biliary excretion.
• Flumazenil is an antagonist.
• Minimal cardiovascular depression.
• Minimal respiratory depression.
• Primarily used for inducing anxiety and anticonvulsant properties.
• Anxiolytic/sedative, anticonvulsant, central muscle relaxation, co-induction agent.

Opioids

• μ, κ, and δ receptors .
• Primary endogenous ligands: β-Endorphin, leucine- and methionine-enkephalin, dynorphin A.
• Full agonists (e.g., methadone, fentanyl), partial agonists (e.g., buprenorphine), antagonists (e.g., naloxone).
• Dose-response relationships.
• IV, IM, SC, poor PO absorption.
• Hepatic metabolism, (remifentanil: plasma esterases).
• Excretion: urine & bile.
• Cardiovascular (bradycardia, respiratory depression, histamine release).
• Urinary retention, gastrointestinal motility (ileus, constipation; horses, colic risk).
• Hyperthermia or hypothermia
• Antitussine properties
• Analgesia properties (mu receptor).
• Potential for inducing or exacerbating hyperalgesia.

Opioid Specifics

• Fentanyl: μ agonist, potency 100x morphine, onset 1-2 mins, short duration (20 minutes), bolus (1-2mcg/kg). CRI(5-10mcg/kg/hr), IV/transdermal patches, respiratory depression, schedule II, licensed in dogs.
• Butorphanol: partial μ agonist, κ agonist, onset 30 mins. long duration (6-8 hours), mild resp/cardio effects, less nausea/vomiting, schedule III, licensed in dog, cat, horse.
• Methadone: full μ agonist, NMDA receptor antagonist, onset 3-4 hrs, not very sedative (except in sick/young patients), schedule II, licensed in dog, cat.

Adjuncts to Premedication

• Trazodone: anxiolytic/sedative, tricyclic antidepressant, serotonin reuptake inhibitors (SSRIs), serotonin 1 & α1-adrenergic receptors antagonist, histamine 1, 3-7 mg/kg PO SID/BID/TID 1-2 h before travel.
• Gabapentin: neuropathic pain/seizures, blocks Ca²⁺ channels, side effects might include sedation, ataxia, vomiting, diarrhea, increased appetite, 100mg/cat 1-2 hrs before vet visit.

Choosing the Perfect Drug Combination

• Tailor anaesthetic protocols to patient needs
• Consider species variations and patient characteristics (aggressive/anxious, age, cardiovascular stability, comorbidities).
• Evaluate the desired outcomes (procedure, duration, pain level).

References

Include the BSAVA and Wiley Blackwell veterinary anesthesia books.

Test your knowledge on veterinary anesthesia, particularly focusing on the use of benzodiazepines and alpha-2 adrenergic receptor agonists. This quiz covers potential side effects, specific drug uses, and the benefits of premedication in animals. Perfect for veterinary students and professionals alike!