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11. Sedatives, Tranquilizers, Anesthetics

Sedatives, Tranquilizers, Anesthetics

Overview

Sedatives, Tranquilizers, and Anesthetics are critical categories of drugs used in veterinary medicine to calm, sedate, or induce insensitivity to pain in animals. These pharmaceuticals play a vital role in ensuring animal welfare during procedures ranging from routine examinations to major surgeries, and their appropriate use can significantly improve both animal and veterinary staff experiences.

Sedatives

Definition

A sedative is a drug that suppresses brain activity and awareness, reducing excitement and inducing sleepiness. They often possess mild analgesic effects, which further aids in the management of pain and anxiety in animals.

Alpha-2 Agonists

Mechanism of Action (MOA)

These drugs attach to alpha-2 adrenergic receptors located on neurons, leading to a decrease in norepinephrine release. This results in a reduced central nervous system (CNS) excitement, promoting sedation and relaxation of the animal.

Clinical Uses

Alpha-2 agonists are used to provide sedation, muscle relaxation, and analgesia. They are particularly useful in veterinary practice for various procedures, such as minor surgeries and diagnostic imaging. Additionally, these drugs can stimulate vomiting, which can be beneficial in certain toxicological emergencies, especially in felines.

Common Drugs

  • Xylazine (Rompun®): Commonly used in multiple species for its sedative and analgesic properties.

  • Detomidine (Dormosedan®): Primarily for horses; offers profound sedation.

  • Medetomidine (Domitor®): Popular for sedation in dogs and zoo animals.

  • Dexmedetomidine (Dexdomitor®): More potent than its predecessor, it is widely employed in both dogs and cats.

  • Romifidine (Sedivet®): Noted for longer sedation duration, used primarily in equine medicine.

Side Effects

Side effects can include bradycardia (slow heart rate), hypotension (low blood pressure), respiratory depression, and vomiting, with felines being particularly susceptible to the latter.

Dosage Considerations

Dosing must vary significantly per species due to differences in body weight and physiology. Importantly, there is a ceiling effect, meaning higher doses will not proportionately increase their sedative effects post a specific threshold. Care must be taken in time management, as analgesic effects may diminish before sedation, necessitating a careful interplay between drug administration and procedure duration.

Specific Alpha-2 Agonists

  • Xylazine: Less selective; dosing will vary across types.

    • Horses: 1.1 mg/kg IV

    • Dogs: 0.2-1 mg/kg IV

    • Cattle: 0.05-0.15 mg/kg IVCaution: Can increase uterine contractility and precipitate rumen stasis in ruminants; vomiting is a possible adverse effect in cats. Reversal agents include Yohimbine and Tolazoline.

  • Detomidine (Dormosedan®): Used mainly in horses; recognized for being more selective in receptor action. Available in both injectable and oral forms. Notable side effects may entail muscle tremors and excessive sweating. It can also lead to bradycardia and a potential excitation response upon recovery. Reversible with Atipamezole.

  • Medetomidine (Domitor®): This drug is preferred for sedation in dogs and is widely popular in zoo anesthesia due to its high receptor selectivity. It is effective in wildlife immobilization and can be reversed with Atipamezole.

  • Dexmedetomidine (Dexdomitor®): As the active isomer of medetomidine, it provides advanced sedative effects than its predecessor and is widely used in both cats and dogs for sedation and as a pre-anesthetic agent in various protocols (i.e., "Kitty Magic" for feline anesthetic cocktails).

Tranquilizers

Phenothiazine Tranquilizers

Mechanism of Action (MOA)

These compounds work by blocking dopamine D2 receptors among others, thereby reducing anxiety and promoting a calm demeanor in animals. This action also lends them antihistaminic and antiemetic properties.

Clinical Uses

Phenothiazine tranquilizers are beneficial in various situations including physical examinations, during transport to reduce anxiety, and as part of preanesthetic protocols to mitigate stress prior to anesthesia.

Common Drugs

  • Acepromazine Maleate: Widely used across several species; it offers sedation without profound analgesia.

  • Chlorpromazine: Historically important yet less common in modern veterinary practice.

Side Effects

Adverse effects may include hypotension, hypothermia, developmental phimosis in stallions, and varied behavioral changes.

Benzodiazepines

Mechanism of Action (MOA)

These drugs enhance the effects of the neurotransmitter GABA (gamma-aminobutyric acid) at GABA-A receptors, leading to increased neuronal inhibition, thus promoting sedation and muscle relaxation effectively.

Clinical Uses

Benzodiazepines are integrated into balanced anesthesia protocols to improve recovery times and reduce the amount of inhaled anesthetic required. They also act as anxiolytics and anticonvulsants.

Common Drugs

  • Diazepam: Known for its safety profile and effectiveness in various species.

  • Midazolam: Similar applications with effective sedative properties.

Reversal Agent

Flumazenil can be used to reverse the effects of benzodiazepines and usually results in minimal cardiovascular side effects.

Anesthetics

Injectable Anesthetics

Barbiturates
Mechanism of Action (MOA)

These drugs enhance the effects of GABA in the CNS, leading to sedation, hypnosis, and anesthesia. Historically relevant, they have largely been supplanted by newer agents but still have specific uses.

Uses

Primarily utilized for euthanasia and as anticonvulsants (e.g., phenobarbital for seizure management).

Side Effects

Barbiturates can cause CNS and respiratory depression and have the potential to irritate surrounding tissues when injected perivascularly.

Propofol
Mechanism of Action (MOA)

Similar to barbiturates, but it is rapidly metabolized, providing a quick recovery compared to others.

Uses

Propofol is preferred for induction of anesthesia, allowing for quick transitions to maintenance anesthetics.

Side Effects

Common issues may include apnea, hypotension, and with frequent use, a risk of Heinz-body anemia in felines.

Dissociative Anesthetics
Common Drugs

  • Ketamine: This drug provides sedation and restraint while potentially elevating cerebrospinal fluid (CSF) pressure.

Side Effects

Can lead to erratic recoveries, apneustic breathing, and other neurologic effects; caution is advised in patients with pre-existing health conditions.

Controlled Substances Schedule/Class

  • Schedule II/Class II: Pentobarbital, highly regulated and often used for euthanasia.

  • Schedule III/Class III: Ketamine, tiletamine.

  • Schedule IV/Class IV: Includes benzodiazepines and phenobarbital, monitored due to potential for misuse.

  • Schedule V/Class V: Includes minimally regulated substances.

Resources

For further study and in-depth references, consult notable texts and journals dedicated to veterinary pharmacology and anesthesia practices.