Anesthestic Agents & Adjuncts_Preanesthestics_NN

Anesthetic Agents and Adjuncts

Preanesthetics

• Purpose of preanesthetics: Prepare the animal for anesthesia, ensuring a smoother induction and recovery process.

Neuroleptanalgesia

• Definition: A profound state of sedation and analgesia induced by simultaneous administration of an opioid and a tranquilizer.

• Opioids:

  • Morphine

  • Buprenorphine

  • Butorphanol

  • Hydromorphone

• Tranquilizers:

  • Acepromazine

  • Diazepam

  • Midazolam

  • Xylazine

  • Dexmedetomidine

Use of Neuroleptanalgesics

• Sedation for minor procedures.

• Induction of general anesthesia, particularly in dogs.

• Not recommended for young, healthy dogs and cats.

Preanesthetic Medications

• Objectives:

  • Calm or sedate an excited animal.

  • Minimize adverse drug effects.

  • Reduce the dose of concurrent drugs.

  • Ensure smoother anesthetic induction and recovery.

  • Provide analgesia and muscle relaxation.

Route of Administration for Preanesthetic Medications

• Affects onset of action and duration of effects:

  • Subcutaneous (SC): Slowest onset, longest duration.

  • Intramuscular (IM): Faster onset, shorter duration.

  • Intravenous (IV): Fastest onset, shortest duration.

Preanesthetic Anticholinergics

• Definition: Parasympatholytic drugs that block acetylcholine.

• Functions:

  • Prevent and treat bradycardia.

  • Decrease salivary secretions.

• Common Agents:

  • Atropine (fast onset, short peak, short duration).

  • Glycopyrrolate (slower onset, longer peak, longer duration).

Anticholinergic Effects

• CNS: Limited effect.

• Cardiovascular: Prevents bradycardia.

• Secretions: Decreases production.

• Eyes: Causes mydriasis and corneal drying.

• Respiratory: Bronchodilation.

Adverse Effects of Anticholinergics

• Cardiac arrhythmia; contraindicated in animals with elevated heart rates or cardiac diseases.

• Temporary bradycardia (Atropine).

• Thickened respiratory and salivary secretions; may lead to airway blockage.

• Inhibition of intestinal peristalsis; may lead to colic in horses or bloat in ruminants.

Tranquilizers and Sedatives

• Main categories include:

  • Phenothiazines

  • Benzodiazepines

  • Alpha2-adrenoceptor agonists

  • Alpha2-antagonists.

Phenothiazines: Acepromazine Maleate

• Also known as acepromazine or "ace".

• Used for preanesthetic sedation; decreases the necessary dose of general anesthetic.

• Facilitates induction and recovery; can combine with opioids.

• Approved for use in horses, dogs, and cats; administered IV or IM.

• No reversal agent available; metabolized by the liver; crosses the placenta slowly.

Effects of Acepromazine

• CNS: Calming, reluctance to move; less pronounced sedation in cats.

• Cardiovascular: Peripheral vasodilation, leading to hypotension, increased heart rate, and hypothermia.

• Respiratory: Worsens the depressive effect of other drugs.

• Gastrointestinal: Antiemetic effect and decreases allergic response.

Adverse Effects of Acepromazine

• CNS: Reduced seizure threshold, potential for aggression or excitement.

• Cardiovascular: Dose-dependent hypotension and penile prolapse in large animals.

• Hematologic: Decreased packed cell volume (PCV) possibly from splenic engorgement.

• Precaution: Avoid in animals undergoing allergy testing.

Special Considerations for Acepromazine

• Adjust dose based on needle placement.

• Increased potency and duration in geriatrics, neonates, and debilitated animals.

• Breed considerations: caution particularly in Australian Shepherds, Boxers, Greyhounds, and Terriers.

Benzodiazepines

• Controlled substances with rapid onset and short duration. Common benzodiazepines include:

  • Diazepam

  • Zolazepam

  • Midazolam.

Effects of Benzodiazepines

• CNS: Effective calming and anticlimactic only in old or ill patients; minimal sedation or analgesia.

• Anticonvulsant effect: Useful for animals having seizures.

• Cardiovascular and respiratory safety: High margin of safety; minimal effects noted.

• Skeletal muscle relaxation: Can potentiate effects of general anesthetics; appetite stimulation in cats and ruminants.

Adverse Effects of Benzodiazepines

• CNS: Disorientation and excitement in young, healthy dogs; dysphoria and aggression in cats.

• Motor Effects: Muscle fasciculations in horses and ataxia in large animals.

• Precautions: Administer Diazepam IV slowly; avoid oral Diazepam in cats due to risk of liver failure.

Use of Benzodiazepines

• Diazepam: Not water-soluble; do not mix with water-soluble drugs; don’t store in plastic. Commonly paired with ketamine for anesthesia induction in small mammals and horses.

• Midazolam: Water-soluble; can be administered IM or SC; good sedative for swine, ferrets, rabbits, and birds.

• Zolazepam: Available as a component of Telazol®.

Alpha2-Adrenoceptor Agonists

• Noncontrolled agents providing sedation, analgesia, and muscle relaxation for various species.

• Administered IM or IV before minor surgical procedures.

• Effects are readily reversed with alpha2-antagonists.

Key Alpha2-Agonists

• Xylazine, Dexmedetomidine, Detomidine, Romifidine: Used primarily in large animals like horses.

Functionality of Alpha2-Agonists

• Stimulates sympathetic nervous system (SNS) receptors, decreasing norepinephrine release, which leads to decreased response in the "fight-or-flight" scenarios.

• Effects include sedation, analgesia, bradycardia, hypotension, and hypothermia with a rapid onset lasting 1-2 hours.

Effects of Alpha2-Agonists

• CNS: Dose-dependent sedation.

• Cardiovascular: Initial vasoconstriction and hypertension followed by decreased cardiac output.

• Respiratory: Potentially significant depressant effect.

• Additional: Muscle relaxation, vomiting, temporary hyperglycemia, hypothermia.

Adverse Effects of Alpha2-Agonists

• CNS: Behavioral changes vary per species.

• Cardiovascular: May see bradycardia and hypotension; concurrent administration with other drugs can magnify effects.

• Gastrointestinal Issues: Can also lead to bloat in dogs and cattle or salivation/regurgitation in cattle.

Caution with Alpha2-Agonists

• Monitor closely, especially in geriatric, diabetic, pregnant, pediatric, or ill patients. Administer anticholinergics beforehand to mitigate effects.

Alpha2-Antagonists

• Functionality: Reverse all effects of alpha2-agonists, correcting analgesia and sedation.

• Adverse effects of overdose: Neurological (excitement), cardiovascular (hypotension), gastrointestinal (salivation).

Specific Alpha2-Antagonists

• Tolazoline: Nonspecific antagonist for ruminants.

• Yohimbine: Used in dogs, cats, horses; species-dependent dose ratio.

• Atipamezole: Specific antagonist for dexmedetomidine; faster recovery for cats compared to dogs.

Opioids

• Derivatives of opium classified into agonists, partial agonists, and antagonists used to provide analgesia and sedation, especially during anesthetic induction.

Commonly Used Opioids

• Agonists: Morphine, hydromorphone, fentanyl.

• Partial agonist: Buprenorphine.

• Agonist-antagonists: Butorphanol and nalbuphine.

• Antagonists: Naloxone, etorphine.

Opioid Pharmacodynamics

• Mimic endogenous opioids affecting pain pathways in the CNS via mu, kappa, and delta receptors.

• Each type provides varying effectiveness based on receptor interaction.

Effects of Opioids

• CNS: Vary significantly based on species with potential for sedation or stimulation.

• Analgesia: Most effective with pure agonists for severe pain during premedication for surgery.

• Cardiovascular effects typically lead to bradycardia; respiratory depression occurs in more sensitive patients.

Other Effects of Opioids

• Behavioral effects such as miosis in dogs and mydriasis in cats, alongside temperature variations and sweating.

Adverse Effects of Opioids

• CNS responses of anxiety or disorientation, cardiovascular effects leading to marked bradycardia, respiratory issues including decreased oxygen levels, and potential GI complications.

Other Adverse Effects of Opioids

• Risk of physical dependence and dermal reactions; significant drug interactions exist.

Use of Opioids

• Employed as preanesthetic agents either alone or alongside tranquilizers and anticholinergics for effective analgesia.

Opioid Antagonists

• Function to reverse the effects of opioids responsible for CNS and respiratory depression, facilitating recovery in sedated patients.

• Naloxone Hydrochloride: Commonly used with rapid effect for reversal of sedation in various animals.

Effects of Opioid Antagonists

• Quick reversal of opioid effects with minimal adverse reactions; however, rapid cessation can induce withdrawal symptoms.

Use of Opioid Antagonists

• Primarily in emergencies for overdose situations, reviving neonates from C-section anesthesia if the dam received opioids.