Premeds

Reasons for this:

• Decrease stress and anxiety (given at home)

• To calm or sedate

• To minimize adverse effects of other drugs

• Reduce nausea and vomiting

• To reduce dose of other drugs

• To produce smoother inductions

• To decrease pain

• To produce muscle relaxation

Classes of Premedication

• Anxiolytics

• Anticholinergics

• Phenothiazines

• Benzodiazepines

• Alpha 2 adrenergic agonists

• Opioids

Anxiolytics

• Administered at home

  • Can be used for any 'stressful' procedure

  • Surgery, radiographs, nail trims, etc.

• Used to minimize negative effects of stress, anxiety, and fear in patients

• Anesthetic adjunct

Drugs: Gabapentin, Trazodone

Gabapentin

• Off-label use in veterinary practice

• Developed as a human anticonvulsant

• Not controlled in Canada (though some will treat it as such due to human use)

• Effects:

  • Analgesia (not sufficient for surgical pain)

  • Reduces anxiety

  • Anticonvulsant properties

• Adverse effects: minimal data due to very few studies, since it is not currently labelled for animals

Trazodone

• Off-label use in veterinary practice

• Developed as a human antidepressant

• Not controlled (though some will treat it as such due to human label)

• Effects:

  • Reduces anxiety, fear & stress

  • Dogs — treatment and relief of situational stress/anxiety (storms, fireworks) and facilitation of confinement

• Adverse effects: minimal data

  • Sedation

Anticholinergics

• Block the binding of acetylcholine at the muscarinic receptor (parasympathetic system)

  • Allow 'fight or flight' to predominate

• Effects:

  • Prevents bradycardia

  • Reduces salivation and secretions of lacrimal, GI, and respiratory tract

  • Mydriasis

  • Bronchodilation

• Adverse effects:

  • May induce first or second degree AV block resulting in sinus tachycardia

  • Atropine may induce temporary bradycardia if given at low doses

  • Increased viscosity of salivary and respiratory secretions

• Atropine

• Onset is 5 min IM, and 1 min IV

• Peak effect in 10-20 min IM and 3-4 min IV

• Duration of 60-90 min

• Choice for CPR

• Glycopyrrolate

• Peak effect in 30-45 min

• Duration of 2-3 hrs

• Less likely to induce arrhythmias

• Suppresses salivation more effectively

• Minimally crosses placental barrier

Phenothiazines

Drugs:

• Acepromazine maleate ('ace' or 'acepromazine')

• Triflupromazine (humans)

• Chlorpromazine (humans)

Routes: IV, IM, SQ, oral NO reversal

Onset of action

• 15 min if given IM in dogs or IV in horses

• Peak effect in 30-60 min

Duration

• 4-8 hrs in small animals

• 1-3 hrs in horses

• Longer in sick, old, or debilitated animals

Phenothiazines effects

Major effects:

• Sedation

• Peripheral vasodilation

  • Hypotension

  • Increased HR

  • Increased heat loss

• Antiarrhythmic

• Antiemetic

Mild effects

• No respiratory depression, but worsens resp depression of other agents

• Antihistamine

• No analgesia

Adverse effects

• Reduction of seizure threshold

• Hypotension

• Penile prolapse

• Reduced PCV (caused by increased uptake of RBC by spleen)

Uses:

• Remember! — Effects are dose dependent; higher doses cause increased hypotension without increased sedation

• Increased potency is noted in geriatrics, neonates, liver and cardiac patients

• Collies, Aussies, sighthounds, boxers, and giant breeds can be more sensitive to effects

Benzodiazepines

Drugs

• Diazepam

• Midazolam

• Zolazepam

• Lorazepam

• Alprozolam

Routes: IV, IM (not diazepam), oral Reversal available: flumazenil — not used commonly due to expense and infrequency of need

Onset of action: 15 min given IM Duration: 1-4 hrs

NOTE: diazepam is mixed with propylene glycol and is NOT water soluble. It can only be mixed with ketamine and should not be given IM.

Benzodiazepines effects

Major effects

• Antianxiety and calming effect

• Anticonvulsant

• Muscle relaxation

• Appetite stimulant in cats and ruminants

Minor effects

• No analgesia

• Minimal effects on respiratory or cardiovascular systems

  • Good use for anesthetic of high-risk and geriatric patients

Adverse effects

• Produce unreliable sedation in young healthy animals (dysphoria, excitement, ataxia)

• Large animals may become ataxic or recumbent; horses may have muscle tremors/twitching

• Can cross placental barrier, therefore may see CNS depression in neonates

• Diazepam given rapidly IV can cause pain, bradycardia, hypotension, and apnea

• Cats receiving oral diazepam can develop hepatic failure

Additional notes

• Diazepam is soluble in plastic

• Diazepam and midazolam are light sensitive

• Commonly used with other agents to help achieve safe, smooth induction to anesthesia

• Midazolam is used commonly in protocols for pocket pets and birds

• Diazepam can be used for behavior modification

• Are controlled agents

Alpha 2 Adrenoceptor Agonists (A-2 agonists)

Drugs

• Xylazine

• Medetomidine

• Dexmedetomidine

• Detomidine

• Romifidine

Routes: IV, IM, (SQ — not as reliable) Reversals are available

Onset of action

• 5-15 min if given IV

• 15-30 min if given IM

Duration: Approx 1-2 hrs

NOTE: These drugs are metabolized in the liver and excreted in the kidneys.

Alpha 2 Adrenoceptor Agonists (A-2 agonists) effects:

Major effects

• Cause dose-dependent sedation that is profound

  • Horses often lower their head and droop their lower lip

• Have analgesic properties (lasts ~20 min in xylazine)

• Major cardiovascular effects:

  • After injection, patient experiences vasoconstriction, hypertension, and bradycardia (pale mucous membranes)

  • May have arrhythmias during this time

  • This phase followed by phase of hypotension and further bradycardia (not seen in medetomidine/dexmedetomidine)

  • Effects more severe if given IV

• Depression of respiratory system (dose-dependent effect) that is most notable in ruminants

• Muscle relaxation

• Vomiting in cats (some dogs) that is most common with xylazine

Minor effects

• Hyperglycemia due to reduced secretion of insulin by pancreas

• Hypothermia occurs due to loss of thermoregulation

• Markedly reduces amounts of other agents required

Adverse effects (most profound if given IV)

• May become hyper responsive or aggressive (especially if excited prior to injection)

• Horses may become more likely to kick

• Cattle may lie down (very sensitive to a2s)

• Dramatic decrease in HR, BP, & CO

• Brachycephalics or animals with upper airway obstruction may suffer dyspnea

• Diuresis

• GI effects: bloat, increased salivation, regurgitation

• Late stage abortion in ruminants

• Sweating in horses

• Intra-arterial injection can cause seizures or collapse

• Absorbed through open skin or mucous membranes

Xylazine

• Available in two concentrations

• Very out of flavor in small animal medicine

• Used in combinations with horses

Dexmedetomidine/Medetomidine

Dexmedetomidine/Medetomidine

• Used in small animals

• Dexdomitor vs domitor

• Greater potency and adverse effects are less pronounced than xylazine

• Recommend dosing based on body surface area

• Often mixed with other agents

• Can be used alone for minor procedures

  • Rads, ultrasound (can be aroused with stimuli)

Detomidine

• Used for equine

• Has twice the duration of xylazine

• Used in horses for sedation, analgesia, and muscle relaxation

• Often combined with other agents

• Most potent of the a-2s

Romifidine

• Used for equine

• Has longest duration of action

• Produces least ataxia and head droop

Alpha 2 Antagonists

Drugs

• Tolazoline

• Yohimbine

• Atipamezole

Routes: IM (IV, SQ) Onset of action: 5-10 min

Adverse effects

• Amount given should be based on amount of agonist given

• If >30 min since injection of agonist, amount of antagonist should be recalculated

• Overdose can cause excitement, muscle tremors, hypotension, tachycardia, salivation, and diarrhea

Use

• Tolazoline most used to reverse xylazine in ruminants

• Yohimbine most used to reverse xylazine in cats, dogs, horses, and exotics

• Atipamezole used to reverse dexmedetomidine

• Will reverse sedation and analgesia

• Give SLOWLY IV

  • IV atipamezole not recommended!

Opioids

Drugs:

• Agonists: Morphine, Hydromorphone, Oxymorphone, Fentanyl, Meperidine

• Partial agonists: Buprenorphine

• Agonist-antagonists: Butorphanol

• Antagonists: Naloxone

Routes: IV, IM, SQ, oral, rectal, transdermal, epidural Reversal: Naloxone

Duration

• In general ½ – 3 hrs

• Buprenorphine = 6-8 hrs

• Morphine = 6-8 hrs

Opioids effects

Major effects

• Can cause sedation or excitement

• Analgesia

• Bradycardia

• Minimal decrease in resp rate and tidal volume

• May cause panting

• May be hyper responsive

Minor effects

• Dogs may become hypothermic

• Cats become hyperthermic

• Sweating in horses

• Decreased urine production with urine retention

Adverse effects

• Anxiety, dysphoria

• Bradycardia — from increased vagal tone

• Decreased resp rate and tidal volume — dose dependent, worsens if given with other respiratory depressant drugs

• Salivation and vomiting

• Initial increased peristalsis then ileus

• Addiction

• Caution in use with head traumas or CNS disorders

Use

• Analgesic

• Premed

• Neuroleptanalgesia

• These are controlled agents (except naloxone)

Neuroleptanalgesia

• A state of profound sedation and analgesia induced by the simultaneous administration of an opioid and tranquilizer

• Can be used for sedation for minor procedures or to induce anesthesia (caution discussed later)

• Do not give rapidly IV

Opioid Antagonists

Drugs: Naloxone Routes: IV, IM Onset of action: 2 min IV, 5 min IM Duration: 30-60 min

Effects

• Reverses desirable and undesirable effects of opioids

Uses

• In cases of overdoses

• Emergencies

• Reversal of neuroleptanalgesia

• Neonates delivered by c-section