Preanesthetic medications

Anticholinergics

noncontrolled drugs that are most commonly used to prevent and treat bradycardia and to decrease salivary secretions arising from parasympathetic stimulation (atropine + glycopyrrolate most commonly used)

Anticholinergics mode of action

block binding of the neurotransmitter acetylcholine at the muscarinic receptors, which prevents bradycardia, bronchoconstriction, excess tear and saliva production, excess production of respiratory system secretions, increased GI motility, and miosis

Atropine

Anticholinergic - used to treat bradycardia. After IM injection, begins to act in about 5 minutes, reaches peak effect in about 10 to 20 minutes, and has a duration of action of 60 to 90 minutes. Give 20-30 mins prior to surgery.

(Or give IV for emergency treatment of Bradycardia during CPR. Onset of action is 1 min, peak effect is 3-4 minutes post injection)

Glycopyrrolate

Anticholinergic - used to decrease salivary secretions. Onset of action 5 minutes, reaches peak effect in about 30 to 45 minutes, duration of action 2-3 hours (may last up to 7). Give 20-30 mins prior to induction

Anticholinergics ADVERSE effects/ contraindications

arrhythmias, thick mucous secretions within the airways of cats,

Contraindications: avoid in animals with preexisting rapid heart rates or heart disease, do not use with ruminants unless intraoperative CPR: saliva becomes thick and ropy; may cause increased anatomic dead space (increased diameter of bronchioles), Mydriasis, Inhibition of peristalsis

Tranquilizers and sedatives

Tranquilizer: drug that reduces anxiety but does not necessarily decrease awareness and wakefulness (diazepam)

Sedative: a drug that causes reduced mental activity and sleepiness (dexmedetomidine)

Acepromazine

Phenothiazine (sedative):

Ace provides sedation, decreases the dose of general anesthetic required, and eases induction and recovery; used alone or in combo with opioids for minor procedures (wound treatment, grooming, radiography) in dogs, cats, horses. no reversal agent available

depression of the reticular activating center of the brain and blockage of alpha-adrenergic, dopamine, and histamine receptors

Onset of action: 15 minutes after IM injection in dogs or IV injection in horses, and peak effect occurs within 30 to 60 minutes. The duration of action is 4 to 8 hours in small animals but may be longer (up to 48 hours); DOA in horses is 1-3 hours.

Acepromazine (Effects/adverse effects)

• Calming, sedation, reluctance to move

• Reduction of the seizure threshold

• Tachycardia or bradycardia

• Antiarrhythmic effects

• Peripheral vasodilation

• Hypotension

• Antiemetic effects

• Penile prolapse in horses and other large animals

• Decreased PCV

Benzodiazepines

“minor tranquilizers”- used for muscle relaxant and anticonvulsant properties (unreliable sedative effects - may instead cause dysphoria, excitement, and ataxia, especially when administered to young, healthy animals.) Schedule IV controlled, reversible drugs.

Benzodiazepines- mode of action

depress the CNS.

relatively rapid onset of action (less than or equal to 15 minutes after IM injection for midazolam and zolazepam) and short duration (1 to 4 hours). Diazepam is not water soluble, can only be mixed with ketamine, can be absorbed by plastic syringes over time.

Benzodiazepines - effects/ADVERESE effects

Major effects and adverse effects of benzodiazepines are as follows:

• Antianxiety and calming only in old or ill patients

• Anticonvulsant activity

• Disorientation and excitement in young, healthy dogs

• Dysphoria and aggression in cats

• Muscle fasciculations in horses

• Ataxia or recumbency in large animals

• Few cardiopulmonary effects

• Skeletal muscle relaxation

• Pain on IM injection of diazepam

Alpha2-Adrenoceptor Agonists (Alpha2-agonists)

group of noncontrolled, reversible agents used alone and in combination with other anesthetics and adjuncts in both large and small animal patients for sedation, analgesia, and muscle relaxation

examples: Xylazine, Dexdomitor

commonly given before minor procedures such as radiography, wound treatment, or bandaging and subsequently reversed with an alpha2 antagonist.

Alpha 2 agonists: mode of action

act on alpha2-adrenoceptors of the SNS (sympathetic nervous system) both within in CNS and peripherally causing a decrease in the release of the neurotransmitter norepinephrine

causes sedation, analgesia, bradycardia, hypotension, and hypothermia.

metabolized in the liver, and the metabolites are excreted in the urine (contraindicated in animals with compromised renal or liver function)

after injection sedation happens rapidly: 5 to 15 minutes after IV injection or 15 to 30 minutes after IM injection) and lasts about 1 to 2 hours in most.

Alpha 2 agonists: Effects and adverse effects

• Dose-dependent sedation that can be profound

• Analgesia

• Agitation or aggression when touched

• Reaction to loud noises

• Muscle tremors in horses

• Cattle often lie down

• Initial hypertension, bradycardia, and pale mucous membranes followed by hypotension, decreased cardiac output, and a further decrease in the heart rate

• Severe decrease in heart rate, blood pressure, cardiac output, and tissue perfusion especially when high doses are given

• Respiratory depression that can be severe

• Muscle relaxation

• Vomiting in dogs and cats

• Adverse GI effects including bloat and colic

• Hyperglycemia

• Hypothermia

• Increased urination

• Premature parturition in cattle

• Horses may sweat

Xylazine

alpha 2 agonist: used alone or in various combinations with ketamine, opioids, and other agents in dogs, cats, and other small animals. It has now been largely replaced by dexmedetomidine in these species. It is still used in large animals as a preanesthetic and sedative, in combination with butorphanol for minor procedures, and in combination with ketamine and guaifenesin (a combination known as “triple drip”) to produce total IV anesthesia.

Dexmedetomidine

most commonly used alpha2-agonist in dogs and cats, having greater potency and fewer adverse effects than xylazine; for sedation, analgesia, and preanesthesia

antagonist: Antisedan

better with iso compared to ace

used in combo with other drugs like tord (i.e. DKT mixed in same syringe)

used for minor procedures like bandage changes, rads, each flush

Detomidine

used in horses to produce sedation, analgesia, and muscle relaxation

-similar to xylazine but twice as long duration of action

-commonly given with butorphanol for standing sedation

-used for analgesia with colic pain

Romifidine

similar to other drugs in this class (alpha 2 agonists)

-produces less ataxia than either detomidine or xylazine

Alpha 2- ANtagonists

reverse the effects of alpha 2 agonists by binding to the receptors thus reversing the effects of the drug

Yohimbine + tolazoline (used to reverse the effects of xylazine), and atipamezole (used to reverse the effects of dexmedetomidine)

Alpha2 ANtagonists - adverse effects

Adverse effects include excitement, muscle tremors, hypotension, tachycardia, salivation, vomiting, and diarrhea.

Anticholinergics should not be given due to possible tachycardia already noted with anticholinergics