Pre-Anesthetics/Muscle Relaxants Flashcards
Pre- and post-anesthetics (peri-anesthetic) are essential to safe anesthetic management. What is their main purpose?
minimize stress, cardiopulmonary depression, and the adverse effects produced by anesthetic administration
What is the purpose of using α2 adrenergic agonists as pre-anesthetics? What 5 are most commonly used?
produce sedation, stupor, analgesia, and muscle relaxation without producing general anesthesia in large and small animals
- Xylazine (Rompun)
- Medetomidine (Domitor)
- Dexmedetomidine (Dexdomitor)
- Detomidine
- Xylazine
What category of drugs are used to reverse α2 adrenergic agonists? What are 2 possible effects?
α2 antagonists
- no effect on respiratory system (beneficial)
- bradycardia (detrimental)
What pharmacological effects do α2 agonists have on the CNS, respiratory system, and GI system?
CNS - dose-dependent sedation, short-acting analgesia
RS: dose-dependent depression
GI: bloat in dogs, cattle, and horses
What unique early and late phase effects do α2 agonists have on the cardiovascular system?
EARLY: dose-dependent vasoconstriction and hypertension, bradycardia, and cardiac arrhthmias
LATE: decrease in cardiac output, hypertension, and further bradycardia
What are 7 other pharmacological effects of α2 agonists?
- muscle relaxation
- potentiates the effect of anesthetics
- vomiting as an immediate response in dogs and cats
- hyperglycemia
- hypothermia
- premature labor (last trimester)
- sweating in horses
In what patients should α2 agonists be avoided? What is commonly administered prior?
- geriatric
- diabetic
- pregnant
- pediatric
- ill
anticholinergics
What are some adverse effects that α2 agonists have on the CNS, cardiovascular system, and respiratory system?
CNS: change in behavior varying with species
CV: bradycardia, hypotension
RS: depression variable in different animals, more severe if given with other drugs
How is Xylazine used as a pre-anesthetic? What is its mechanism of action? What effect does this have?
sedative that provides pain relief and muscle relaxation in dogs
stimulates α2 receptors of the sympathetic NS, decreasing the release of NE
- no “fight or flight” response
- bradycardia
- hypotension
- hypothermia
Where is Xylazine metabolized and excreted?
METABOLIZED: liver
EXCRETED: urine
How is the effect of Xylazine different in ruminants? Which one in particular?
ruminants are more sensitive to the effects than most other species
goats are the most sensitive of ruminants —> low concentrations of small animal formulations should be used
How does Medetomidine (Domitor) compare to other α2 agonists? What is it registered for use in?
more potent, selective, and commonly used in vet med (safer than Xylazine)
registered for use in animals only, on prescription (most common in dogs and cats)
How are animals able to act when sedated with Medetomidine (Domitor)?
despite being completely sedated, animals can still move, kick, bite, or scratch in response to sharp stimulation
What are 4 possible adverse effects of Medetomidine (Domitor)?
- slowed heart rate
- low body temperature
- slowed breathing rate
- vomiting
In what 3 animals should Medetomidine (Domitor) usage be avoided?
- animals < 12 weeks of age
- animals in the last trimester of pregnancy
- animals with heart disease, low blood pressure, shock, breathing problems, severe liver/kidney disease, or known seizure disorders
What are 3 common effects of α2 antagonist overdose?
- excitement and muscle tremors
- hypotension and tachycardia
- salivation and diarrhea
What drugs are specifically used to reverse Xylazine and Medetomidine?
Yohimbine reverses the cardiovascular and sedative effects; used in dogs, cats, horses, and exotic species
Atipamezole
What is the purpose of pre-anesthetic anticholinergics? What is their mechanism of action?
prevent or limit salivary secretions, emesis, and bradycardia
parasympatholytic: block the action of Ach via M receptors
What are the 2 most common pre-anesthetic anticholinergics? How do their different properties affect their use? What animals is this class of drugs used on?
- Atropine - faster onset, shorter peak, shorter duration used in emergency situations and for bradycardia
- Glycopyrrolate - slower onset, longer peak, longer duration
dogs and cats; NO horses or rabbits
What are 5 common anticholinergic effects?
- limited effect on CNS
- prevent bradycardia
- decrease GI secretions
- mydriasis (dilated pupils) and corneal dryness
- bronchodilation to increase airflow to the lungs
What are 4 possible adverse effects of anticholinergics?
- cardiac arrhythmia in animals with elevated heart rates or cardiac disease
- temporary bradycardia
- thickened respiratory and salivary secretions that can lead to airway bockage
- intestinal peristalsis inhibition that leads to colic in horses and bloat in ruminants
What causes the different duration and onset of the 2 pre-anesthetic anticholinergics?
- Atropine - plant-derived, able to cross BBB
- Glycopyrrolate - synthetic, 5-6x potency, prolonged duration, doesn’t cross the BBB
What adverse effect is a common result of IV administration of Atropine?
slower heart rate - bradycardia
What is spasticity?
an increased muscle tone (spasm) common in a variety of neurological conditions
What is the goal to using pre-anesthetic muscle relaxants (spasmolytics)?
reduction of excessive muscle tone WITHOUT reducing muscle strength
What are the 2 classes of muscle relaxants?
- centrally acting
- peripheral acting (neuromuscular blocking agents)
What are centrally acting muscle relaxants (spasmolytics)?
drugs that act on CNS to mitigate tension and spasm of skeletal muscles by acting heterogeneously on a variety of receptors as CNS depressants
- sedative
- musculoskeletal relaxants
What is the site of spasmolytic actions of centrally acting muscle relaxants? What 3 classes of drugs are most commonly used in this way?
spinal cord
- Benzodiazepines - Diazepam (GABAa)
- GABA agonists - Baclofen (GABAb)
- α2 agonists - Tizanidine
What are common side effects of the pre-anesthetic spasmolytics, Diazepam, Baclofen, and Trizanidine?
DIAZEPAM: significant sedation, but milder than that produced by other sedative-hypnotic drugs at doses that induce equivalent muscle relaxation
BACLOFEN: less sedation that Diazepam
TIZANIDINE: asthenia, drowsiness, dry mouth, hypotension
How is Guaifenesin used as a pre-anesthetic spasmolytic? What are its 4 main effects?
IV with thiobarbiturates and ketamine for induction of anesthesia in horses, cattle, and swine
- mild sedation accompanies muscle relaxation
- mild analgesia
- decreased blood pressure in horses, with minimal effect on heart rate
- minimal respiratory effects
What components make up the Triple Drip protocol? What is its purpose?
- Guaifenesin
- α2 agonist
- ketamine
prolongs anesthesia up to an hour
What are 3 possible adverse effects of Guaifenesin?
- allergic reaction in horses
- overdose can cause bradycardia, hypotension, extensor rigidity, apnea, and cardiac arrest
- perivascular injection causes inflammation, necrosis, and sloughing at injection site
What is Mathocarbamol? What is it used for?
Guaifenesin analog
treats muscle spasms in dogs and cats - NOT pre-anesthetic
What is the mechanism of action of Baclofen? What is it typically used to treat?
GABAa agonist - mimics GABA within the spinal cord and works by depressing afferent reflex activity at the spinal cord level to reduce skeletal muscle spasm
urinary retention by reducing urethral resistance and gastroesophageal reflex
Baclofen has a mild margin of safety, making clinical use infrequent. What does Baclofen intoxication cause? What are some additional signs?
loss of gag reflex, putting dogs at high risk for aspiration
- excessive drooling leading to aspiration of saliva
- severe vomiting
- seizures
- cardiac arrest
