Download preanesthetic agents - Dr. Roberta Dev Anand

HOW DO YOU
CHOOSE YOUR
AGENT?
Choosing anesthetic agents is something
that is not taken lightly.
In addition to your minimum data base of
your history, physical exam, and
diagnostics, the following affect how an
agent should be chosen
CHOOSING YOUR
AGENT
AVAILABILITY
-An anesthetic agent is chosen by what is available to the
veterinarian/clinic.
-Most clinics will not have the ability to choose from every
anesthetic option.
-An anesthetic machine would not be practical to take on a
farm call. Injectables are used.
FAMILIARITY
-The safest anesthetic protocol is the safest one that the
doctor is familiar with.
-A doctor wouldn’t use the drug that he/she just read about
on a Class V anesthetic risk.
CHOOSING YOUR
AGENT
PROCEDURE
-A short-acting anesthetic should not be chosen for
a long procedure. An animal that is undergoing
major surgery requires more than a sedative.
-The anesthetic must be evaluated for duration of
action, amount of pain control, and facilitation of
restraint.
-Look at what the procedure is.. Drugs that are
acceptable for a spay may not be acceptable for
a C-section because of its affect on the fetuses
CHOOSING YOUR
AGENT
COST
-Some anesthetics are pricey. It is ok to use a
cheaper anesthetic, as long as it is as safe as
the other, more expensive options.
-Don’t substitute safety for cost.
SPEED
-If an emergency surgery is about to be performed,
it is not wise to choose a premedication that will
take 20 minutes to take affect.
*** The anesthetic protocol, dose, and route
are chosen by the veterinarian.
*** A vet/clinic may have a routine protocol,
but ALL PATIENTS ARE DIFFERENT and
the best anesthetic for a patient may not
be the one that is used frequently
*** Point out all of your findings – they will
help the veterinarian with drug selection
PREANESTHETIC AGENTS



Drugs that are administered to an animal before
general anesthesia.
Can either be a single drug or a combination of
drugs.
Dosages are adjusted according to the patient’s
need/health.
SO WHY ARE WE GIVING AN ANIMAL
DRUGS BEFORE IT IS ANESTHETIZED?
1) To calm or sedate an excited, fractious, or
vicious animal

This makes the patient more comfortable and your
job a lot easier

The dose is adjusted according to the patient’s
need and health status
-some animals require minimal dosing,
however some are unaffected by a high dose that
was given to them.
2) To counteract the effects of other
injectable or inhalant anesthetics

If the ideal agents being used for anesthesia produce
effects of hypersalivation (ketamine) or bradycardia
(opiods), atropine can be given because it can treat both
of these side effects
3) To reduce the amount of general
anesthesia required
The animal should not be so sedate from it’s
premedications that it can be intubated, but it will not
need a large amount of anesthetics to produce surgical
anesthesia.
 To use low doses of preanesthetics and general
anesthetics in combination to reduce the effects on the
animal from each is called BALANCED ANESTHESIA.

4) To reduce pain/discomfort
 If
the anesthetic procedure is brief, the
premedications may still be in effect during
recovery
BESIDES SURGERY AND
ANESTHESIA, PREANESTHETICS
CAN BE USED IN OTHER WAYS:
Calming nervous patients for travel, reducing
the aggressiveness during a physical exam,
minor procedures, thunderstorm anxiety,
antiemetics, cough suppressants.
OTHER THINGS TO NOTE:




Preanesthetics are usually given about 20
minutes before the induction anesthetic is given.
If you try to induce anesthesia too soon, you
may not see any effect from the pre-meds
Usually given IM or SQ. Many patients do not
have catheters in place until they are sedated,
depending on the protocol of the clinic. Most
preanesthetics are too potent to be given IV
Every preanesthetic has a side effect
All anesthetics are not safe for all animals
CLASSES OF PREANESTHETIC
AGENTS

ANTICHOLINERGICS

TRANQUILIZERS
AND SEDATIVES

OPIODS
ANTICHOLINERGICS
SO WHAT IS AN
ANTICHOLINERGIC?

The anticholinergics are also called
parasympatholytics, meaning they block acetylcholine
at the muscarinic receptors in the parasympathetic
portion of the autonomic nervous system.

The muscarinic receptors are in the heart, GI tract,
bronchi, glands, and eye. When the parasympathetic
system is activated by anesthesia, bradycardia, GI
stimulation, salivation, and pupil constriction may
occur.

Anticholinergics like ATROPINE and
GLYCOPYRROLATE can help to stop these effects.

Both ATROPINE AND GLYCOPYRROLATE can be given SQ, IM
(preanesthetic purposes) or IV (emergency bradycardia/cardiac arrest).

The IV dose is much lower than the SQ, IM doses

Atropine comes in a variety of concentrations that vary from 0.4mg/mL to
1/120 grain/mL to 15 mg/mL. When calculating and drawing up doses, be
sure you are using the correct concentration!

Often used in a preanesthetic mixture (often with Butorphanol and
Acepromazine)

Onset of SQ is 20 minutes, IV is 30 sec – 2 min.

Duration of action: 60-90 minutes. Will notice heart rate decreasing as it
wears off.

Can be mixed with most other premeds except Diazepam
EFFECTS OF ATROPINE
- Increases heart rate
-blocks stimulation of the vagus nerve
which causes increased
parasympathetic activity, bradycardia,
and reduced cardiac output.
-things that can stimulate the vagus
nerve during anesthesia and cause
decreased heart rate: endotracheal
intubation, handling of abdominal
organs in surgery, and other
anesthetic agents
-Atropine protects the heart, but care
should be taken because tachycardia can
occur. Do not use if the patient has CHF or
hyperthyroidism, or if an animal is
tachycardic before anesthesia (140 bpm for
dogs, 180 bpm for cats)
-Decreases salivation (antisialagogic
agent)
-Some agents cause increased salivation
(Ketamine). This can be problematic when
trying to intubate the patient and can even
compromise the airway.
-Reduces GI activity
-Inhibits peristalsis whereas other agents
can cause flatulence, vomiting, and diarrhea
-Don’t use if you know that the
patient has ileus- will make it
worse.
- Pupil
Dilation
-More common in cats with preanesthetic
dosages. Makes PLRs less reliable in
assessing anesthetic depth. More sensitive
to light.
-Reduces Tear Secretion
-Reduces tear production dramatically.
Contraindicated with glaucoma. ALWAYS
LUBRICATE THE EYES (no matter what anesthetic
agent is used). Eyes don’t close while they are
under anesthesia + decreased tears = dry corneas.
-Bronchodilation
-Causes increased dead space (parts of the
respiratory system that contain air, but
oxygen and carbon dioxide are not
exchanged. If dead space is too large,
animal can get hypoxic.
-Increased airway mucous
secretions in cats.
-may cause airway blockage
TOXICITY
 Overdose = drowsiness, excitement, dry
mucous membranes, ataxia, muscle
tremors, dilated pupils, hyperthermia, and
tachycardia
 Dogs are more susceptible
 Treated with physostigmine
GLYCOPYRROLATE
Similar to Atropine
 Effects last much longer (up to 3 hours
when giving SQ)
 Less tachycardia and arrhythmias seen
 Better antisialagogue
 More expensive
 Safer for pregnant animals
 Atropine is better at rapidly treating
bradycardia

TRANQUILIZERS AND
SEDATIVES
SO WHAT DOES A
TRANQUILIZER/SEDATIVE DO?
Tranquilizers (Diazepam) reduce anxiety,
but may not decrease awareness
 Sedatives (Medetomidine) reduce mental
activity and causes sleepiness
 The terms are often interchanged
 These drugs are given to calm the patient,
facilitate handling, have a quieter
induction, less stressful recovery

Do not leave these patients unattended
unless in a kennel.
 These animals can be very easily aroused
or aggressive is stimulated suddenly
 Act on the central nervous system
 3 CLASSES: PHENOTHIAZINES,
BENZODIAZEPINES, ALPHA-2
AGONISTS
 May cause 3rd eyelid prolapse or ataxia
 Only Alpha-2 agonists have analgesic
effects

PHENOTHIAZINES
Includes Acepromazine and
Chlorpromazine
 Not a controlled substance
 Do not cause significant respiratory or
cardiac depression
 Wide margin of safety
 May be combined with other agents
(commonly Atropine, Ketamine, and
Opiods)
 Can administer orally, SQ, IM, IV (not
common and with caution)

EFFECTS OF PHENOTHIAZINES

Sedation
 Affect
the reticular activating center of the
brain
 May last up to 24 hours at higher doses
 Less sedation seen in cats
 Not as deeply sedating as medetomidine
 Useful to also sedate for thunderstorms, travel

Antiemetic
Help prevent vomiting (Chlorpromazine)

Antiarrhythmic effect
 Can
antagonize arrhythmias caused by other
anesthetics

Antihistamine effect
 Prevent
the release of histamine and help
decrease allergic reactions. *Don’t use with
allergy testing

Peripheral Vasodilation
 May
get a reflex tachycardia to compensate,
hypothermia may accompany vasodilation,
hypotension may follow.

Personality Effects
 Occasionally
may see excitement that can
take 48 hours to resolve. Make owners aware.

Penile Prolapse
 Occurs

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in horses. Rarely permanent.
NO Analgesia
No reversing agent
Manufacturer’s listed dose may be as much as
50% too high
High doses do not cause increased levels in
sedation, but can induce significant hypotension
Reduce the dose in geriatrics, neonates, and
those with liver dysfunction
Caution in Boxers, hypotensive, anemic, or
dehydrated animals
BENZODIAZEPINES




Includes Diazepam (Valium), Zolazepam (in
Telazol), Midazolam
Use of Diazepam is off-label
Thought to exert their effects by releasing
GABA, in inhibitory neurotransmitter in the
brain
Diazepam and Midazolam can be used as
appetite stimulants in cats or to correct
inappropriate urination.
EFFECTS OF
BENZODIAZEPINES

Anti-anxiety, calming
Don’t cause significant sedation in healthy animals
unless used in combination with other drugs
 Doesn’t cause drowsiness, is still alert but is less
anxious.
 May get more effect in a geriatric or debilitated
animal.
 Enhances sedation of other drugs
 Relatively safe
 NO ANALGESIA


Skeletal muscle relaxation


Can be used to counteract rigidity seen with other
agents (ketamine)
Anticonvulsant
Excellent
 Used often, good for animals with a seizure history
 Can be combined with drugs that lower the seizure
threshold to counteract (NOT WITH SEIZURE
PATIENTS)


Minimal Adverse Effects
High margin of safety
 Good for geriatrics or high risks
 Flumazenil can reverse
 Not good for animals with liver dysfunction or Csections

 DIAZEPAM
 Preferred route of administration is slowly IV
(can cause arrhythmias if too quickly).
(can also give per rectum for seizing
animals)
 CAN ONLY BE MIXED WITH KETAMINE
-Equal volumes in same syringe,
prolonged storage is not
recommended (absorbed by syringes
and IV bags)
-Not water soluble, may precipitate out
with other meds
 Can’t induce anesthesia by itself
-usually administered with Ketamine
 CONTROLLED DRUG
MIDAZOLAM
 Can be mixed with other anesthetic agents
 More readily absorbed after IM or SQ routes
 See effects in 3 minutes
 Isn’t used as the only anesthetic agent
-Animal is not inducible or easy to restrain
 Combined with Ketamine, Thiopental, or an
opiod
 Minimal cardiovascular effects
ALPHA 2 AGONISTS






Xylazine and Medetomidine most commonly
used (xylazine often in large animals)
Also called thiazine derivatives
Fast onset, good effect, REVERSIBLE,
ANALGESIA produced, not controlled
Stimulate alpha 2 adrenoreceptors on
sympathetic nerves in the brain and spinal cord
causing decrease in norepinephrine. Also affects
receptors in the heart and blood vessels.
Result is sedation, analgesia, muscle relaxation
Standard recommended doses are only given to
young, healthy patients
EFFECTS OF ALPHA 2
AGONISTS



If combined with other agents, surgical
procedures can be performed (usually minor
procedures)
Sedation may last hours, but analgesia can be
as short as 20 minutes if another analgesic is
not used with it
Humans should be careful when administering


Can be absorbed through skin abrasions and mucous
membranes (causing hypotension and sedation)
Metabolized in the liver, excreted in the urine. Be
sure both systems are functioning adequately
before administering.
 XYLAZINE
 2% solution for small animals, 10% solution for
large animals
 If used with another anesthetic, the dose of the
other medication can be reduced up to 80%
 IV or IM gives best effect – avoid SQ
 MEDETOMIDINE
 Usually called DOMITOR
 Approved for dogs, but used in many other species
 Usually combined with Butorphanol to be given IV
(preferred) or IM
 Allow the drug 20 minutes to take effect – let the
animal be in a quiet environment
 Great sedative effect. Often combined with a local
anesthetic for minor surgical procedures
 TOO AWAKE TO INTUBATE (but you can
always provide oxygen in the mask)
 Can be suddenly aroused (especially with
deep ear cleanings or as medication is
wearing off)
 For a major surgery, a general anesthetic is
also used but at a much lower dose than
usual
ADVERSE EFFECTS

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
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Usually seen when given IV
BRADYCARDIA, reduced cardiac output, arrhythmias.
DON’T use in animals that are debilitated or have
heart disease
-usually best treated with reversal agent. Atropine
may not be as effective and may make the
animal hypertensive
Blood pressure initially rises, and then falls making the
animal potentially hypotensive
RESPIRATORY SUPPRESSION is common. Don’t use
with respiratory diseases
Cyanosis is sometimes seen (careful in Brachycephalics)
Can cause vomiting in a large # of animals (especially
cats and more so with Xylazine). Atropine may help to
prevent this

Xylazine depresses GI activity. Can cause
bloat in ruminants and GI stasis in dogs.
Use with caution or not at all in deep
chested breeds that are prone to GDVs.
Temporary behavior changes
 Reduces insulin secretion. May cause
transient hyperglycemia.
 Xylazine can increase intrauterine
pressure in cattle and may cause abortion
in the last trimester.

REVERSAL AGENTS

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


Commonly performed
Yohimbine reverses Xylazine. Administered IV and
results are seen within a few minutes.
Tolazoline also reverses Xylazine. Administered IV and
is more effective than Yohimbine in cattle. May cause
vasodilation and hypotension.
Both Tolazoline and Yohimbine should be administered
slowly. Tremors, excitement may result.
Medetomidine is reversed with Atipamezole
(Antisedan).



Given IM. Dose is 5 times greater than Medetomidine, but the
drug is 5 times more concentrated, so the volume amounts are
equal.
Cats are more sensitive, so they are given ½ of the reversal
dosage.
Reversal occurs within 20 minutes. Bradycardia and analgesia
should no longer exist. An analgesic is usually administered with
it.