Premedication Dr. Ambrisko Flashcards
what is sedation
induction of CNS depressioun & drowsiness by use of drugs (not unconsciousness)
What is tranquillization
used somewhat synonymously to sedation
what is analgesia
aka antinociception
loss or reduction of pain sensation
what is general anesthesia
controlled & reversible loss of consciousness, nociception & mobility
what is balanced anesthesia
using multiple drugs to achieve the goals of general anesthesia w/ less side effects than using a single agent (e.g. a combo of an anesthetic, analgesic & muscle relaxant)
what is dissociative anesthesia
form of general anesthesia characterized by a catatonic state
what is neuroleptanalgesia
state similar to general anesthesia produced by a sedative & an analgesic agent (e.g. phenothiazine + opioid)
- very deep sedation + very deep pain mgmt*
- usually no unconsciousness*
what is inhalationa anesthesia
general anesthesia using inhalational drugs
what is TIVA
Total Intravenous Anesthesia
achieved soley by injectable drugs
what is PIVA
combination of injectable drugs & lower concentration of inhaled anesthetic agent
basically balanced anes!
what is local anesthesia
loss of sensation in a smaller, circumscribed body area
what is regional anethesia
loss of sensation in larger but limited body area
what is purpose of premedication
sedation & analgesia
anesthetic sparing effect
reduction of stress & catecholamine release
types of premedication d2rugs
anticholinergics
alpha2 agonists
phenothiazines & butryophenones
benzodiazepines
opiods
muscle relaxants
antihistamines & antibiotics
what do anticholinergics do
name 2
inhibit PSNS
antagonists on muscarinic ACh receptors
Atropine
Glycopyrrolate
Atropine
solubility
route
crosses BBB & placental barrier
lipid soluble
IM, SC, PO, IV
Glycopyrrolate
solubility
route
onset slower & duration longer than atropine
doesn’t cross BBB & placenta
H2O soluble
IM SC, PO
indications for use of anticholinergics
Routine use not recommended
incr HR
- opioid induced bradycardia*
- tx of reflex bradycardia*
- young & brachycephalic breeds*
decr salivation & bronchial seccretion
contraindications for anticholinergics
tachycardia
hyperthyroidism
most cardiac dz except when tx bradycardia
narrow angle glaucoma
CV side effects of anticholinergics
2° AV block
bradycardia
cardiac arrest
tachycardia
hypertension
what happens if you combine an alpha2 agonist + atropine
vasoconstriction
tachycardia
hypertension
T or F
Routine co-administration of an alpha2 agonist & anticholinergic is contraindicated
TRUE!
other effects of anticholinergics
relaxes lower esophageal sphincter (regurg)
mydriasis (may trigger acute glaucoma)
bronchodilation (increases airway dead space)
dries airway secretions
Intestinal paralysis (may cause colic in horses)
CNS toxicity: crosses BBB may cause sedation, coma
Tx w/ physostigmine (not neostigmine)
What is special about rabbits & atropine
most rabbits have high levels of atropinase enzyme so atropine not effective
Glycoyrrolate is preferred!
What are alpha2 agonists
MoA
strongest available sedatives (except pigs)
have important CV side effects
have specific antagonists
competitive agonist of alpha2 adrenergic receptors
suppress NE release by negative feedback
what are effects of alpha2 receptors
cause sedation
analgesia
reduction of sympathetic outflow from brain
decr stress response
where are important (to anesthesia) alpha2 receptors located & what do they do
wall of arteries & veins (along with alpha1) - mediate vasoconstriction
pancreas ß cells - inhibit insulin release ⇒ hyperglycemia
what are CNS effects of alpha2 agonists
sedative effect spp specific:
strong: dogs, cats, horses, ruminants
weak: pigs (alpha2 + benzo + opioid best combo)
some anlagesic effects synergistic w/ opioids
muscle relaxation
what are CV effects of alpha2 agonists
strong vasoconstriction ⇒ reflex bradycardia ⇒ low CO & tissue perfusion
what are respiratory effects of alpha2 agonists generally
in horses
mild respiratory depression
In horses: upper airway resistance increases
- relaxation of larynx, pharynx & narse
- head dropping in horses: nasal edema
- V/Q mismatch
what are respiratory effects of alpha2 agonists in ruminants
Bronchoconstriction
V/Q mismatch
lung edema & hypoxemia
any alpha2 in any ruminant spp
xylazine in sheep is contraindicated
xylazine spp differences, issues
duration
Equine: better than for other spp
intraarterial injection may cause excitement (seizure) Don’t do it!
Cattle: sensitive ≈ 10% of horse dose needed
may cause uterine contraction & abortion in cattle
Dogs: less preferred alpha2 for this spp
duration: 20-40 min
indications for use of alpha2 agonists
sedation of aggressive animals
contraindications for alpha2 angonists
hemodynamic instability
not suitable for risk patients
Alpha2 agonists/antagonists (reversal agents)
Xylazine/Yohimbine (LA w/ exceptions)
Dexmdetomidine/Atipamezole
Medetomidine
Detomidine
Romifidine
In the order of highest to lowest, what are the alpha2/alpha1 receptor specificity of the alpha2 agonists?
Dexmedetomidine >>> detomidine > xylazine
the main effects are very similar even with the specificity differences
In the order of highest to lowest, what are the alpha2/alpha1 receptor specificity of the alpha2 antagonists?
atipamezole >>> yohimbine >> tolazoline
yohimbine & tolazoline are “dirty drugs” acting on other receptors & have many side effects
Use atipamezole to antagonize any alpha2 agonist whenever possible, give IM except for emergency (IV)
Medetomidine
Not available in US
50:50 racemic mix of demedetomidine & levomedetomidine
Dexmedetomidine 2 x as potent as medetomidine
Dexmedetomidine
most specific drug for alpha2 receptors
mostly for SA use but may be used in horses
Detomidine
RoA
Duration
LA use
IM, IV, sublingual
90-120 min
Romifidine
use
RoA
duration
used in horses
IM, IV
45-90 min
thought to produce less ataxia than xylazine, preferred for standing dental surgery
Alpha2 agonists In order of duration of action (minutes) from shortest to longest
Xylazine (20-40) < Romifidine < (45-90) < Detomidine (90-120) < Dexmedetomidine (30-120)
What are the premeds in the phenothiazine class
how do they work?
Acepromazine
Chlorpromazine, etc
antagonist of multple receptor types:
Dopamine
Serotonin
Alpha1
Histamine
Acepromazine
duration of action
4-8 hours
48 hours in liver pts
CNS effects of phenothiazines
weaker sedative effects compared to alpha2 agonists
NO analgesic effect
antiemetic
mild respiratory depression
CV effects of phenothiazines
Alpha1 antagonist:
causes vasodilation & hypotention
other effects of phenothiazines
antihistaminic
anty arrhythmogenic
inhibits platelet function
Penile prolapse in horses
seizures?
Indications for phenothiazines
Contraindications
mild sedation for premed or post op
prevent/tx opioid dysphoria
prevent emesis by morphine
sedation of dogs w/ laryngeal paralysis
enhance sedation of xylazine in horses
- Hypovolemia/hemodynamic instability*
- vWD in dobes*
- Boxers (bradycardia)*
- Breeding stallions (penile prolapse)*
What drug family is similar to phenothiazines
drugs in family
spp most used with
effects/side effects
Butyrophenones
Droperidol & azaperone (Stresnil®)
Azaperone used for Pigs
sedative similar to aceprom
less hypotensive, stronger antiemetic than aceprom
less platelet effect
antiarrhythmogeni, not seizurogenic
More likely to cause behavioral side effects
Characteristics of benzodiazepines
GABA receptor agonists
Sedative, anticonvulsant, muscle relaxant effects
Minimal CV & respiratory effects
NO ANALGESIA!
What are GABA receptors
GABA main inhibitory neurotransmitter in CNS
Benzodiazeines allosterically activate (no maximal effect)
Site of most anesthetics (notable exception is ketamine!)
Benzodiazepines used in vet med
Agonists
Midazolam
Diazepam
Zolazepam (Zoetil or Telazol)
Antagonists
Flumazenil
spp specific sedation effects of benzos
Dogs, cats, horses: disorientation, excitation may occur when used alone
Better sedative effects in ruminants, camelids, pigs, birds & ferrets
Rarely used alone
indications for use of benzos
as Premed:
Combine with opiods a/o alpha2 agonists
for Induction:
Combine w/ dissociatives, barbiturates or propofol
Tx of seizures
Diazepam characteristics
RoA
duration
cautions
Lipid soluble
formulated in propylene glycol or lipid emulsion
chemical compatibility limited
IV, poor absorbtion & pain IM
hepatic metabolism
1-4 hours
Oral may cause liver damage in cats
Midazoam characteristics
RoA
duration
caution
Water soluble (no propylene glycol)
Good chemical compatibility
more potent than diazepam
hepatic metabolites inactive
IM, IV or TM
Opioid characteristics
strongest available systemic analgesics
best choic for tx of acute pain
decr anesthetic dose
MINIMAL CV effects
Suitable for most risk pts
Types of opioid receptors
Mu (µ): strong analgesis, resp. depression, dependency
Kappa (K): weaker analgesia
Delta (ð): weaker (human)
classification of opioids
Full agonists: activate receptors & trigger full tissue response
Partial agonists: activate receptors but do not trigger full tissue response even at high doses (butorphanol)
Antagonists: bind to receptors but do not activate them
Mixed agonist antagonist: activate one receptor type & inhibit another one
e.g. Butorphanol mu antagonist & kappa agonist
CNS effects of opioids
Analgesia
decr MAC
spp dependent:
primates > dogs > cats > pigs > horses
may incr MAC in horses!
T or F
Opioids may trigger or inhibit vomiting
TRUE
stimulate CRTZ outside BBB triggering vomiting then cross BBB & inhibit
water soluble opioids (morphine) enter BBB slowly: cause more vomiting
lipid soluble (fentanyl) enter BBB fast: no vomiting
CV effects of opioids
Minimal CV effects
No direct negative inotropy or vasodilation
Suitable for most risk pts
Respiratory effects of opioids
May depress
Antitussive effect
May inhibit airway protective reflexes (coughing)
GIT effects of opioids
defecation
obstipation
Other effects of opioids
Hypothermia
Cats: post OP hyperthermia
Myosis (dogs), mydriasis (cats)
inhibition of urination
noise sensitivity
indications for opioids
Premedication alone or in combo
Perioperative analgesia
Tx acute & chronic pain
RoA of opioids
IM, SC
IV/CRI
PO
Epidural/spinal
Transdermal (fentanyl)
Oral (buprenorphine for cats)
Morphine
Preferred
full mu agonist
water soluble
slow onset (30-45min) long duration (4-6 hr)
May cause hitamine release esp post high IV dose
hydromorphone, oxymorphone
full mu agonist
4 hr duration
No histamine release
Methadone
similar to hydromorphone
also acts as NMDA antagonist!
Fentanyl
full mu agonist
fast onset, short duration (15-20 min)
no histamine release
may accumulate after long infusions
Remifentanil
similar potency to fentanyl
very short acting (5 min), does not accumulate
Butorphanol
mu antagonist, kappa agonist
may worsen pain in case of strong pain
weak & short acting analgesic
may antagonize full mu agonists
Buprenorphine
partial mu agonist
stronger analgesic than butorphanol but weaker than full mu
slow onset (20-40min IV) long acting 6-8 hours
often given to cats because may cause less excitation than full mu agonists
Owners can give TM
Tramadol
Weak analgesic
hepatic metabolite is mu opioid agonist
tramadol itself inhibits NE & serotonin reuptake (analgesia)
PO
Opioid antagonists
Naloxone
30 min duration
Naltrexone
long acting (≈10 hrs)
used to antagonize wild animals after long acting opioids e.g. carfentanyl or etorphine
Muscle relaxants
Centrally acting:
Guaifenesin
Peripherally acting
NMBD’s
Dantrolene
Guaifenesin
aka glyceryl guaiacolate (GG)
large animal anesthesia
minimal CV or resp effects
NO ANALGESIA or Unconsciousness SHOULD NOT be used as sole agent!
most common: 5% GG in dextrose
>10% = thrombophlebitis
Part of TIVA “Triple Drip” : GG + alpha2 + ketamine
Dantrolene
muscle cells
used for tx Malignant hyperthermia (pigs mainly)
