anesthetic agents and adjuncts

routes of administration

• inhalant

• injectable

• oral

• topical

• GV20 (SQ)

what are agents and adjuncts classified by

1. route of administration

2. principal effect

3. time of administration in the anesthetic period

4. chemistry

agonists

bind to, and stimulate target tissue

antagonists

bind to target tissue but DONT stimulate

• reversal agents

example of a partial agonist

Buprenorphine

example of an agonist-antagonist

Butorphanol

pre-anesthetic meds should provide ____ properties

anxiolytic

purpose of a balanced, multi-modal anesthesia

• reduce the MAC

• decrease dose of concurrent drugs to minimize adverse effects

• smooth induction and recovery

• provides preventive analgesia and muscle relaxation

what would be an IDEAL pre-med

1. provide reliable sedation and anxiolysis

2. provide analgesia

3. minimal effects of cardiopulmonary system

4. be reversible

which drugs can be given to prevent peri-anesthetic vomiting

• Famotidine (Pepsid)

• Maropitant (Cerenia)

• Metoclopramide

MOA of famotidine (Pepsid)

an antacid, Histamine H2 receptor blocker which decreases stomach acid

MOA of maropitant (Cerenia)

acts on the CNS vomiting center to prevent vomiting

MOA of metocloprimide

increases muscle contractions to increase gastric emptying

SQ, IM and IV onset of action and durations of effect

SQ - slowest onset, longest duration

IM - faster onset, shorter duration

IV - fastest onset, shortest duration

MOA of anticholinergics

prevent acetylcholine from binding to receptors, stopping action potentials from occurring

• parasympathetic drugs

examples of anticholinergics

1. Atropine

2. Glycopyrrolate

which anticholinergic has faster onset

Atropine - faster onset, shorter peak, shorter duration

which anticholinergic has slower onset

Glycopyrrolate - slower onset, longer speak, longer duration

main effects of anticholinergics

• prevent and treat bradycardia

• bronchodilation

• decreases secretions

• mydriasis and corneal drying

adverse effects of anticholinergics

• cardiac arrhythmias

• bradycardia

• thickened respiratory and salivary secretions

• inhibits intestinal peristalsis → may cause colic (horses) or bloat (ruminants)

when do you use an opioid in anesthesia

on induction when combined with other drugs to produce analgesia and sedation

routes of admin for opioids

IV, IM, SQ, PO, rectal, transdermal, subarachnoid, and epidural

opioid agonist examples

• morphine

• hydromorphine

• oxymorphine

• fentanyl

• meperidine

• carfentanil

• etorphine

MOA of pure mu agonists

bind to and stimulate mu and kappa receptors

• best for moderate to severe pain

where are mu and kappa receptors found

in the CNS, GIT, urinary system and smooth muscle

kappa - found in cerebral cortex and spinal cord

MOA of partial agonists

bind to and partially stimulate receptors

• strong affinity but low potency for certain receptors

• best for minimal to moderate pain

MOA of agonist-antagonist drugs

bind to mu and kappa receptors but ONLY stimulate kappa receptors (cerebral cortex and spine)

• best for minimally painful procedures

example of an antagonist opioid

Naloxone

MOA of an antagonist opioid

binds to but does NOT stimulate mu and kappa receptors

which opioids would you use for pre-anesthetic

• agonists

• partial agonists

• agonist-antagonist

what else can you combine an opioid with for pre-anesthetic

tranquillizers and anticholinergics

CNS effects of opioids

• anxiety

• disorientation

• excitements

• dysphoria

• analgesia

cardio and respiratory effects of opioids

• bradycardia

• panting in dogs

• decreased RR and tidal volume (Vt)

• decreased PaO2 and increased PaCO2

• ceiling effects with some agents

GI and other effects of opioids

• salivation and vomiting

• initial diarrhea and flatulence

• GI stimulation → colic in horses

• miosis (dogs) and mydriasis (cats)

• decreased urine production/urine retention

• addiction

• increased IOP and ICP

• drug interactions

neuroleptanalgesia

a profound state of sedation and analgesia induced by simultaneous administration of an opioid and a tranquillizer

neuroleptanalgesia combos

• pure mu agonist +

• acepromazine, diazepam, midazolam, zylazine, dexmeditomidine

when is neuroleptanalgesia used

1. sedation for minor procedures

2. premed for GA

3. induction for GA (not as common)

what can sudden analgesia loss lead to after admin of an opioid antagonist

excitement, anxiety, and sypathetic NS stimulation

duration of action of naloxone

30-60 min

when are antagonists used

in emergencies and overdoses

• to reverse neuroleptanalgesia

• to revive neonates delivered by C-secation

purpose of injectable anesthetics

• facilitate ETT intubation and deliver gas anesthetic

• create a multi-modal anesthesia to decrease the MAC

injectable anesthetics are ____ to effect

titrated

examples of injectable anesthetics

• propofol

• alfaxalone

• barbituates

what properties would an IDEAL induction agent have

1. provide analgesia

2. minimal cardiopulmonary effects

3. be reversible or short acting

when is the ketamine + diazepam combo used

IV induction in dogs and cats

onset and duration of ketamine + diazepam

onset: 30-90 sec

duration: 5-10 min

recovery: 30-60 min

advantages of ketamine + diazepam combo

• minimal cardiac depression

• good muscle relaxation

• some analgesia

disadvantages of ketamine + diazepam

• possible respiratory depression

• doesn’t work well IM

what must guaifenesin be administered with

Dextrose solution (5-10%) to prevent hemolysis

MOA of inhalant anesthetics

1. liquid anesthetic is mixed with O2 gas

2. delivered to patient and travels to lungs and is diffused into bloodstream

3. distributed to tissues is dependent on blood supply

4. depth of anesthesia is dependent on partial pressure of anesthetic in the brain

5. maintenance depends on sufficient quantities of anesthetic delivered to lungs

6. eliminated by lungs

what does the rate of diffusion depend on for inhalant gases from the alveoli to capillaries

concentration gradient and lipid solubility

when is the concentration gradient the greatest

during initial induction

CNS, cardio and respiratory effects of inhalant anesthetics

• reversible CNS depression

• hypothermia

• increased ICP in head trauma patients

• decreased cardio function

• vasodilation, decreased BP and may decrease renal blood flow

• HR effects vary

• dose dependant ventilation depression

• CO2 retention and respiratory acidosis

vapor pressure

the tendancy of an inhalation anesthetic to vaporize to its gaseous state

• the amount of pressure everted by the gas form at equilibrium

what does vapor pressure determine and what does it depend on

how readily an inhalant will evaporate in the vaporizer

depends on:

• temperature and the agent

volatile agents

HIGH vapor pressure

• Iso, sevo, des, and halothane

• delivered from a precision vaporizer

non-volatile agents

LOW vapor pressure

• methoxyflurane

• delivered by non-precision vaporizer

Blood-Gas Partition Coefficient (BGPC)

the measure of solubility of an inhalation anesthetic in blood compared to alveolar gas

what does the BGPC indicate

the speed of induction and recovery for an inhalant agent

low BGPC

agent is more soluble in alveolar gas than in blood at equilibrium

• faster induction and recovery

high BGPC

agent is more soluble in blood than in alveolar gas at equilibrium

• slower induction and recovery

minimun alveolar concentration (MAC)

the measure of potency of the inhalant

what is the MAC used to determine

the average setting on the vaporizer that produces surgical anesthesia

• the lower the MAC = the more potent to agent

isoflurane’s BGPC and MAC

BGPC: 1.46

MAC: 1.3% (dogs), 1.63% (cats)

effects of isoflurane

• maintains cardiac output, HR and rhythm

• decreases resp system

• maintains cerebral blood blow

sevoflurane’s BGPC and MAC

BGPC: 0.68 → faster induction and recovery

MAC: 2.34% (dogs), 2.58% (cats) → less potent than iso

desflurane BGPC and MAC

BGPC: 0.42 → very fast induction and recovery

MAC: 7.2% (dogs), 9.8% (cats) → least potent

what are the most common alpha-2 agonists we use

1. dexmedetomidine

2. xylazine

3. detomidine

MOA of alpha-2 agonists

stimulate alpha-2 receptors on SNS

explain the biphase effect of dexmeditomidine

phase 1 - increased BP caused by vasoconstriction

• decreased CO

• vasoconstriction → baroreceptor response → bradycardia (dogs)

phase 2 - less vasoconstriction; BP returns to normal or just below normal

• bradycardia maintained until reversal given

• arrhythmias, less CO

• dose dependent, predictable effects

how are most alpha-2’s metabolized

in the liver, excreted in the urine

why should you avoid giving an anticholinergic under sedation after giving an alpha-2

arrhythmogenic

Is dexmedetomidine or xylazine more potent

dexmedetomidine

side effects of xylazine

• noise sensitivity

• vomiting

• hyperglycemia

• hypothermia

• polyuria

• premature parturition (cattle), sweating (horses)

general side effects for alpha-2 agonists

• sedation, analgesia, bradycardia, ± hypotension, hypothermia

• change in behaviour

• bloat (dogs, cattle, horses)

• salivation and regurgitation

examples of alpha-2 antagonists

• atipamezole

• yohimbine

• tolazoline

MOA of alpha-2 antagonists

reverse all effects of the alpha-2 agonists

side effects of alpha-2 antagonists

• reverse analgesia and sedation

• reverse bradycardia

overdose effects:

• excitement and muscle tremors

• hypotension, tachycardia

• salivation, diarrhea

what is the reversal for dexmeditomidine, what is the dose

atipamezole (Antisedan)

• 10:1 → dose is 1/10th the dose of the agonist

what is the reversal for xylazine, and the dose

Yohimbine

• 10:1 (dogs/horses)

• 2:1 (cats)

examples of barbituates

1. Pentobarbital

2. Phenobarbital

which barbituate is short/long acting

short: pentobarbital

long: phenobarbital

what does pentobarbital treat

status epilepticus in small animals

what does phenobarbital treat

long term seizure medication

examples of benzodiazepines (controlled drugs)

1. Diazepam (not water soluble)

2. Midazolam (water soluble)

side effects of benzodiazepines

• calming and anti-anxiety (old and ill)

• disorientation and excitement (young and healthy)

• dysphoria and aggression (cats)

• NOT analgesic

• anticonvulsant

• minimal cardiopulmonary effects

what are benzodiazepines used for

• minor tranquillizers

• potentiate GA effects

• appetite stimulant in cats and ruminants

• skeletal muscle relaxant

examples of dissociatives (controlled drugs)

1. Ketamine

2. Tiletamine

MOA of dissociatives

decrease wind-up through NMDA inhibition

when can you use dissociative for anesthesia

• can be used alone or with tranquillizers, benzo’s or opioids to induce GA

• can use CRI for analgesia

CNS side effects of dissociatives

• trance-like state → appears awake but immobile and unaware

• painful on IM injections

• cataleptoid state

• intact reflexes

• analgesia

cardio and respiratory side effects of dissociatives

• increased HR, CO, and MAP

• may decrease inotropy

• arrhythmias

• apnea at higher doses

• increased salivation

what is tilatemine combined with

zolazepam (Telazol) - benzodiazepine

• reduce seizure risk

• skeletal muscle relaxation

• less apnea than ketamine

how are dissociatives metabolized

in the liver or excreted unchanged in urine

route of admin: IV or IM

when are dissociatives contraindicated

• cranial trauma

• increased CSF pressure

• ocular surgeries

example of a phenothiazine

Acepromazine

side effects of a phenothiazine

• calming

• reduces seizure threshold

• aggression or excitement

• vasodilation → hypotention

• increased HR, decreased CO

• hypothermia

• anti-arrhythmic properties

• antiemetic properties

• penile prolapse in horses

when would you use a phenothiazine

• pre med sedation

• to decrease MAC

• used with opioids for minor procedure

disadvantage of a phenothiazine

there is no reversal agent

examples of ultra-short acting non-barbituates

1. Propofol

2. Alfaxalone

3. Etomidate