Injectable Anesthetics - Dr. Roberta Dev Anand
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Transcript Injectable Anesthetics - Dr. Roberta Dev Anand
Injectable Anesthetics
Ch. 3
Injectable anesthetics
Can produce unconsciousness when given alone
In general, don’t provide analgesia or muscle relaxation
Administered IV, titration method
Classes include:
Barbiturates
Non-barbiturates
Dissociatives
Injectable anesthetics
Barbiturates
Ultra-short acting
Short-acting
Intermediate acting
Long-acting
Non-barbiturates/Hypnotics
Propofol
Etomidate
Dissociatives
Ketamine
Tiletamine
BARBITURATES
ULTRA-SHORT ACTING
SHORT ACTING
INTERMEDIATE ACTING
LONG ACTING
Barbiturates
Derivatives of barbituric acid
Controlled
No analgesia
No reversal agent
Classes of Barbiturates
Based on duration of action
Ultrashort
Thiopental sodium, methohexital, and thiamylal
Used in dogs, cats, horses
Short
Pentobarbital
Used in laboratory animals
May be used to treat status epilepticus and for
euthanasia
Classes of Barbiturates
Most intermediate and long acting barbiturates
are no longer used as anesthetics
Intermediate
Long-acting
Phenobarbital – used as a sedative &
anticonvulsant
Barbiturates
Subclasses based on chemical structure
Oxybarbiturates
Phenobarbital, pentobarbital, and methohexital
Thiobarbiturates
Thiopental and thiamylal
Action of Barbiturates
Mimics the inhibitory neurotransmitter GABA
Depresses nerve impulses to cerebral cortex resulting in
CNS depression and loss of consciousness
Termination of action
After agent leaves brain:
Is metabolized, excreted, or redistributed
Pharmacodynamics of Barbiturates
Factors that affect potency, onset, and duration of action
Ionization
Non-polar (non-ionized) forms pass through the cell membranes
Acidosis (blood pH <7.4)
Increases non-ionized form of the drug
Increased drug amounts to brain
Exaggerated patient response
Lower doses should be used to anesthetize an acidotic animal
Pharmacodynamics of Barbiturates
Factors that affect potency, onset, and duration of action
Protein binding
Travels in blood bound to proteins
Free (unbound) drug enters the brain
So…
Hypoproteinemia results in more free drug
More drug goes to the brain
Normal drug dose may actually produce prolonged
unconsciousness or death
Pharmacodynamics of Barbiturates
Factors that affect potency, onset, and duration of
action
Lipid solubility (partition coefficient)
Is the tendency of the drug to dissolve in fats,
oils, and lipids
Affects the ability to penetrate the cell
membrane fatty layer
Pharmacodynamics of Barbiturates
High solubility results in ultra–short-acting drug
Passes into the brain cells more quickly=faster onset of action
High solubility results in rapid tissue redistribution
Moderate solubility results in short-acting drug
metabolized by the liver – takes longer than redistribution
Low lipid solubility results in long acting drug
excreted primarily through the kidneys – longest process
Redistribution: How it works
Ultrashort acting Thiopental sodium is given IV. It then travels to the
brain (vessel rich). It is highly lipid soluble and crosses into brain cells
quickly.
Patient is now unconscious ~30 seconds
Once the levels in the brain are higher than in the blood, the
molecules will move back down the concentration gradient
Drug re-enters circulation
Redistributes to muscle, fat and other body tissues
Patient begins to recover in 10-15 minutes
Over the next couple of hours thiopental is released from muscle and
fat and eliminated from the body by liver metabolism and excretion of
metabolites in the urine
Barbiturate Redistribution
Variations of
Barbiturate Redistribution
Thiopental—ultra–short-acting
Released from muscle and fat and metabolized by liver,
excreted by kidneys
Continuous or repeated dosing may lead to “full” muscle
and fat and increased brain levels = prolonged recovery
and possible death
Methohexital—ultra–short-acting
Released from muscle and fat but metabolized faster
Muscle and fat don’t get “full” so there is no prolonged recovery with
continuous or repeated doses
Variations of
Barbiturate Redistribution (Cont’d)
Phenobarbital—long acting- low lipid solubility
Sustained effect caused by slow uptake and release from the brain
Release is dependent on kidney excretion, which is slowest
Pentobarbital—short acting- intermediate solubility
Brain levels decrease based on liver metabolism
Faster than kidney excretion
Barbiturates
EFFECTS:
ADVERSE EFFECTS:
EFFECTS:
ADVERSE EFFECTS:
EFFECTS:
ADVERSE EFFECTS:
Barbiturates
Effects on sighthounds:
Effect on critically ill animals:
Effect on tissues:
Barbiturates
Effects during induction/recovery:
Interaction with other drugs:
Thiopental
Ultra–short-acting
Small animals and horses
duration of action:10-15minutes
Reconstitute with sterile water, normal saline, or 5%
dextrose in water
Shelf life: 1 week refrigerated or 3 days at room temperature
Don’t use if a precipitate is present
Sighthounds – avoid use
Methohexital
Ultra–short-acting
Can be useful on an unfasted animal
Rapid induction and intubation
Decreased risk of vomitus aspiration
A powder that must be reconstituted (sterile water)
Shelf life—6 weeks without refrigeration
More expensive than thiopental
Can be used in sighthounds
Excitement and seizures during induction and/or recovery
Premedicate with tranquilizer
Control postoperative seizures with diazepam IV
Don’t use in animals with epilepsy
Pentobarbital
Short acting
Largely replaced by propofol
Administered IP to rodents for general anesthesia
Status epilepticus- treatment, persistent seizure
Administer IV to stop seizure and produce heavy sedation
Narrow margin of safety
Euthanasia
NON-BARBITURATES
PROPOFOL
ETOMIDATE
Propofol
Ultra–short-acting, non-barbiturate anesthetic
Most commonly used injectable anesthetic
Give IV for anesthetic induction and short-term maintenance
affects GABA receptors similar to barbiturates
Other use
IV bolus and CRI to treat status epilepticus in dogs and cats
Propofol
Available in an egg lecithin/glycerin/soybean oil aqueous
solution
Milky white appearance
Highly lipid soluble rapid onset, re-disribution, and rapidly
metabolized
Onset of action:30-60 seconds
Duration of action:5-10 minutes
Complete recovery
20 minutes in dogs
30 minutes in cats
Effects of Propofol
EFFECTS:
ADVERSE EFFECTS:
EFFECTS:
ADVERSE EFFECTS:
Effects of Propofol
EFFECTS:
ADVERSE EFFECTS
EFFECTS:
Other Effects of Propofol
Some dogs may exhibit muscle twitching during
induction
Safe in animals with liver or kidney disease because of
its rapid metabolism
Use of Propofol
IV slowly, give ¼ dose every 30 seconds, but don’t give
too slowly because it might cause excitement
IM administration produces mild sedation and ataxia
only
Highly protein bound
Don’t use in hypoproteinemic animals
Propofol Handling and Storage
Poor storage characteristics
Egg lecithin, glycerol, and soybean oil support bacterial
growth
Use aseptic technique- always write time and date on bottle
Discard unused drug within 6 hours of opening
May keep in refrigerator up to 24 hours*
more expensive than ketamine-diazepam or thiopental
Now there is propofol-28
Lasts up to 28 days
Etomidate
Noncontrolled, ultra short acting nonbarbiturate, sedative-hypnotic
Used for induction—dogs, cats, exotics
Minimal effects on the cardiovascular and respiratory systems
Expensive
Pain with IV injection
Etomidate Mode of Action
Similar to barbiturates and propofol
Increased GABA inhibitory action- hypnosis with a
little analgesia
Wide margin of safety
Effects of Etomidate
EFFECTS:
ADVERSE EFFECTS
EFFECTS:
ADVERSE EFFECTS
Effects of Etomidate
EFFECTS:
ADVERSE EFFECTS
EFFECTS:
Adverse Effects of Etomidate
Painful IV injection
Perivascular sterile abscesses
Hemolysis with rapid administration (cats)
Nausea, vomiting, involuntary excitement during
induction and recovery
Guaifenesin (GG)
Noncontrolled muscle relaxant
Common use in large animals
Muscle relaxation
Facilitate intubation
Ease induction and recovery
Not an anesthetic or an analgesic
Mode of action is not understood- blocks nerve impulses to
the CNS
Effects of Guaifenesin
EFFECTS:
EFFECTS:
EFFECTS:
EFFECTS:
Adverse Effects of Guaifenesin
Few adverse effects at therapeutic doses
Overdose
Muscle rigidity
Apneustic respiration
Perivascular tissue irritation
Use of Guaifenesin
Used with ketamine in anesthetic induction protocol
Premedicate with alpha2-agonist or acepromazine
Triple drip: GG, ketamine, xylazine
Used in horses
Maintain anesthesia for less than an hour
Not a sedative or analgesic
Must premedicate
May cause excitement if not
Increased risk of side effects if not
DISSOCIATIVES
KETAMINE
TILETAMINE
Mode of action
Disrupts nerve transmission in some brain
sections and has selective stimulation in other
parts of the brain
Decreases “windup” through NMDA inhibition
(N-methyl-D-aspartate)
Windup is exaggerated response to low-intensity pain
stimuli that results in worsening of post op pain
Dissociative Anesthetics
Ketamine hydrochloride
Derivative of Phencyclidine PCP
Can be used alone in cats for minor procedures or to facilitate
restraint
Mostly used to compliment other drugs such as Tranquilizers and
opioids to induce general anesthesia
Subanesthetic dose can be used as CRI for analgesia
Dissociative Anesthetics
Tiletamine hydrochloride
Combined with benzodiazepine zolazepam (Telazol® )
Tiletamine is a controlled substance
No reversal for Telazol
Provides limited somatic analgesia
Dissociative Effects
EFFECTS:
ADVERSE EFFECTS
EFFECTS:
ADVERSE EFFECTS
Dissociative Anesthetic Trancelike
State
Other Adverse Effects of Dissociatives
Pain after IM injection due to tissue irritation
Increased intracranial and intraocular pressure
Ketamine
Duration of effect: 20-30 minutes
Increased dose prolongs duration but doesn’t increase
anesthetic effect
All dissociatives are either metabolized in the liver or
excreted unchanged in the urine
Avoid use in animals with liver or kidney disease
Ketamine
Approved for use in cats and subhuman primates
Also used in dogs, birds, horses, and exotic species
Schedule III drug (United States) prescription drug (Canada)
Administer IV or IM or orally (cats)
Elimination
Hepatic metabolism—dogs
Renal metabolism—cats
Ketamine and Diazepam Combination
IV induction in dogs and cats
Equal volumes of diazepam and ketamine
Can be mixed in one syringe
Watch for possible precipitate
Alternative combination for IM injection: midazolam and
ketamine
Minimal cardiac depression
Superior recovery and some analgesia
Tiletamine
Similar to ketamine
Sold only in combination with zolazepam (Telazol®)
Telazol®—sold as a powder to reconstitute
Stable for 4 days at room temperature, or 14 days if refrigerated
A class III drug
Possible long and difficult recoveries
Tachycardia, and cardiac arrhythmias
Increased salivation
Avoid in patients with ASA P3 rating
Advantages of Telazol®
(as compared to Ketamine)
Decreased apneustic respiratory response
Can be administered SC, IM, or IV
Used effectively in some wildlife