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Test Review: Anesthesia. Jenifer Sweet, B.A., S.R.S., L.A.T. MPI Research in coordination with The Academy of Surgical Research Testing Committee. Overview. General Anesthesia Definition Stages of Anesthesia Considerations Pharmacokinetics Method of action Modifying factors

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test review anesthesia
Test Review: Anesthesia

Jenifer Sweet, B.A., S.R.S., L.A.T.

MPI Research

in coordination with

The Academy of Surgical Research Testing Committee

overview
Overview
  • General Anesthesia
      • Definition
      • Stages of Anesthesia
      • Considerations
  • Pharmacokinetics
      • Method of action
      • Modifying factors
  • Types of Anesthesia
      • Pre-anesthetic Agents and Adjuncts
      • Injectable Anesthetic Agents and Adjuncts
      • Inhalation Anesthesia
      • Local and Regional Anesthesia
      • Physical Methods of Anesthesia
  • Equipment
  • Review
general anesthesia
General Anesthesia
  • What is general anesthesia?
  • Doses based on “average” animal
  • Biological variations
      • Metabolic rate
      • % fat
      • General health
      • Sex
      • Genetics
      • Time of day
      • Species
      • Individualized sensitivity
    • The perfect anesthetic agent does not exist
stages of anesthesia
Stages of Anesthesia

4 Stages of Anesthesia

  • Stage I: “The stage of voluntary movement”
    • Initial administration of anesthetic to the loss of consciousness
    • Tachycardia and hypertension
    • Irregular / increased respiration
    • Breath holding
    • Pupils dilate
    • Struggling as animal becomes ataxic
    • Some analgesic effects
  • Stage II: “The stage of delirium or involuntary movement”
    • CNS depression
    • Loss of voluntary control
    • Exaggerated reflexes
    • Struggling, breath holding, tachypnea, hyperventilation
    • Cardiac arrhythmias may occur
    • Eyelash and palpebral reflexes present
    • Vocalization
    • Salivation
    • Laryngeal spasm
stages of anesthesia5
Stages of Anesthesia
  • Stage III: “Stage of Surgical Anesthesia”
    • Pulse rate returns to normal
    • Muscles relax
    • Swallowing and vomiting reflexes lost
    • 3-4 planes
      • Plane I:
        • Eyeball movement ceases
        • Normal BP with strong pulse
        • Decrease of respiratory rate and depth
        • Pupils less dilated
        • Eyeball may rotate
        • Palpebral reflex present
        • Slight reaction to surgical manipulation
        • Loses jaw tone
  • Plane II:
    • Surgical Anesthesia
    • Bradycardia
    • Hypotension
    • Capillary refill slows
    • Palpebral reflex diminishes and disappears
    • Eyeball rotates ventrally
    • Abdominal muscle tone lost
    • Minimal jaw tone
    • Pedal reflex absent
    • Dysrhythmia possibility low
stages of anesthesia6
Stages of Anesthesia
  • Stage III (cont): “Stage of Surgical Anesthesia”
    • Plane III:
      • Deep surgical anesthesia
      • Intercostal and abdominal muscle tone minimum
      • Weak corneal reflexes
      • Diaphragmatic breathing
      • Profound muscle relaxation
      • Centered and dilated pupils
      • Bradycardia intensifies
      • Hypotension increases
      • Respiratory rate and depth decrease
  • Plane IV:
    • Deep/ Overdose
    • Dysrhythmia probability
    • Respirations slow and irregular
    • Lowered HR
    • Cyanosis
    • Widely dilated pupil and unresponsive to light
    • Flaccid muscle tone
    • Jaw tone lost
    • Sphincter control lost
pharmacokinetics
Pharmacokinetics
  • Action of anesthetic on CNS
      • Partial pressure gradients
  • Inhalants vs. Injectables
  • Distribution and clearance
      • Modifying factors
        • Concentration
        • Plasma pH
        • Protein binding
        • Hydration
        • Multiple drugs present
effects of disease
Effects of Disease
  • Cardiovascular dysfunction
      • Most anesthetics cause CV depression
      • Animals prone to fluid overload & arrhythmias
  • Pulmonary dysfunction
      • Most anesthetics cause pulmonary depression
      • Balancing between lowering doses and preventing anxiety
      • Intubation and ventilation are key
      • Nitrous oxide contraindicated
  • Neurologic disease
      • Loss of ICF and CBF regulation
      • Watch for respiratory depression
      • Nitrous oxide contraindicated
  • Renal disease
      • Stress and anesthetic agents decrease rate of filtration
      • Reduction in elimination = increase in acidity and plasma concentrations
      • Lingering effects
      • K+ increases in serum
effects of disease9
Effects of Disease
  • Hepatic disease
      • Acepromazine, thiobarbiturates and α-2-adrenergic agents contraindicated
      • Propofol, ketamine and inhalation the safest
      • Lowered elimination rate and coagulation
  • Gastrointestinal disease
      • Damaged GI can release toxins
      • Decrease in cardiac function and ventilation
  • Endocrine disorders
      • Select anesthesia for easiest reversibility
pre anesthetic agents and adjuncts
Pre-anesthetic Agents and Adjuncts
  • Anticholinergics
  • Tranquilizers
  • Opioids
  • Alpha2adrenergic agonists
  • Alpha2adrenergic antagonists
  • Tranquilizer-opioid combinations
  • Paralytic agents
anticholinergics
Anticholinergics
  • Block acetylcholine receptors
  • Reduce secretions
  • Prevent vagal inhibition and GI stimulation
  • Reduce vagus nerve response (vomiting and laryngospasm)
  • Promote bronchodilation
  • Dilate the pupil
  • Treatment of choice for opioid, xylazine and vagal reflex activity induced bradycardia
anticholinergics12
Anticholinergics
  • Atropine Sulfate
      • Contraindicated with tachycardia, constipation and obstruction
      • May cause thick mucus secretions in cats
      • Atropine esterase occurs in cats, rats, and rabbits
      • Minimally effective in sheep and goats
      • Increased incidence of bloat
      • Prolongs thiopental anesthesia
      • Overdose: dry mucous membranes, thirst, dilated pupils and tachycardia (dogs most susceptible)
      • Can be treated with physostigmine IV over several minutes
  • Glycopyrrolate
      • Reduces diffusion over blood brain or placental membranes
      • Lasts longer than atropine
      • Prevents ketamine/xylazine associated bradycardia in rabbits
      • Longer onset of action in ruminants
tranquilizers
Tranquilizers
  • NO ANALGESIC EFFECTS
  • Relieve anxiety
  • Decrease anesthetic dosages
  • Reduce histamine release and vomiting
  • Make anesthetic recovery smoother
  • Promote skeletal muscle relaxation and vasodilatation
  • May lead to hypotension and excessive heat loss
  • May raise seizure thresholds/ act as anticonvulsants
tranquilizers14
Tranquilizers

Acepromazine Maleate

Phenothiazine

May reduce or prevent malignant hypothermia in swine

Droperidol

Butyrophenone

Alpha-adrenergic antagonist

May prevent epinephrine induced dysrhythmias

Decreases barbiturate doses

Primarily used as a component of InnovarVt in a mixture with fentanyl

Diazepam

Benzodiazepine

Prevents seizures

Rapidly passes blood-brain and placental barriers

Should be injected slowly to prevent venous thrombosis and should not be injected IA

IM injection not recommended- painful

tranquilizers15
Tranquilizers

Midazolam

Benzodiazepine

Shorter duration of action and clearance than diazepam

May cause behavioral changes in dogs and cats

Suitable for IM injection

Can be mixed with other preanesthetic agents

Flumazenil

Reverses CNS action of benzodiazepine without anxiety, tachycardia, or hypertension

Rapid action (24 minutes)

Replaced aminophylline and physostigmine

opioids
Opioids

Depress CNS

Lower the amount of anesthetic agents needed

Do not cause unconsciousness at therapeutic levels

Addictive

Most are controlled substances

Best for continuous dull pain

opioids17
Opioids

Methadone hydrochloride (Methadone, Dolophine)

Synthetic opioid unrelated to morphine

2-6 hours of analgesia

Decreases barbiturate dose by 50%

Oxymorphone hydrochloride

(Numorphan)

Semi synthetic

10 times more potent than morphine

Provided effective epidural analgesia

Morphine sulfate

Stimulates vomiting

Decreases BMR and body temp

Variable effects

Poor effects on neuropathic pain

Meperidine hydrochloride

(Demerol, Pethidine)

Analgesic effect 1/10 of morphine

Rapidly excreted

Does not cause vomiting

Slow administration recommended

opioids18
Opioids

Fentanyl citrate

250 times more potent than morphine

Rapid onset of action

Short duration; peak at 30 minutes

Depressed respiration

Exaggerated response to loud noise

Little cardiac output or BP effects

Carfentanil citrate

10,000 times more potent than morphine

Used primarily for capture of wild animals

Sufentanil

5 to 10 times as potent as fentanyl

Provided unpredictable anesthesia in dogs

Provides neuroleptanalgesia when combined with tranquilizers and glycopyrrolate

Alfentanil

1/5th to 1/10th as potent as fentanyl

80-1000 times more potent than morphine SC

More rapid onset than fentanyl or sufentanyl

Used primarily for the capture of wild animals

opioids19
Opioids

Pentazocine lactate (Talwin)

1/3rd as effective as morphine

Minimal CV effects

  • Buprenorphine (Buprenex)
      • 25 to 30 times as potent as morphine
      • Max analgesic effect less than morphine
      • Slow onset of action (20-30 minutes)
      • Excreted in feces
alpha 2 adrenergic agonists
Alpha 2 Adrenergic Agonists

Produce sedation, muscle relaxation and analgesia

Not potent respiratory depressant

Non-addictive

Anticonvulsants

Wide range of drug interactions

Barbiturate, inhalant and dissociative anesthetic doses should be lowered used in combination with alpha 2 adrenergic agonists

alpha 2 adrenergic agonists21
Alpha 2 Adrenergic Agonists

Xylazine hydrochloride (Rompun)

Most common sedative/analgesic in horses and cattle

Short term surgical anesthetic when combined with ketamine

Effects within 10-15 minutes IM or 3-5 minutes IV

IV bolus causes bradycardia, hypotension followed by decreased CO and BP

Poor efficacy in swine

Wide margin of safety

May cause emesis in cats and dogs

Reduces insulin secretion, effecting blood glucose levels

Medetomidine

More potent than xylazine

BP and RR decreases dose dependent

Detomidine

Sedative with analgesic properties

Cardiac, respiratory and antidiuretic effects

Primarily used in horses

Dexmedetomidine (Precedex)

More potent than medetomidine

Sedative, analgesic, sympatholytic and anxiolytic effects

Sedation without respiratory depression

Shortens time to extubation

Reduces anesthetic dosages

Clonidine

Alpha-methyldopa

alpha 2 adrenergic antagonists
Alpha 2 Adrenergic Antagonists

Used as reversal agents for injectable anesthetics

Yohimbine

Reverses xylazine

Also reverses ketamine and pentobarbital combinations when combined with 4-aminopyridine.

Tolazoline

Reverses xylazine and some anesthetic drug combinations with xylazine

Atipamezole

Selectivity ration 200 to 300 times higher than yohimbine

Rapid IV doses may cause death or severe hypotension and tachycardia

tranquilizer opioid combinations
Tranquilizer-Opioid Combinations

Provide neuroleptanalgesia

Intense analgesic action with short duration

Fentanyl citrate Droperidol (Innovarvet)

Wide margin of safety with easy recovery

Partially reversed with opioid antagonists

paralytics
Paralytics

Provide superior muscle relaxation as an adjunct to general anesthesia

DO NOT PROVIDE ANALGESIA OR UNCONSCIOUSNESS

Prohibited as a sole anesthetic by the Guide

Mechanical ventilation required

More difficult anesthesia management

paralytics25
Paralytics

Succinylcholine

Depolarizing neuromuscular paralytic

Marked twitching for 30 minutes before muscle relaxation

Muscle pain and stiffness associated

Rise in intraocular pressure

Cats, swine and ponies resistant

May not be reversible

Pancuronium

Lasts 20 to 30 minutes

Causes increased HR

Metabolized in liver, excreted via kidneys

Vecuronium

More potent and shorter acting than pancuronium

rapid recovery

no effect on HR

Widely used

do not use with renal or hepatic failure

Pipecuronium

Long acting- twice duration of pancuronium

2 to 4 times as potent as pancuronium

Rapid onset

Retained in kidneys for days

no effect on HR

paralytics continued
Paralytics (continued)
  • Rocuronium
    • 20% as potent as vecuronium
    • Rapid recovery
  • Curare (dTubocurarine)
    • Long acting
    • Increases HR
  • Metocurine
    • Safer than curare
  • Gallamine
    • Long acting
    • Produces tachycardia
    • The only non-depolarizing agent to cross the placenta
  • Atracurium
    • Unstable- refrigerate
    • Intermediate muscle relaxant
    • Widely used
  • Doxacurium
    • Long acting
    • No autonomic side effects
  • Mivacurium
    • Lasts slightly longer than succinylcholine and ½ the duration of vecuronium
    • No autonomic side effects
paralytic reversal agents
Paralytic Reversal Agents
  • Anticholinerases
    • Bradycardia, arrhythmias, secretions
    • CNS stimulation
    • Edrophonium, neostigmine, pyridostigmine
    • 4 Aminopyridine and Guanidine
  • Calcium
    • Only partially effective
injectable anesthetic agents and adjuncts
Injectable Anesthetic Agents and Adjuncts

Enter blood stream for transport to target tissues

Require redistribution

Generally detoxified in liver and excreted via kidneys

Metabolism based on first order kinetics

Constant fraction metabolized in a given period

Less control of elimination process

Barbiturates

barbiturates
Barbiturates

Divided into Ultra short, Short, Intermediate and Long acting

Depress CNS neurons

May lead to respiratory depression, central and peripheral CV depression, decreased BP and BMR, reduced stroke volume and increased HR

Hypnotic sedatives

Cross cell walls and placental membrane

Glucose effect in some animals

Should not be administered to animals less than 3 months old

IV administration preferred

Barbiturate slough may occur

Oxybarbiturates

Thiobarbiturates

oxybarbiturates
Oxybarbiturates

Phenobarbital Sodium

Long acting

Effective anticonvulsant

Excreted slowly and cumulative

Pentobarbital Sodium

Short acting

Initial spike in HR followed by a decrease in HR and BP

Prolonged use leads to decreased systolic BP, stroke volume, pulse pressure, CO, pH, and BT (shock-like)

Crosses placenta

Tranquilizers advised for smooth recovery

Methohexital Sodium (Brevital)

Ultra short acting (redistribution)

Respiratory failure with overdose

Good for induction

thiobarbiturates
Thiobarbiturates

Thiopental sodium

Ultra short acting

Most secreted in urine within 4 days

Initial respiratory depression

Increase in HR, BP and vascular resistance

Thiamylal sodium

Ultra short acting

IV bolus lasts approx. 15 minutes

Less cumulative than thiopental

Less CV effects than thiopental

non barbiturate anesthetics
Non-Barbiturate Anesthetics

Chloralose

Minimal CV depression

Less depression of neuronal function

Long duration, acute procedures

Urethane, N.F.

Carcinogenic

Magnesium sulfate

Globally depresses CNS

Means of euthanasia after unconsciousness

Althesin

Don’t use with barbiturates

Good muscle relaxation

May cause allergic reaction

Chloral Hydrate, U.S.P.

Oral admin may cause vomiting

Depresses cerebrum

Good hypnotic/poor anesthetic

Amount needed for anesthesia close to lethal dose

non barbiturate anesthetics33
Propofol

Supports microbial growth

Rapid uptake into CNS

Quick and smooth recovery

Minimal analgesic effects

Propanidid

Extremely short duration of action

Difficult to administer fast enough

Severe respiratory depression and hypotension in dogs

Tricaine Methanesulfonate (MS222)

Anesthesia of fish and amphibians

Metomidate (Hypnodil)

Hypnotic w/ relaxant properties

Sleep without anesthesia

Etomidate

No depression of CV or respiratory centers

Does not trigger MH in swine

Anticonvulsant properties

Venous pain during injection

Non-Barbiturate Anesthetics
dissociative anesthetics
Dissociative Anesthetics

Interrupts transmission from the unconscious to the conscious brain

Characterized by a cataleptic state in which eyes remain open and nystagmus present

  • Ketamine
      • Least potent
      • Rapid onset of action
      • Rapid redistribution
      • Tissue irritation due to low pH (3.5)
      • Analgesic effects greater for somatic pain than visceral pain
      • Transient decrease in respiratory rate
      • Hallucinatory behavior
  • Telazol
      • Tiletamine hydrochloride and Zolazepam
      • Wide safety margin
      • Rapid and smooth induction/recovery
      • Good muscle relaxant
      • Lingering analgesic effects
      • May cause increased HR and respirations
      • Decrease in MAP
inhalation anesthesia
Inhalation Anesthesia

Administration and elimination through lungs

Dependent upon:

Vapor pressure

Boyle’s law

Dalton’s law

Temperature

Charles’ law

Solubility

Partition coefficients

Pharmacokinetics

Biotransformation

MAC

Much more control

inhalation anesthetics
Inhalation Anesthetics
  • Historical Inhalant Agents
    • Chloroform
    • Cyclopropane
    • Diethyl ether
    • Fluroxene
    • Trichlorethylene
inhalation anesthetics37
Inhalation Anesthetics

Nitrous oxide

Rapid onset

Minimal cardiovascular, liver and kidney effects

May cause pneumothorax, blood embolus, increase in middle ear pressure

Must be combined with another agent

Beware of diffusion hypoxia

Halothane

Potent and rapid onset

High volatility

Respiratory depression

Mixed with thymol for stability

Ether

Explosive

Highly irritating

Methoxyflurane

Low volatility

High solubility

Extensively metabolized

Respiratory depressant

Isoflurane

Potent and low solubility

Rapid induction and recovery

“Safer” than halothane

Coronary vasodilator

inhalation anesthetics38
Inhalation Anesthetics

Desflurane

Very rapid induction and recovery

Lower solubility than isoflurane

Respiratory irritant

Requires heated vaporizer

Sevoflurane

Very rapid induction and recovery

Lower solubility than isoflurane, halothane or methoxyflurane

local and regional anesthesia
Local and Regional Anesthesia

Administration

Topical

Solution in gel or aerosol

Injectable local

Ring block

Brachial plexus block

Epidural

IV regional block

Intercostal nerve blocks

Affects 2 adjacent intercostal spaces

Muscle nerve blocks

For extensive surgical manipulation

Interpleural admin

Examples

Lidocaine

Proparacaine

Benzocaine

Tetracaine

Butacaine

physical methods of anesthesia
Physical Methods of Anesthesia

Hypothermia

Some vital organs can survive for longer periods at low temps with reduced blood supply

Risks profound CNS and vital organ depression

<28°C may cause VF

Prolonged clotting time

3 methods of hypothermia

Surface

Body cavity

extracorporeal

Electronarcosis

Delivered via electrodes applied to head

Convulsions during induction

Difficult to monitor and questionably humane

Acupuncture

Useful for chronic pain

equipment
Equipment

Anesthesia machine

Components

Vaporizer in circuit or out of circuit?

Rebreathing, non-rebreathing, semi-closed circuits

CO2 absorber/ Scavenging

Medical gas cylinders

Color codes

Airway maintenance

Endotracheal tubes

Laryngoscope blades

review what do you need to know
Review: What do you need to know?

Know your drugs- what group they belong to and what they do

Know the stages of anesthesia

Have a basic understanding of the pharmacokinetics behind anesthesia

Know your patient and how biological variations can effect anesthesia

Be familiar with anesthetic equipment

Areas not covered in depth: fasting, thermoregulation, fluids and acid/base balance