General Anesthesia I

Question 1

Define general anesthesia.

Answer 1

controlled and reversible lack of consciousness, lack of pain sensation, lack of memory, depressed reflexes

Question 2

What are the four anesthetic procedure phases?

Answer 2

pre-anesthesia, induction, maintenance, recovery

Question 3

Define induction.

Answer 3

animal leaves normal state of consciousness and enters the anesthetized state

Question 4

What are two routes of anesthetic induction?

Answer 4

injectable - often followed by gas for maintenance

inhalation - face mask, induction chambers, intubation

Question 5

Define maintenance.

Answer 5

stable level of anesthetic depth, stage during which the surgical procedure is performed

Question 6

Define recovery.

Answer 6

when concentration of anesthetic in the brain begins to decrease

Question 7

How are injectable anesthetics excreted by the body?

Answer 7

most metabolized by the liver and excreted by the kidneys, but there are exceptions

Question 8

How are inhalent anesthetics excreted by the body?

Answer 8

most commonly used agents are eliminated by the respiratory tract, some older agents have variable amounts of liver metabolism

Question 9

What is the most important factor in anesthetic safety?

Answer 9

monitor

there is no substitute for using your senses

Question 10

What are the classic stages and planes of anesthesia?

Answer 10

Stage I - beginning of induction
Stage II - excitement phase
Stage III - 
Plane 1 - can intubate at this point
Plane 2 - heart rate, breathing rate steady
Plane 3 - 8 breaths per minute or less, 
Plane 4 - dying
Stage IV

Question 11

Define endotracheal intubation.

Answer 11

placement of breathing tube in airway, which minimizes dead space, decreases risk of aspiration, allows direct delivery of oxygen o assist respiration, more efficient delivery with less waste gas

Question 12

What patients are most at risk for aspiration?

Answer 12

oral surgery/dentistry, unfasted patients

Question 13

What are the risks of endotracheal intubation?

Answer 13

• stimulates parasympathetic nervous system
• brachycephalic breeds
• laryngospasm, especially cats
• species problems -> blind intubation
• tube too far in, past tracheal bifurcation
• increased dead space - trim length of tube
• cuff inflation - too much -> pressure necrosis/occlusion of tube lumen
• tube obstruction
• loss of tube into airway during recovery
• infection transfer -> disinfect between patients

Question 14

What brachycephalic traits cause anesthetic risks?

Answer 14

stenotic nares
elongated soft palate
everted pharyngeal ventricles

Question 15

Which dog breed has the highest anesthetic risk?

Answer 15

English bulldog

Question 16

What is the inhalation anesthesia mechanism of action?

Answer 16

gas anesthesia within the brain is poorly understood

Question 17

What is the distribution/elimination route of inhalation anesthesia?

Answer 17

liquid anesthesia -> vaporized into oxygen -> air passages -> alveoli -> bloodstream -> brain

Question 18

What is the distribution/elimination of inhalation anesthesia control mechanism?

Answer 18

concentration gradient from alveoli -> bloodstream

lipid solubility -> agents leave bloodstream -> brain

Question 19

What does inhalation anesthetic maintainance require?

Answer 19

requires that enough gas be delivered to alveoli to maintain concentrations in blood and brain

Question 20

inhalation anesthetic recovery

Answer 20

reduce flow to alveoli -> concentration gradient now favors flow to alveoli from bloodstream and then from brain to bloodstream

running 100% oxygen during recovery will speed process

Question 21

inhalation anesthetic agent vapor pressure

Answer 21

measure of the amount of liquid that will evaporate at 20 C

Question 22

high vapor pressure

Answer 22

volatile - vaporizes easily
need precision vaporizers wth a maximum delivery of 5-8% depending on the agent used

examples - sevoflurane, isoflurane, halothane > 30%

Question 23

blood to gas solubility coefficient

Answer 23

also known as partition coefficient - measure of distribution of age between blood and gas phases of the body

Question 24

low solubility coefficent

Answer 24

tends to remain in gas phase in lungs rather then dissolving into tissues and blood, steep concentration gradient, rapid induction/recovery

ex. isoflurane, sevoflurane

Question 25

high solubility coefficient

Answer 25

"sponge effect" - slow induction and recovery

Question 26

endotracheal intubation materials

Answer 26

3 tubes in size range with cuffs checked for leaks - stylet for small tubes - lubricant (water soluable) - lidocane/cotton tipped applicators - oral speculum - laryngoscope - 2-3 gauze sponges - roll gauze cut to length appropriate to tie in place - syringe to inflate cuff - eye ointment - gas machine checked and ready to attach to trach tube

Question 27

Minimum Alveolar Concentration (MAC)

Answer 27

Lowest concentration that produces no response in 50% of patients exposed to a painful stimulus. A measure of strength or potency of an agent Low MAC value = more potent than a high MAC value

Question 28

Factors Influencing Absorption & Elimination

Answer 28

concentration, ventilation, diffusion, pulmonary blood flow, tissue absorption, lipid content of tissues

Question 29

concentration

Answer 29

Greater the concentration, the greater the pressure gradient -> more rapidly anesthetic will diffuse across alveoli

Question 30

ventilation

Answer 30

Increased rate/depth will aid in moving more anesthetic vapor across the alveoli

Question 31

diffusion

Answer 31

Is a physical process determined by: The agent’s solubility coefficient The molecular weight of the gas The pressure gradient from the alveoli and plasma

Question 32

pulmonary blood flow

Answer 32

The more blood exposed to the anesthetic gas, the more molecules will move into the blood

Question 33

tissue absorption

Answer 33

Highly perfused tissues receive and absorb most of the gas taken up by the alveoli (brain, heart, lungs, liver, kidneys, intestine, endocrine glands)

Question 34

lipid content of tissues

Answer 34

Lipid-rich cells take up more of an anesthetic than lipid-poor cells Brain highly lipid-rich

Question 35

halogenated organic compounds

Answer 35

``` Modern common inhalation agents Isoflurane – most common Sevoflurane - common Halothane – not used today Methoxyflurane – not used today Enflurane – little use vet med Desflurane – little use vet med ```

Question 36

Isoflurane

Answer 36

``` Good margin of safety High vapor pressure Low solubility coefficient Rapid induction/recovery High MAC value (low potency) Stable at room temp with no preservative Fewest adverse effects on heart, lungs, etc. Eliminated by lungs No post-operative analgesia provided ```

Question 37

sevoflurane

Answer 37

Rapid induction/recovery (faster than isoflurane) Best agent for mask/chamber inductions (use 6-8%) Also good agent for C-sections Rapid/quiet recovery in horses Some myocardial depression and vasodilation Sensitizes myocardium to catecholemine-induced arrhythmias Causes some respiratory depression Depresses temperature regulation Avoid in patients susceptible to malignant hyperthermia No analgesic effect in recovery period

Question 38

halothane

Answer 38

``` Introduced in 1956, use dwindling today Up to 20% eliminated by liver metabolism Mixed with preservative thymol Sensitizes heart to catecholemines (i.e. epinephrine) Some myocardial depression/vasodilation Increases intracranial pressure Avoid in head trauma patients Associated with malignant hyperthermia Increased temp, muscle rigidity, cardiac arrhythmias, may die ```

Question 39

Methoxyflurane

Answer 39

Introduced in 1959, not available today Low vapor pressure (use in non-precision vaporizer) Slow induction/recovery Low MAC (most potent of class) Considerable solubility in rubber/plastics

Question 40

enflurane

Answer 40

Used in human medicine Rapid induction/recovery Profound respiratory depression, with mechanical ventilation generally required In dog, may produce seizure-like muscle spasms

Question 41

Desflurane

Answer 41

``` Little use in veterinary medicine Lowest solubility coefficient Therefore, most rapid induction/recovery Extremely volatile (high vapor pressure) Requires special high-cost vaporizer ```

Question 42

Nitrous Oxide

Answer 42

Little used in vet med today Stored in blue cylinders as compressed gas Administered via flowmeter, like oxygen Mixed with oxygen at concentrations of 33-67% Was used to speed induction and recovery, and provide additional analgesia Because newer agents have rapid induction/recovery, not used much today except in human dentistry