General Anesthesia I Flashcards
Define general anesthesia.
controlled and reversible lack of consciousness, lack of pain sensation, lack of memory, depressed reflexes
What are the four anesthetic procedure phases?
pre-anesthesia, induction, maintenance, recovery
Define induction.
animal leaves normal state of consciousness and enters the anesthetized state
What are two routes of anesthetic induction?
injectable - often followed by gas for maintenance
inhalation - face mask, induction chambers, intubation
Define maintenance.
stable level of anesthetic depth, stage during which the surgical procedure is performed
Define recovery.
when concentration of anesthetic in the brain begins to decrease
How are injectable anesthetics excreted by the body?
most metabolized by the liver and excreted by the kidneys, but there are exceptions
How are inhalent anesthetics excreted by the body?
most commonly used agents are eliminated by the respiratory tract, some older agents have variable amounts of liver metabolism
What is the most important factor in anesthetic safety?
monitor
there is no substitute for using your senses
What are the classic stages and planes of anesthesia?
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
Define endotracheal intubation.
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
What patients are most at risk for aspiration?
oral surgery/dentistry, unfasted patients
What are the risks of endotracheal intubation?
- 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
What brachycephalic traits cause anesthetic risks?
stenotic nares
elongated soft palate
everted pharyngeal ventricles
Which dog breed has the highest anesthetic risk?
English bulldog
What is the inhalation anesthesia mechanism of action?
gas anesthesia within the brain is poorly understood
What is the distribution/elimination route of inhalation anesthesia?
liquid anesthesia -> vaporized into oxygen -> air passages -> alveoli -> bloodstream -> brain
What is the distribution/elimination of inhalation anesthesia control mechanism?
concentration gradient from alveoli -> bloodstream
lipid solubility -> agents leave bloodstream -> brain
What does inhalation anesthetic maintainance require?
requires that enough gas be delivered to alveoli to maintain concentrations in blood and brain
inhalation anesthetic recovery
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
inhalation anesthetic agent vapor pressure
measure of the amount of liquid that will evaporate at 20 C
high vapor pressure
volatile - vaporizes easily
need precision vaporizers wth a maximum delivery of 5-8% depending on the agent used
examples - sevoflurane, isoflurane, halothane > 30%
blood to gas solubility coefficient
also known as partition coefficient - measure of distribution of age between blood and gas phases of the body
low solubility coefficent
tends to remain in gas phase in lungs rather then dissolving into tissues and blood, steep concentration gradient, rapid induction/recovery
ex. isoflurane, sevoflurane
high solubility coefficient
“sponge effect” - slow induction and recovery
endotracheal intubation materials
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
Minimum Alveolar Concentration (MAC)
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
Factors Influencing Absorption & Elimination
concentration, ventilation, diffusion, pulmonary blood flow, tissue absorption, lipid content of tissues
concentration
Greater the concentration, the greater the pressure gradient -> more rapidly anesthetic will diffuse across alveoli
ventilation
Increased rate/depth will aid in moving more anesthetic vapor across the alveoli
diffusion
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
pulmonary blood flow
The more blood exposed to the anesthetic gas, the more molecules will move into the blood
tissue absorption
Highly perfused tissues receive and absorb most of the gas taken up by the alveoli (brain, heart, lungs, liver, kidneys, intestine, endocrine glands)
lipid content of tissues
Lipid-rich cells take up more of an anesthetic than lipid-poor cells
Brain highly lipid-rich
halogenated organic compounds
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
Isoflurane
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
sevoflurane
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
halothane
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
Methoxyflurane
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
enflurane
Used in human medicine
Rapid induction/recovery
Profound respiratory depression, with mechanical ventilation generally required
In dog, may produce seizure-like muscle spasms
Desflurane
Little use in veterinary medicine Lowest solubility coefficient Therefore, most rapid induction/recovery Extremely volatile (high vapor pressure) Requires special high-cost vaporizer
Nitrous Oxide
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
