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In general which is safest - inhalation anesthesia or injectable anesthesia?
List the characteristics of an ideal inhalation anesthetic agent (there are 10).
- minimal toxicity to patient and personnel
- ease of administration
- rapid and gentle induction and recovery
- anesthetic depth easily controlled and altered
- good muscle relaxation
- adequate potency
- no expensive equipment
List 3 classes of inhalation agents.
- nitrous oxide
- halogenated compounds
List 4 halogenated compounds
Which 2 of the halogenated are in common use at this time?
What are the 2 theories of the mechanism of action of inhalation agents?
Theory 1: may inhibit breakdown of GABA in brain (gamma amino butyric acid - inhibitory neurotransmitter)
Theory 2: lipid soluble anesthetic may dissolve in nerve cell membranes - nerve cells lose ability to conduct impulses
Describe the induction on inhalant anesthetics.
- patient inhales mixture of vaporized anesthetic agent and O2 into alveoli of lungs
- anesthetic molecules diffuse from alveoli across cell membranes into the blood
- anesthetic molecules diffuse into brain from blood (lipid structure)
- anesthesia is induced
What controls the rate of diffusion of an inhalant anesthetic from the alveoli into the blood?
- lipid solubility of the anesthetic agen
- concentration gradient
Maintenance of anesthesia continues as long as adequate concentration of anesthetic agent is maintained in _____.
Describe how a patient recovers from inhalation anesthesia.
- concentration of inhaled anesthetic agent is reduced or stopped
- alveolar levels of agent decrease through exhalation
- blood levels of the agent are now higher than alveolar levels (agent diffuses from blood to alveolil to then be exhaled)
- blood levels of the anesthetic agent decrease
- brain levels of the agent are now higher than blood levels (so anesthetic agent diffuses from brain into blood)
- brain levels decrease
- anesthetic lightening or recovery occurs
What happens if the anesthetic agent is very lipid soluble?
- can accumulate in body fat
- from fat, agent can be slowly released into blood resulting in long term low level blood concentration
- liver metabolism and renal excretion occur
- causes prolonged recover and a hangover
Define Vapor Pressure
- amount of a liquid anesthetic that will evaporate at 20 degrees Celcius
- expressed as a percent
What is a high volatility anesthetic agent? What are some examples? These require what kind of vaporizer?
- reaches high concentration because evaporates easily
- isoflurane, sevoflurane
- requires precision vaporizer
What are some non-volatile agents and what are some examples of these?
- low vapor pressure, non-precision vaporizer
- methoxyflurane, ether, chloroform
Can you mix vaporizers?
Can you switch patients from one inhalation agent to another udring a procedure? How?
yes - must switch vaporizers....do not mix them
Define Solubility Coefficient.
the tendency of an inhalation agent to dissolve in blood
What are the different inhalation solubilities?
- low solubility
- medium low
- high solubility
What inhalation drug do we use for low solubility?
What inhalation drug do we use for medium low solubility?
What inhalation drug do we use for high solubility?
The commonly used agents today are low or high solubility agents?
What is the relevance of this to clinical use of these agents?
doesn't want to dissolve into blood, so builds up to a high level in alveoli first, then enters blood rapidly. also, tends to stay in blood because doesn't enter other tissues easily - except for brain, which has large blood supply, high lipid content.
End result: enters blood rapidly, enters brain rapidly, anesthestizes patient rapidly. also the process reverses rapidly when the administration of the agent stops so recovery occurs rapidly.
Describe the alveolar concentration of low solubility gas anesthetic agents.
- anesthetic agent would rather stay as a gas
- does not want to dissolve into blood
- alveolar concentration of gas builds up
- steep concentration gradient develops between alveoli and blood (very high level in alveoli, very low level in blood)
- steep gradient forces diffusion of anesthetic agent into blood
- blood concentration rises quickly
Describe the blood concentration of low solubility gas anesthetic agents
- rises quickly
- agents with low blood solubility do not enter other body tissue very fast
- tend to stay in the blood
Do lipid soluble agents dissolve into muscle quickly?
Do lipid soluble agents dissolve into fat quickly?
Do lipid agents dissolve into the brain? Why?
yes because the brain has a large blood supply and is a lipid structure
Describe the process of a low soluble agent.
- enter blood quickly
- enter brain quickly
- enter other tissues slowly
- rapid induction
- rapid recovery
Summarize low solubility agents
low solubility agents only need to "fill up" the circulatory system in order to get high enough levels to affect the brain
Summarize high solubility agents
high solubility agents must "fill up" the entire body in order to get high enough blood levels to affect the brain
Define minimum alveolar concentration, or MAC
concentration of an anesthetic agent in the alveoli at which 50% respond to pain and 50% do not
What does MAC mean to us in a clinical setting?
an agent with a low MAC is more potent (requires a lower % of the gas) than an agent with a high MAC
What patient factors influence MAC?
- body condition
- body temperature
- concurrent drugs
How does hypothermia affect MAC and how is this clinically relevant?
hypothermia decreases MAC and causes the patient to get more deeply anesthetized on the same concentration of gas. cold patients require less anesthesia, have longer recovery times