Inhalational Agents Part 1 Flashcards
What are the three A’s of anesthesia?
Amnesia- loss of memory
Analgesia- loss of sensation and pain control
Areflexia- lack of movement (minimizes sympathetic and parasympathetic changes in vital signs)
What is a benefit to using gas over other agents?
Gas has the three properties of amnesia, analgesia, and areflexia versus other drugs would need to be combined to get all of these effects
Gases affect what areas of the body?
cerebral cortex, brain stem arousal centers, central thalamus, and spinal cord
Gases crosses
the blood brain barrier quickly because it is lipophilic, carbon based, and diffuses quickly
List the stages of anesthesia in order:
Stage 1: amnesia and anesthesia
Stage 2: delirium and excitation
Stage 3: surgical anesthesia
Stage 4: anesthetic overdose
What occurs in stage 1 of anesthesia?
light plain of anesthesia; initiation of anesthesia to the loss of consciousness; patient able to follow simple commands, protective reflexes remain intact, eyelid reflex intact
What occurs in stage 2 of anesthesia?
loss of consciousness and lid reflex, irregular breathing pattern, dilated pupils; neurons that inhibit excitation are not functional and can lead to vomiting, laryngospasm, cardiac arrest, and emergence delirium
Would not want to manipulate anything in this stage due to hyper-reactivity
What occurs in stage 3 of anesthesia?
cessation of spontaneous respirations; absence of eyelash response and swallowing reflexes
risk for aspiration because there’s no airway reflexes
What is stage 4 of anesthesia?
cardiovascular collapse requiring provider intervention
too much of a good thing
What does the respiratory pattern look like under gas?
smaller, more frequent breaths
What patient population tends to be exaggerated for their stages of anesthesia?
children
When we discuss the stages of anesthesia, we are typically discussing
emergence because we give them induction agents as well so we skip some stages
When we give a combination of agents,
we bypass stages such as stage 2 using sedation (benzodiazepines, alpha 2 agonists) and induction/maintenance (barbiturates, propofol, etomidate, ketamine, TIVA
What drugs are synergistic with gases?
analgesics: opioids, non-opioids
Paralytics: non-depolarizing & depolarizing
Adjuncts: regional anesthetics
Would need to give less gas in this situation
Choice of anesthesia is based on:
proposed surgery
patient comorbidities
provider experience
surgeon
Perioperative considerations include:
preoperative- anesthetic based upon assessment, proposed surgery, patient comorbidities, provider experience, and surgeon
intraoperative- married to our plan, need to have back up plan if surgeon is taking longer
postoperative: responsibility to follow up and make sure anesthesia worked
Absorption of inhalational agents are related to:
ventilation, blood uptake, cardiac output, blood solubility, alveolar to blood partial-pressure difference
Where do we measure the concentration of inhalational agents?
at the lungs because we make the assumption that what’s in the lungs is what’s in the brain
Administration of an inhalation agent involves
taking a liquid, vaporizing it, and delivering it to the brain
The main factors in anesthetizing a patient are
technical and machine related, drug specific, and patient factors such as respiratory, circulatory and tissue
What is MAC?
the minimum alveolar concentration (%) required to produce anesthesia (lack of movement) in 50% of the population
What conditions require increased MAC?
hyperthermia, drug-induced increases in CNS activity, hypernatremia, chronic alcohol abuse
assuming that we are using gas only
The dose of gas is expressed as
minimal alveolar concentration or MAC
MAC is
age dependent; peaks at 6 months and decreases with age
The faster the lung concentration rises
the faster anesthesia is achieved
MAC awake means
patients are not getting full anesthetic gas
Factors that decrease MAC include
hypothermia, increasing age, alpha-2 agonists, acute alcohol ingestion, pregnancy, hyponatremia
What are machine-related factors that impact our gas anesthesia?
Rubber and plastic machine pieces and CO2 absorbent can retain gas delaying initial uptake
-can retain small quantities of anesthetic gases so must be flushed
All inhalational agents can trigger
malignant hyperthermia (except nitrous oxide); a thorough flush at 10 L/min for 20 minutes, replacement of all breathing circuits and CO2 absorbent and removal of vaporizers
LIter flow of carrier gas is related to
air, oxygen, and nitrous oxide (less soluble so is able to bring gas along with it)
What accelerates agent intake?
increasing liter flows
Why would we give 100% oxygen prior to induction?
we denitrate them because we’re creating a situation where they’re oxygenated prior to intubating them
What is blood: gas solubility?
describes the amount of gas that will dissolve or bind to the blood versus the amount that will diffuse into the tissues
soluble agents remain in the blood longer, so less is released into the tissues during uptake (it will be slower)
Isoflurane has a solubility coefficient of 1.4 this means that
1.4 times more gas is bound to blood than is available to the tissues
If you have a gas with a solubility coefficient of 1.4 versus a solubility coefficient of 0.42, which drug would put you to sleep quicker?
0.42
If you have a gas with a solubility coefficient of 1.4 versus a solubility coefficient of 0.42, which drug would you wake up quicker from?
0.42 drug
A gas that is more potent, has
a lower MAC
The lower the blood gas solubility coefficient
the faster the rate of rise in the lung and brain concentrations
How could you make a gas with a high solubility coefficient more like desflurane with a low solubility coefficient?
add nitrous oxide
The rate and depth of ventilation influences
the uptake on induction and removal on emergence
if we hyperventilate someone, you will increase the
amount of gas more quickly; you can also
hyperventilate someone to get rid of gas but this also
blows off CO2 so they don’t want to breathe
Ventilation perfusion defects alter
the rate of uptake
gases with a low blood solubility coefficient
affected to a greater extent
Oil:gas solubility is
an indicator of potency
indicator of ability to get into the tissues
highly lipid-soluble drugs tend to be more potent
isoflurane most potent; nitrous oxide least potent
Compare blood:gas to oil:gas solubility
blood: gas- ability to get into body
Oil: gas- ability to get into the tissues
The second-gas effect is
co-administration of a slower agent with nitrous oxide to speed the onset of the slower agent
can also be used during emergence to quickly remove a slower gas
The concentration or dose effect is
administration of a higher concentration of gas than necessary to speed up initial uptake
greater effect on high solubility gases
An increase in cardiac output
slows uptake of gas because blood spends less time in the lungs which slows the rise in lung/brain concentration
high solubility anesthetic gases are affected more
(isoflurane)
The circulatory system has two major influences on anesthetic gases:
uptake and distribution
Majority of blood leaving the heart is distributed to the vessel rich organs or central compartment
Decreases in temperature results in increased:
potency & solubility
Hypothermia decreases tissue perfusion resulting in
slowed induction
Hypothermia increases tissue
anesthetic capacity which leads to slow recovery
Hyperthermia increases
cardiac output and anesthetic requirement which also slows induction
How are modern anesthetics metabolized?
they’re minimally metabolized so toxic metabolites do not present a clinical issue
Which modern anesthetics are not metabolized?
nitrous oxide, desflurane, and isoflurane
Where is sevoflurane metabolized?
approximately 5-8% of sevoflurane is metabolized by the liver releasing free fluoride ions
What was Ether day?
March 30, 1842 William T.G. Morton successfully removed a tumor at Mass Gen
When was nitrous oxide discovered?
1790s by Joseph Priestley
1842 Horace Wells used it for a dental procedure but not as successful when he demonstrated use at Mass Gen
Halothane was developed in
1956 & was the first halogenated anesthetic agent
it was a major break through because it was non-flammable but it is metabolized in the liver so halothane hepatitis was possible
Chloroform was discovered in
1831 by Dr. Samuel Guthrie
non-flammable but highly potent which resulted in many deaths
Anesthetic leaves the blood via
the lungs with ventilation
Diffusion hypoxia can occur
Patients who were receiving nitrous oxide during a case should receive 100% oxygen on emergence in order to get rid of nitrous from the lungs but this can cause diffusion atelectasis (utilizing 100% fiO2)
Emergence tends to be smoother than induction
after longer cases
less so in younger patients
The longer an anesthetic gas is used during the case
the slower the emergence
The higher the solubility
in regards to emergence
the slower the emergence (isoflurane>sevoflurane>desflurane>nitrous oxide)
Definition of diffusion hypoxia
when high concentrations of insoluble anesthetics such as nitrous oxide are delivered, it quickly exits the lungs during emergence and is replaced by nitrogen resulting in dilution of less soluble gases such as oxygen and carbon dioxide
delivering 100% oxygen for several minutes prior to
emergence prevents this phenomenon
What happens in emergence phase 1?
cessation of anesthetic drugs, reversal of NMBD, transition from apnea to breathing, increased alpha and beta waves on EEG
What happens in emergence phase 2?
increased HR and BP, return of autonomic responses, responsiveness to pain, salivation, tearing, grimacing, swallowing and gagging, defensive posturing
What happens in emergence phase 3?
eye opening, responds to verbal commands, awake EEG patterns, extubation possible
How does obesity impact anesthetics?
minimal impact on uptake, prolonged cases may prolong emergence due to deposit of anesthetic into adipose tissue
How does pregnancy impact anesthetics?
higher minute ventilation but higher CO counterbalances
How does cardiopulmonary bypass impact anesthetics?
higher concentrations required than under normal lung ventilation
uptake and elimination is inversely related to blood:gas solubility
How does left-to-right shunts impact anesthetic gas?
increases uptake of anesthetic in mixed venous blood
slight increase in uptake to the brain, tissue, and muscle
How do right-to-left shunts impact anesthetic gas?
(PE) shunted blood mixes and dilutes blood coming from ventilated alveoli reducing anesthetic partial pressure and slows induction
insoluble agents are affected more
Because nitrous oxide is 34 times more soluble than nitrogen, when nitrous oxide is administered during a case,
it will readily diffuse into air-containing cavities, increasing the volume
Rigid air-containing spaces will undergo increased pressure
What cases are we concerned with usage of nitrous oxide and air cavities?
ENT- tympanic space/ eyes intracranial vault air emboli cases air filled cuffs on ET tube and pulmonary artery catheters free air in bowel (bowel obstruction)
Pediatric-related factors of anesthetic gases include:
anesthetic uptake greater in pediatric patients than adults d/t decreased muscle mass so higher concentration in vessel rich tissue
anesthetic agents less soluble in children than adults
a six month old has 1.5-1.8 times the MAC
requirement of a 40 year old adult
Emergence delirium is
more common in infants, children, and young adults and can be attenuated with administration of dexmedetmoidine, fentanyl, and ketamine
