Anesthetic Inhalants Flashcards
what is the maineneance phase of anesthesia
- to keep the patient in an unconsious state as long as required
- usually switch from IV to an inhaled general anesthetic
how to achieve “balanced anesthesia”
sedatives, analgesics and general anesthetics are each administered at a low to moderate dose - safer than high dose single-drug protocols
what are the goals during maintenence anesthesia
- keep patient in stage 3 sugical anesthesia
- adequate analgesia during surgical procedures
- physiologic homeostasis (monitor vitals)
true or false: inhalent anesthetics are cleared much faster than IV general anesthetics
true
what determines the safety of a general anesthetic
the extent to CV and respiratory functions are impaired at concentrations required to maintain unconsciousness
Characteristics of an ideal inhalent anesthetic
- produces unconsciousness while maintaining brainstem function
- negligible visceral toxicity
- non-flammable
- non-pungent odour
- compatible with anesthetic machinery
- does not react woth soda lime
- chemically stable
- potent
why have we switched from using Ether as an inhaland anesthetic to using halogenated hydrocarbons
- ether is flammable
- substituting halogens for hydrogens reduces flammability and improves safety
Halothane as an inhaled anesthetic
- safer than ether but still unstable because it contains Br
- must be combines with stabilizing molecule: thymol
- problem is thymol accumulates on machine valves, causing malfunction
- halothane breaks down in the presence of soda lime and releases a toxic compound
- sensitizes the myocardium to epinephrine - promotes arrhythmia
what is soda lime
a CO2 absorbant used in most inhalent machines - removes CO2
hepatic and renal toxicity of inhalants (halothane)
- halothane undergoes extensive hepatic biotransformation into toxic metabolites
- metabolites are at highest concentration in the liver and kidney
- toxicity is directly related to the extent which an nhalent is metabolized
- Halothane = 40% metabolized, Isofluorane = 0.2% metabolized (no organ toxicity)
Evolution of halogenated hydrocarbons from Halothane
- switch to those that have an ether linkage
- switch bromine for more fluorines
- ether + fluorine = safer inhalent
- e.g. isoflurane and sevoflurane
what are the main inhalents we use today
isoflurane
sevoflurane
- theraputic index = 2 to 4 (narrow)
most likely Mechanism of action for inhalant anesthetics
- facilitate the interaction of GABA with the GABAa receptor in the brain
why are inhalent anesthetics a volatile liquid before administration
- they are poured into a vapour chamber that is connected to the oxygen floe system of the anestheric machine
- vapor chamber turns this liquid to gas
Anesthetic machine as a rebreather recycling machine
- anesthetic is breathed in by patient
- ## patient breathes out
which part of the anesthetic machine enters the patient
- endotracheal tube: delivers the anesthetic
- inflatable cuff: creates a seal in the trachea to prevent vapor from escaping into the room and prevent inhalation og saliva or stomach contents
Pathway of anesthetic vapor in the body
anesthetic vapor is delivered in pure oxygen - absorbed from lungs into systemic circulation - districuted to tissues
how is anesthetic removed from the body
- mainly though exhalation
- small amount may be metabolized by liver
how can we remove inhalants rapidly from the body in an emergency
- reduce the anesthetic concentration
- apply artificial respiration
- drug concentration in the body falls as drug is exhaled
what determines the speed at which anesthetics enter and leave the brain and cause their effects?
solubility and concentration
solubility of halothane vs isoflurane
halothane = highly soluble in tissues
isoflurane = less soluble in tissues
solubility vs. concentration of an inhalent
if a drug is poorly soluble in tissues, we must administer a high concentration of it in the inspired air to achiece the same concentration in the brain as a more soluble drug
what is the depth of anesthesia proportional to
the partial pressure created by the anesthetic
high solubility volatile anesthetic (halothane) =
- low partial pressure within tissues
- don’t need high concentration to cross from alveolar air to blood
- slow diffusion
- slower onset and recovery
poorly soluble volatile anesthetic (isoflurane) =
- high partial pressure within tissues
- need higher concentration to cross from alveolar air to blood
- fast diffusion
- faster onset and recovery
why is partial pressure more critical than concentration for inhalant anesthetics
- creates an equilibrium across tissues to produce the anesthetic effect
- uptake and distribution driven by pressure gradients
- correlates with clinical effects pg hypnosis, amnesia and immobility
conventration vs. partial pessure
concentration - the rate at which the partial pressure rises
partial pressure - determines the effectoveness of an anesthetic
what factors influence the delivery of inhalants from the lungs to the brain
- ratio of the concentration of volatile anesthetic in blood compared to alveolar gas once PP has been equilibrated (BGPC)
- Concentration of anesthetic in the inspired gas mixture (MAC)
- Ventilation rate of patient
what is the blood:gas partition coefficient (BGPC)
- a value that indicates the relative solubility of the drug in blood vs alveolar gas
- highere BGPC = highly soluble
if an anesthetic has a high BGPC…
a large amount of it will have to be taken up in the body’s blood before being passed on to the tissues of the brain
isoflurane has a BGPC of 1.4, what does this mean?
at partial pressure equilibrium, the concentration will be 1.4x higher in tissues than in the alveolar air
a low BCPG means…
- anesthetic is poorly soluble in blood
- has a high partial pressure
- fast onset/recovery
why do we prefer to use anesthetics with low tissue solubility
- we want to be able to adjust anesthetic depth rapidly
what is the minimum alveolar concentration (MAC)
- measure of potency at the level of the lungs
- the concentration in the inspired gas that will prevent 50% of petients from being awakened in response to mild pain
- expressed as a percent vapour in the inspired gas
MAC is inversly realted to potency and solubility…
low MAC = high potency
low MAC = high solubility
- therefore would need a higher concentration of isoflurane to achieve the same level of unconsciousness as halothane
how can we control the inspired anesthetic concentration
- using a precision vapour
- MAC tells us what to set the vapourizer dial to
in situations where it is not ideal to premedicate patients what happens to the MAC?
- we start woth the vapour setting at 1.3 so it is the concentration at which 95% of non-premedicated patients will not respond to pain, then reduce to 50% for maintenence
what influence the elimination of inhalents from the body
- solubility in tissues (MAINLY)
- respiration rate
- pulmonary blood flow
adverse effects of inhalants on thermoregulation
- depress thermoregulation and cause hypothermia
adverse effects of inhalants on metabolism
- extent of metabolizm related to toxicity
- can lead to hepatotocicity and renal toxicity
- only a problem in long procedures with halothane or very long procedures with sevoflurane
Rare adverse effect of inhalant - malignant hyperthermia
- an inherited susceptibility caused by mutations in a gene that codes for skeletal muscle calcium release channels
- when exposed to a halogenated hydrocarbon it triggers uncontrolled muscle contraction
- body temp rises and brain damage may occur
- can be treated by administering a skeletal muscle relaxant
Characteristics of Isoflurane
- most commonly used inhaled anesthetic
- no hepatic or renal toxicity
- low solubility in tissues - rapid chamges in depth are possible
- low depression of CO, but causes vasodilation which causes dose-dependent drop in BP
- highly stable - wont react with soda lime
what is the only real drawback of isoflurane
has a pungent odour - can cause gagging in non premedicated patients
safety of isoflurane compared to halothane
- isoflurane has a much larger safety margin at the lungs
- the lethal effects of both drugs begon at the same brain drug concentration
- isoflurane still produces its desired effect at lower brain concentrations
Characteristics of Sevoflurane
- similar to isoflurane - even less soluble in tissues so faster onset and recovery
- minimal metabolism = low toxicity
- less potent than isoflurane = higher MAC
- main advantage = non-pungent odour
drawbacks of sevoflurane
- humans can metabolize up to 5%, but fluoride ions appear to damage renal tubules whem hey accumulate during long procedures
- breaks down in the presence of soda lime to form toxic compound during long procedures
Characteristics of nitrous oxide
- aka laughing gas
- inhibits the NMDA receptor in the brain
- a weak CNS depressant - cannot produce anasthesia
- used to reduce the amount of hydrocarbon inhalant needed to produce unconciousness
- provides excellent analgesia and alleviates anxiety
- very safe
