Transcript Inhalation Anesthetics

Inhalation Anesthetics
CHAPTER 3
ISOFLURANE AND SEVOFLURANE
(HALOGENATED COMPOUNDS)
NITROUS OXIDE AND DESFLURANE
ENFLURANE
HALOTHANE
METHOXYFLURANE
DIETHYL ETHER
Diethyl Ether
 1st inhaled anesthetic
 No longer used as an anesthetic agent
 Classic stages and planes of anesthesia described
using ether
 Desirable characteristics
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Stable cardiac output, rhythm, and blood pressure
Stable respirations
Good muscle relaxation
Diethyl Ether (Cont’d)
 Undesirable characteristics
 Tracheal and bronchial mucosal irritation
 Prolonged induction and recovery
 Postoperative nausea and vomiting
 Flammable and explosive
Halogenated Organic Compounds
 Isoflurane and sevoflurane are the most commonly
used agents in this class
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Others include Desflurane, Halothane, Methoxyflurane, and
Enflurane, but these are not commonly used
 Liquid at room temperature
 Stored in a vaporizer on an anesthetic machine
 Vaporized in oxygen that flows through the vaporizer
 Exception being Desflurane- has a special injection type
vaporizer
Uptake and Distribution of
Halogenated Organic Compounds
 Liquid anesthetic is vaporized and mixed with
oxygen
 Mixture is delivered to the patient via a mask or
endotracheal tube (ET tube)
 Mixture travels to lungs (alveoli) and diffuses into
the bloodstream
 Diffusion rate is dependent on concentration
gradient (alveoli/capillary) and lipid solubility of the
anesthetic gas
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Concentration gradient is greatest during initial induction
Uptake and Distribution of Halogenated
Organic Compounds
 Distribution to tissues is dependent on blood
supply
Tissues with greater blood flow (brain, heart, kidney) are
more quickly saturated with anesthetic gas
 Lipid solubility of the gas determines entry into tissues
through cell walls
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 Maintenance of anesthesia is dependent on
sufficient quantities of anesthetic delivered to the
lungs
Elimination of
Halogenated Organic Compounds
 Reducing amount of anesthetic administered
reduces amount in the alveoli
 Anesthetic will move from the brain into the blood
and then into the alveoli where it is finally
breathed out
 Patient wakes up
Inhalant Anesthetics
EFFECTS:
ADVERSE EFFECTS:
EFFECTS:
ADVERSE EFFECTS:
EFFECTS:
ADVERSE EFFECTS:
Physical and Chemical Properties
of Inhalant Anesthetics
 Important properties to consider
 Vapor pressure
 Partition coefficient
 Minimum alveolar concentration (MAC)
 Rubber solubility
Vapor Pressure
 Is the amount of pressure exerted by the gaseous
form of a substance when in equilbrium
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i.e. – it’s ability to evaporate
 Determines how readily an inhalation anesthetic will
evaporate in the anesthetic machine vaporizer
 Temperature and anesthetic agent dependent
Vapor Pressure
 Volatile agents
 High vapor pressure- evaporates readily
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Isoflurane, sevoflurane, desflurane, and halothane
Delivered from a precision vaporizer to control the
delivery concentration
 All precision vaporizers are made to deliver only one
specific halogenated agent
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 Nonvolatile agents
 Low vapor pressure- no need for precision vaporizer
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Methoxyflurane
*Vaporizers are specific to that gas, and is unacceptable to
combine agents in the same vaporizer. Although it is safe
to switch patient from one gas to another
Blood:Gas Partition Coefficient
 The measure of the solubility of an inhalation
anesthetic in blood as compared to alveolar gas (air)
 Indication of the speed of induction and recovery for
an inhalation anesthetic agent
 Low blood:gas partition coefficient
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Agent is more soluble in alveolar gas than in blood at
equilibrium
Agent is less soluble in blood
Faster expected induction and recovery
Blood:Gas Partition Coefficient
 High blood:gas partition coefficient
 Agent is more soluble in blood than in alveolar gas at
equilibrium
 Agent is less soluble in alveolar gas
 Agent is absorbed into blood and tissues (sponge effect)
 Slower expected induction and recovery
Blood:Gas Partition Coefficient
 Blood: gas partition coefficient determines the
clinical use of the anesthetic agent
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Induction: Can a mask be used?
Maintenance: How fast will the anesthetic depth change in
response to changes in the vaporizer setting?
Recovery: How long will the patient sleep after anesthesia?
Minimum Alveolar Concentration
(MAC)
 The lowest concentration of which 50% of patients
shows no response to a painful stimulus
 The measure of the potency of a drug
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Used to determine the average setting on the vaporizer that
will produce surgical anesthesia
 The lower the MAC, the more potent the anesthetic
agent and the lower the vaporizer setting
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MAC may be altered by age, metabolic activity, body
temperature, disease, pregnancy, obesity, and other agents
present
 Every patient must be monitored as an individual
 Age, disease, temperature, pregnancy, obesity, pre medications
Isoflurane
 Most commonly used inhalant agent in North
America
 Approved for use in dogs and horses; commonly
used in other species
Isoflurane
 Properties
 High vapor pressure: need a precision vaporizer
 Low blood:gas partition coefficient: rapid induction and
recovery
 Good for induction with mask or chamber
 MAC = 1.3% to 1.63%: helps determine initial vaporizer setting
 Low rubber solubility
 Stable at room temperature; no preservatives needed = no
build up in the machine
Effects and Adverse Effects
 Maintains cardiac output, heart rate, and rhythm
 Fewest adverse cardiovascular effects
 Depresses the respiratory system
 Maintains cerebral blood flow
 Almost completely eliminated through the lungs- 0.2%
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metabolized by the liver
Induces adequate to good muscle relaxation
Provides little or no analgesia after anesthesia
Difficult to mask patient
Can produce carbon monoxide when exposed to a
desiccated carbon dioxide absorbent
Sevoflurane
 High vapor pressure: need a precision vaporizer
 Low Blood:gas partition coefficient = rapid induction
and recovery
 Good for induction with a mask or chamber
 High controllability of depth of anesthesia
 MAC = 2.34% to 2.58%
 Cost about 10x more than Isoflurane
 Easier to mask a patient, more pleasant
smelling
Effects and Adverse Effects of Sevoflurane
 Minimal cardiovascular depression
 Depresses respiratory system
 Eliminated by the lungs, minimal hepatic metabolism- 2-
5%
 Maintains cerebral blood flow
 Induces adequate muscle relaxation
 Some paddling and excitement during recovery
 No post-op analgesia
 Can react with potassium hydroxide (KOH)
or sodium hydroxide (NaOH) in desiccated
CO2 absorbent to produce a chemical
(Compound A) that causes renal damage
CNS and Respiratory Stimulants
 Doxapram
 Analeptic agent (CNS stimulant)
 Stimulates respiration and speeds recovery
 Acts at the carotid sinus and the aortic arch
 Used in neonate puppies and kittens after C-section
 IV administration or sublingual drops (neonates)
Adverse effects
 Wide margin of safety, but the following may
be seen:
 Hyperventilation and hypertension
 Lowers seizure threshold
 CNS damage
Use of Doxapram
 Repeat injections may be necessary
 Reverses respiratory depression from inhalant
agents and barbiturates
 1-5 ggt under the tongue of a puppy, or 1-2 ggt in a
kitten if needed after C-section or dystocia
UNCOMMONLY USED
INHALANT ANESTHETICS
Desflurane
 Closely related to isoflurane
 Expensive
 Lowest blood:gas partition coefficient: very rapid
induction and recovery
 Used with a special heated electronic precision
vaporizer
 MAC = 7.2% and 9.8%
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Least potent inhalant agent
 Eliminated by the lungs- 0.02% metabolized in liver
Effects and Adverse Effects of Desflurane
 Strong vapors cause coughing and holding the
breath= difficult to mask
 Other effects are similar to isoflurane
 Transient increase in heart rate and blood pressure
(humans)
 Produces carbon monoxide with spent soda lyme
Other Halogenated Inhalation Agents
 Halothane (Fluothane)
 Not available anymore
 replaced by isoflurane and sevoflurane
 B:G -2.54
 20-46% metabolized in the liver
 MAC- 0.87-1.19
 Sensitizes heart to catecholamine and induces
arrhythmias
 Cardiac, respiratory depression
 Increased cerebral blood flow
 Increased temperature- malignant hyperthermiaDantrolene is used for treatment
Methoxyflurane
 Methoxyflurane
 No
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longer available in North America
B:G- 151!
Used in a non precision vaporizer- wick
50-75% metabolized by the liver, excreted by the
kidneys!!
Fluoride ions and other potentially toxic metabolites
produced by the liver= renal damage
 Enflurane
 Used primarily in human medicine
Nitrous Oxide
 Nitrous oxide
Used primarily in human medicine; some veterinary use
 A gas at room temperature; no vaporizer is required
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 Mixed with oxygen at 40-67%, then delivered to patient
 Reduces MAC 20-30%
Used with Halothane and Methoxyflurane
to reduce the adverse effects of these gases
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