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, as well as
Chloroform and NO
 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
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used agents in this class
Also Desflurane, Halothane, Methoxyflurane, and
Enflurane
Liquid at room temperature
Stored in a vaporizer on an anesthetic machine
Vaporized in oxygen that flows through the vaporizer
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Exception being Desflurane- has a special injection type
vaporizer
Uptake and Distribution of
Halogenated Organic Compounds
 Liquid anesthetic is vaporized and mixed with
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oxygen gas
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
Concentration gradient is greatest during initial
induction
Uptake and Distribution of Halogenated
Organic Compounds (Cont’d)
 Distribution to tissues is dependent on blood supply
 Lipid solubility determines entry into tissues through cell walls
 Depth of anesthesia is dependent on partial pressure of
anesthetic in the brain
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Partial pressure in the brain is dependent on partial pressure of the
anesthetic in blood and alveoli
 Maintenance of anesthesia is dependent on sufficient
quantities of anesthetic delivered to the lungs
Elimination of
Halogenated Organic Compounds
 Reducing amount of anesthetic administered
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reduces amount delivered to the alveoli
Blood level is initially higher than alveolar level
Concentration gradient now favors anesthetic
diffusion from blood into the alveoli
Blood levels drop quickly as patient breathes out
anesthetic from the alveoli
Brain levels drop as less anesthetic is delivered by
blood
Patient wakes up
Effects of
Halogenated Organic Compounds
 CNS
 Dose-related reversible CNS depression
 Hypothermia
 Cardiovascular system
 Depress cardiovascular function
 Effects on HR variable
 Respiratory system
 Dose-dependent ventilation depression
Adverse Effects of
Halogenated Organic Compounds
 CNS (Cont’d)
 Increased intracranial pressure in patients with head trauma
or brain tumors
 Considered safe for epileptic animals
 Cardiovascular system
 Decrease blood pressure and may decrease renal blood flow
 Respiratory system
 Hypoventilation
 Carbon dioxide retention and respiratory acidosis
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
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High vapor pressure- evaporates readily
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
 Nonvolatile agents
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Low vapor pressure- no need for precision vaporizer
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
 Solubility is a ratio of concentration of an agent in 2
substances
 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 (Cont’d)
 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 (Cont’d)
 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
 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 the 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 of 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
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 commonly used anymore
 Being 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
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 toxid 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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CNS and Respiratory Stimulants
 Doxapram
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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
 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