Transcript Chapter 10 outline

Chapter 10:
General Anesthetics
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Chapter 10 Outline

General Anesthetics

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History
Mechanism of action
Adverse reactions
General anesthetics
Balanced general anesthesia
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2
General Anesthetics

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A heterogeneous group of central nervous
system (CNS) depressants


Haveles (p. 127)
Produce a reversible loss of consciousness and
insensitivity to painful stimuli
Contemporary techniques use balanced
anesthesia using a combination of drugs to
minimize adverse reactions

The patient must be monitored constantly for
respiratory depression and loss of protective
reflexes associated with general anesthesia
cont’d…
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3
General Anesthetics

Oral and maxillofacial surgeons have used
general anesthetic drugs for many years with
an excellent safety record
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
Nitrous oxide is not useful alone as a general
anesthetic
Other general anesthetic drugs, in less than
anesthetic doses, are used to provide conscious
sedation in the dental office
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4
History


About the middle of the 1800s, true general
anesthetics were discovered in the Unite States


Haveles (pp. 127-128)
Colton began giving public demonstrations of
“laughing gas” (nitrous oxide)
Horace Wells, a Connecticut dentist saw nitrous
oxide/oxygen (N2O/O2) sedation demonstrated

He persuaded William Morton, a former dental partner
studying medicine, to arrange for a demonstration at
Harvard
• It failed because of the low potency of nitrous oxide
cont’d…
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5
History

William Morton experimented with ether
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It was used successfully to anesthetize the patient
Morton spent the end of his life attempting to be
compensated for the discovery
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6
Mechanism of Action


Haveles (p. 128)
Overview

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Many theories have been proposed to explain the
mechanism of action of general anesthetics, but
none of them does so completely
We know they depress the CNS, but the way in
which they depress normal function is complicated
by a lack of knowledge of physiologic and
biochemical events governing arousal and
unconsciousness
cont’d…
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7
Mechanism of Action

Overview

Proposed mechanisms for action of general
anesthetics
• An increase in threshold for firing
• Facilitation of inhibitory γ-aminobutyric acid (GABA)
• Decrease in duration of opening of nicotinic receptor
activated cation channel
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8
Stages and Planes of Anesthesia
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
Haveles (pp. 128-129) (Table 10-1)
The degree of CNS depression must be
carefully titrated to prevent excessive
cardiorespiratory depression

Guedel created a system of stages and planes to
describe the effects of anesthesia in 1920
 Modern techniques seldom show these exact
stages, but Guedel’s terminology is still used to
describe the depth of anesthesia
cont’d…
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9
Stages and Planes of Anesthesia
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
Haveles (p. 128)
Stage I: analgesia

Reduced pain sensation
 The patient is still conscious and can respond
 Reflexes are present, respiration remains regular
 Some amnesia may be evident
 The end of stage I is marked by loss of
consciousness
cont’d…
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10
Stages and Planes of Anesthesia
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
Haveles (p. 128)
Stage II: delirium or excitement
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Stage begins with loss of consciousness and is
associated with involuntary movement and excitement
Respiration becomes irregular, and muscle tone
increases
Sympathetic stimulation produces tachycardia,
mydriasis, and hypertension
Emesis (vomiting) and incontinence (defecation) can
occur
cont’d…
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11
Stages and Planes of Anesthesia
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
Haveles (p. 128)
Stage III: surgical anesthesia

This is the stage in which most major surgery is
performed
• Divided into four planes differentiated by eye movements,
depth of respiration, muscle relaxation
• The onset of stage III (planes I and II) is typically
characterized by the return of regular respiratory movements,
muscle relaxation, and normal heart and pulse rates
• Beginning in plane III and progressing to plane IV is
characterized by intercostal muscle paralysis (diaphragmatic
breathing remains), absence of all reflexes, and extreme
muscle flaccidity
cont’d…
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12
Stages and Planes of Anesthesia


Haveles (p. 129)
Stage IV: respiratory or medullary paralysis
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Characterized by complete cessation of
respiration and circulatory failure
Pupils are maximally dilated, and blood pressure
falls rapidly
If this stage is not reversed immediately, the
patient will die
Respiration must be artificially maintained
cont’d…
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13
Stages and Planes of Anesthesia


Haveles (p. 129) (Fig. 10-1)
Modern techniques (Flagg’s approach)
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
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Induction: all preparation and medication up until
the operation begins, including preoperative
medications, adjunctive drugs to anesthesia, and
anesthetics required for induction
Maintenance: begins with depth of anesthesia
sufficient for surgery and continues until the
completion of the procedure
Recovery: termination of the procedure until the
patient is fully responsive
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14
Adverse Reactions to General
Anesthetics
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

Haveles (p. 129) (Table 10-2)
Cardiovascular system: collapse, arrest
Arrhythmias: ventricular fibrillation
Blood pressure: hypertension (stage II), hypotension
Respiration: depressed (stage III) or arrest (stage
IV), laryngospasm (ultrashort-acting barbiturates),
“boardlike” chest (neuroleptanalgesia)
Explosions/flammability: cyclopropane, ether
Teratogenicity (either male or female exposure):
fetal abnormalities, spontaneous abortion
Hepatotoxicity (repeated exposure): for operating
room personnel, halogenated hydrocarbons
Other: headache, fatigue, irritability, addicting
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15
General Anesthetics
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Classification of anesthetic agents
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

Haveles (pp. 129-135)
Inhalation anesthetics
Physical factors
Intravenous (IV) anesthetics
Nitrous oxide
Halogenated hydrocarbons
Other general anesthetics
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16
Classification of Anesthetic
Agents


Haveles (pp. 129-130) (Table 10-3)
General anesthetics can be classified
according to chemical structure or route of
administration
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17
Inhalation Anesthetics


Haveles (p. 129)
Inhalation agents are divided into gases and
volatile liquids
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
The liquids are vaporized and carried to the
patient in the form of a gas
Inhalation agents are often used in combination,
with oxygen as a carrier gas
cont’d…
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18
Inhalation Anesthetics

Volatile general anesthetics are liquids that
evaporate at room temperature because of
their low boiling points

They are classified as halogenated hydrocarbons
or halogenated ethers
• halothane (Fluothane), a halogenated hydrocarbon, and
methoxyflurane (Penthrane), a halogenated ether, are
used infrequently
• enflurane (Ethrane) and isoflurane (Forane) are
halogenated ethers used more frequently
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19
Physical Factors
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The concentration of anesthetic in the
inspired mixture is proportionate to its partial
pressure or tension


Haveles (pp. 129-130) (Table 10-4)
The depth of anesthesia is a function of the
tension (partial pressure) of anesthetic agent in
the brain
Induction can be hastened with high initial
anesthetic concentration and hyperventilation
cont’d…
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20
Physical Factors

The term minimal alveolar concentration
(MAC) is used to compare potency of general
anesthetic inhalation agents

MAC is the defined as the minimum alveolar
concentration of anesthetic at 1 atmosphere
required to prevent 50% of patients from
responding to a supramaximal surgical stimulus
cont’d…
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21
Physical Factors
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MAC of nitrous oxide > 100; MAC of
isoflurane is 1.15
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
Lower MAC values indicate a more potent
anesthetic
Volatile anesthetics are given in combination
with nitrous oxide to reduce the concentration
of each while improving MAC values
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22
Intravenous Anesthetics
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
Haveles (p. 130)
IV anesthetics include opioids, ultrashortacting barbiturates, and benzodiazepines


ketamine (Ketalar) can also be given
intramuscularly
These drugs are most commonly used for
induction of general anesthesia but may be used
as single agents for short procedures
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23
Nitrous Oxide
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A colorless gas with little or no odor


Haveles (pp. 130-133)
The least soluble in blood of all inhalation
anesthetics
Because of its low potency (MAC > 100), it is
unsatisfactory as a general anesthetic when
used alone

If, however, anesthesia is first induced with a
rapidly acting IV agent and N2O/O2 is administered
in combination with a volatile anesthetic, excellent
balanced anesthesia is produced
cont’d…
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24
Nitrous Oxide
 Haveles

(p. 131) (Table 10-5)
Administration of N2O/O2 has become a
primary part of dental office anxiety reduction


The intent is to provide for a lightly sedated and
relaxed patient
It provides anxiety relief with analgesia
cont’d…
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25
Nitrous Oxide

At the termination of the procedure, the
patient should be placed on 100% oxygen for
at least 5 minutes to avoid diffusion hypoxia


This occurs because of the rapid outward flow of
nitrous oxide accompanied by oxygen and carbon
dioxide
The loss of carbon dioxide, a stimulant to
respiratory drive, could decrease ventilation with
resultant hypoxia
cont’d…
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26
Nitrous Oxide


Haveles (p. 131)
Advantages of the N2O/O2 technique

Rapid onset: under 5 minutes
 Easy administration: no injection required
 Close control: percentage of nitrous oxide easily
adjusted
 Rapid recovery
 Acceptability for children (but hygienist in MI
cannot administer to people under 18)
 Relaxed dental team
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27
Pharmacologic Effects
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CNS sedation
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Resulting in analgesia and amnesia
Cardiovascular effects


Haveles (pp. 131-132)
Peripheral vasodilation
Gastrointestinal effects

Nausea and vomiting are uncommon but may occur
cont’d…
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28
Pharmacologic Effects
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The best indicator of the degree of sedation is
the patient’s response to questions
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Haveles (pp. 131-132) (Table 10-5)
The patient may exhibit slurred speech or a slow
response
The patient is relaxed and cooperative and
reports a feeling of euphoria
The patient is easily able to maintain an openmouth position in the desired plane
The patient’s eyes may be closed but can be
opened easily
The respiration, pulse, rate, and blood pressure
are within normal limits
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29
Adverse Reactions


Haveles (p. 132)
Complications have been the result of misuse or
faulty installation of equipment

Nitrous oxide cylinders are blue and oxygen cylinders
are green
 Cylinders are “pin coded” to prevent mixing of cylinders
and lines

Nitrous oxide concentration should be
automatically limited and have a fail-safe system
that shuts off automatically if the oxygen runs out
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30
Contraindications and Dental
Issues
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Respiratory obstruction
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
Haveles (pp. 132-133)
Contraindication for technique
Chronic obstructive pulmonary disease (COPD)

For a normal person, breathing is stimulated by
elevated carbon dioxide levels; for persons with
COPD the patient’s ventilation may be driven by a
decrease in the partial pressure of oxygen [typo in
book: should be PaO2 not PaCO2]
• If a patient with COPD is given oxygen and the oxygen
pressure rises, the stimulant to breathing is removed and the
possibility arises of inducing apnea
cont’d…
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31
Contraindications and Dental
Issues

Emotional instability

Fanciful dreams during the procedure may be
interpreted as actually having occurred
• Have a female staff member in the room when a male is
treating a female patient

Pregnancy considerations

Safety of use in pregnant patients or administration by
pregnant operators is in question
• The incidence of spontaneous abortion or miscarriages is
higher in female operating personnel chronically exposed to
anesthetic agents or in wives of male operators
cont’d…
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32
Contraindications and Dental
Issues
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
Haveles (p. 133)
Abuse


Case histories of chronic abuse have reported
examples of neuropathy
Symptoms include numbness and paresthesia of
the hands or legs that progresses to more severe
neurologic symptoms
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33
Halogenated Hydrocarbons
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

Haveles (pp. 133-134)
Halothane
Enflurane
Isoflurane
Desflurane and sevoflurane
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34
halothane
(Fluothane)


Nonflammable and nonexplosive
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
Relatively rapid induction and recovery
Nonirritating to bronchial mucous membranes
Bradycardia and peripheral vasodilation that
lowers blood pressure


Haveles (p. 133) (Fig. 10-2)
Sensitizes myocardium to cardiac stimulatory effects
of injected epinephrine, leading to cardiac
arrhythmias
Causal relationship with postanesthetic hepatitis
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35
enflurane
(Ethrane)


A halogenated ether



Haveles (p. 133)
Rapid induction and recovery
Good analgesia and muscle relaxation, but
supplemental muscle relaxants are still required
Alteration in electroencephalographic activity:
may cause excessive motor activity during
anesthesia
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36
isoflurane
(Forane)


Low tissue solubility allows for rapid induction
and recovery



Haveles (p. 133)
Effects include respiratory depression, reduced blood
pressure, and muscle relaxation
Respiratory acidosis associated with deeper
levels of anesthesia
A useful and popular drug for general anesthesia
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37
Desflurane and Sevoflurane



Low blood/gas partition coefficient for rapid
onset and shorter duration of action
Desflurane’s low volatility requires a special
vaporizer


Haveles (p. 134)
Because it induces cough and laryngospasm, it
cannot be used for induction
Sevoflurane is chemically unstable when
exposed to carbon dioxide absorbents,
producing a potentially nephrotoxic compound
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38
Other General Anesthetics

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

Haveles (pp. 134-135)
Ultrashort-acting barbiturates
Propofol
Ketamine
Opioids
Droperidol plus fentanyl
Benzodiazepines
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Ultrashort-Acting Barbiturates


Have a rapid onset when given intravenously


Haveles (p. 134) (Fig. 10-3)
If used alone for short surgical procedures, the
patient will respond to painful stimuli; most
effectively used with a local anesthetic
Other complications include laryngospasm
and bronchospasm
cont’d…
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40
Ultrashort-Acting Barbiturates

Extravascular injection can lead to necrosis
and sloughing


Intraarterial injection can lead to arteriospasm
associated with ischemia of the arm and fingers
and severe pain
Absolute contraindications include an
absence of suitable veins, status asthmaticus,
porphyria, or known hypersensitivity
cont’d…
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41
Ultrashort-Acting Barbiturates

These drugs can be used alone for very short
dental procedures or as part of a balanced
anesthesia to induce surgical anesthesia

Resuscitative equipment should be available
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42
Examples of Ultrashort-Acting
Barbiturates




Haveles (pp. 130) (Table 10-3)
methohexital (Brevital)
thiamylal (Surital)
thiopental (Pentothal)
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43
propofol
(Diprivan)


An IV anesthetic unrelated to any other general
anesthetic




Patients “feel better” and are back on their feet
sooner than with other agents
Can be used for induction and maintenance of
balanced anesthesia
Popular for outpatient surgery
Can produce a marked decrease in blood
pressure during induction; produces vasodilation



Haveles (p. 134)
Apnea occurs in 50% to 80% of patients
Bradycardia and pain at injection site
Relatively costly
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44
ketamine
(Ketalar)


Chemically related to phencyclidine (PCP), a
hallucinogen



Haveles (pp. 134-135)
Produces dissociative anesthesia because it
appears to disrupt association pathways in the
brain
The patient appears catatonic and has amnesia;
ketamine produces analgesia without actual loss
of consciousness
May be given intravenously or intramuscularly
cont’d…
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45
ketamine
(Ketalar)


Principal drawback is “emergence
phenomenon” including delirium and
hallucinations during recovery
Specific contraindications include a history of
cerebrovascular disease, hypertension, and
hypersensitivity to the drug

Psychiatric problems present a relative
contraindication
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46
Opioids


May be used as anesthetic agents



Haveles (p. 135)
Include morphine, fentanyl (Sublimaze), sufentanil
(Sufenta), and alfentanil (Alfenta)
Do not significantly alter cardiac function or
peripheral resistance
Prolonged respiratory depression is a major
disadvantage
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47
droperidol plus fentanyl
(Innovar)


Neuroleptanalgesia: a so-called wakeful
anesthetic state produced by the combination
of a neuroleptic drug, droperidol (Inapsine)
and the opioid analgesic fentanyl (Sublimaze)


Haveles (p. 135)
Produces marked sedation and a catatonic state
Respiratory depression and extrapyramidal
tremors have occurred
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48
Benzodiazepines


Used for conscious sedation and preanesthetic
medication for years



Haveles (p. 135)
diazepam (Valium) has been used intravenously
midazolam (Versed), which is water soluble, does
not need solvent for solution, thus one of
diazepam’s side effects, thrombophlebitis, can be
avoided
Benzodiazepines are used as adjunctive drugs
in balanced anesthesia or for conscious
sedation
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49
Balanced General Anesthesia


The goals of surgical anesthesia are good
patient control, adequate muscle relaxation,
and pain relief


Haveles (p. 135)
Many agents can produce general anesthesia;
each has its own adverse reaction profile
When balanced anesthesia is used, the patient
passes from stage I to stage III, skipping over
the signs of stage II
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50