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ANESTHETICS

I. LOCAL ANESTHETICS

II. GENERAL ANESTHETICS

Major Classes of Anesthetic

Agents

Local

– Are injected at the operative site to block

nerve conduction

General

– Are given either as inhaled or intravenous

agents

– Primarily have CNS effects

Local Anesthesia

• Is the condition that results when sensory transmission from a local area of the body to the CNS is blocked

Local Anesthetics

Esters

Long-acting

– Tetracaine

Short-acting

– Procaine (Novocain)

– Chloroprocaine

(Nesacaine)

Surface-acting

– Cocaine

– Benzocaine (Cetacaine)

Amides

Long-acting

– Bupivacaine

(Marcaine)

– Ropivacaine

– Mepivacaine

(Polocaine/Carbocaine)

– Etidocaine (Duranest)

– Prilocaine

Medium-acting

– Lidocaine (Xylocaine)

Mechanism of Action of Local

Anesthetics

Blockade of voltage-dependent sodium

channels on the neuronal membrane

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Mechanism of Action of Local

Anesthetics

Blockade of voltage-dependent sodium

channels on the neuronal membrane

Vasoconstrictors added to

Local Anesthetics

Epinephrine (1:2000,000 or 5 ug/ml) – Increased uptake of the local anesthetic

– Higher anesthetic concentration near nerve fibers (increased local anesthetic concentration in the vicinity of sensory nerves

– Increased duration action: prolongation conduction blockade by about 50% longer

Epinephrine

– Reduced systemic absorption by about 33% (advantage: absorption rate more likely to match metabolic rate resulting in less local anesthetic systemic toxicity)

Increased cardiac irritability which may cause an increased risk of cardiac arrhythmias

Increased possibility of hypertensive response in susceptible patients

Dextran

Low-molecular-weight

When added to local anesthetic solutions

result in increased peripheral nerve block

anesthesia duration

Factors Influencing the Effectiveness of

EPI on local anesthesia

Lipophilicity – more lipophilic local anesthetics will tend on their own to associate strongly with tissues such as Mepivacaine (Carbocaine) and Etidocaine (Duranest) than with Lidocaine (Xylocaine), which is less lipophilic

Factors Influencing the Effectiveness of

EPI on local anesthesia

Level of sensory blockade needed for spinal or epidural anesthesia

Duration of lower extremity sensory anesthesia extended by epinephrine or phenylephrine (Neo-Synephrine) which is not observed for abdominal region anesthesia

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Clinical Uses of Local anesthetics

To abolish painful

stimulation prior to surgical,

dental (tooth extraction), or

obstetric (delivery)

procedures.

Commonly found as

ingredients in many OTC

preparations for sunburns,

insect bites, and

hemorrhoids

Common Side Effects of Anesthetics

How does metabolism of the ester

and amide anesthetics differ?

Esters

– More rapidly metabolized by blood and

tissue esterases, which gives them shorter

half-lives

Amides

– Are metabolized by hepatic microsomal

enzymes, which results in a longer half-life

GENERAL

ANESTHETICS

- Are CNS

depressants which

abolish pain by

inhibiting the function

of the CNS through

an unknown

mechanism

PROPERTIES OF GENERAL

ANESTHESIA

All sensation (hearing, sight, touch, smell

and pain) is ABSENT

Primarily used to prevent the reaction to

painful stimuli associated with surgery

All of the major areas of the CNS are

suppressed except the medullary centers

which regulate the vital organs

SIGNS AND STAGES OF

ANESTHESIA

STAGE 1 – ANALGESIA

The cerebral cortex is gradually inhibited

Euphoria, giddiness, and loss of

consciousness

STAGE 2 – DELIRIUM AND EXCITEMENT

Affects the thalamus

Increase sympathetic tone, increase BP

and heart rate; irregular respirations

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SIGNS AND STAGES OF

ANESTHESIA

STAGE 3 – SURGICAL ANESTHESIA

Plane 1 – Sleep, normal BP and respiration

Plane 2 – dilated pupils; loss of corneal reflex

Plane 3 – skeletal muscle relaxation

Plane 4 – paralysis of the diaphragm

STAGE 4 – MEDULLARY PARALYSIS

Respiratory paralysis leading to circulatory collapse and death

* Clinical signs associated with each stage may vary with the agent used

INDUCTION OF ANESTHESIA

– Is the time required to take the patient from

consciousness to stage 3.

MAINTENANCE OF ANESTHESIA

– Is the ability to safely keep the patient in

stage 3

THEORIES OF GENERAL

ANESTHESIA

BIOCHEMICAL HYPOTHESIS

– Decrease cellular function by decreasing ATP production

HYDRATE THEORY

– Anesthetic molecules form gas hydrates or structured water which inhibit brain function

IONIC PORE THEORY

– Blockade of ionic channel by interaction of anesthetic molecule with the membrane

TYPES OF ANESTHETICS

I. INHALATIONAL ANESTHETICS

VOLATILE LIQUIDS

HALOTHANE

ISOFLURANE

DIETHYL ETHER

GASEOUS

NITROUS OXIDE

II. INTRAVENOUS ANESTHETICS

BARBITURATES

METHOHEXITAL

THIAMYLAL

THIOPENTAL

BENZODIAZEPINES

DIAZEPAM

LORAZEPAM

MIDAZOLAM

ETOMIDATE

OPIOIDS

FENTANYL

MORPHINE

DROPERIDOL AND FENTANYL

CITRATE (Innovar)

NEUROLEPTANESTHESIA

KETAMINE – DISSOCIATIVE

ANESTHETIC

PROPOFOL

PHYSIOLOGICAL EFFECTS OF

GENERAL ANESTHETICS CNS EFFECTS

All nervous tissues are depressed

Voluntary (motor) and involuntary (autonomic) systems are inhibited

Respiratory function is depressed

Some cause pituitary secretion of ADH resulting to post operative urinary retention

CARDIOVASCULAR EFFECTS

Myocardium and BP are depressed

Increase heart rate due to vagal inhibition

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PHYSIOLOGICAL EFFECTS OF

GENERAL ANESTHETICS

RESPIRATORY SYSTEM

Inhaled anesthetics irritate the mucosal lining of

the respiratory tract

Increase mucous secretion, coughing and

spasm of the larynx

SKELETAL MUSCLES

Causes relaxation due to depression of

pyramidal system and spinal reflexes

Some causes relaxation by inhibiting the

neuromuscular function

PHYSIOLOGICAL EFFECTS OF

GENERAL ANESTHETICS

GI TRACT

Nausea and vomiting (occurs during

recovery)

Decrease intestinal motility (post operative

constipation)

LIVER

Halothane (high risk), Enflurane and

chloroform cause liver toxicity

INHALATIONAL ANESTHETICS

Primarily used for the maintenance of anesthesia

Produce all stages of anesthesia except Nitrous oxide

Excreted through the lungs

Depth of anesthesia can be rapidly altered by changing the concentration and providing hyperventilation

Don’t cause respiratory depression

Advantages and Disadvantages of

Inhalational anesthetics

Anesthetic ADVANTAGES DISADVANTAGES

NITROUS

OXIDE

Good analgesia

Rapid recovery

Safe, non irritating

No muscle relaxation

Must be used with other

anesthetics for surgical

anesthesia

HALOTHANE Best agent in pediatric

patients

Bronchial smooth muscle

relaxation; good for patients

with asthma

Lowers blood pressure

Reduces renal and hepatic

blood flow

Hepatic toxicity

Arrhythmias

ISOFLURANE

Good muscle relaxation

Rapid recovery

Does not raise intracranial pressure

No sensitization of heart to Epinephrine

INTRAVENOUS ANESTHETICS IV Anesthetics Advantages Disadvantages

Thiopental Rapid onset

Potent anesthesia

Poor analgesia

Little muscle

relaxation

Laryngospasm

Ketamine Good analgesia

Fentanyl Good analgesia

Propofol

Rapid onset

Lowers intracranial

pressure

Poor analgesia

INTRAVENOUS ANESTHETICS

BARBITURATES

Produce residual CNS depression, mental

disorientation and nausea

May cause laryngospasm/bronchospasm

Accumulate in adipose tissue

BENZODIAZEPINES

Midazolam and Lorazepam are more potent

than diazepam

Facilitate amnesia while causing sedation

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INTRAVENOUS ANESTHETICS

ETOMIDATE Hypnotic

Lacks analgesic property

Can cause uncontrolled skeletal muscle activity

OPIOIDS

Morphine + Nitrous oxide provide good anesthesia for cardiac surgery

Are not good amnesics

Can cause hypotension, respiratory depression and muscle rigidity as well as post op. nausea and vomiting

INTRAVENOUS ANESTHETICS

KETAMINE

Produces dissociative anesthesia ( patient

appears awake but is unconscious and does not

feel pain)

Provides sedation, amnesia and immobility

Increase BP and cardiac output

Vivid dreams and hallucinations occur during

recovery period

Employed mainly in children and young adults

INTRAVENOUS ANESTHETICS

FENTANYL CITRATE AND DROPERIDOL (Innovar)

Narcotic analgesic + neuroleptic = neuroleptanalgesia

Produces neurolepthanesthesia (provides excellent analgesia while the patient remains conscious)

Can cause extrapyramidal muscle movements

INTRAVENOUS ANESTHETICS

PROPOFOL

Sedative-hypnotic used in the induction or

maintenance of anesthesia

Supplementation with narcotics for

analgesia is required

Depress CNS, decrease BP without

depressing the myocardium

Decreases intracranial pressure