Local anesthetics

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Local Anesthetics Local Anesthetics www.freelivedoctor.com www.freelivedoctor.com

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Transcript of Local anesthetics

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Local AnestheticsLocal Anestheticswww.freelivedoctor.comwww.freelivedoctor.com

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Local AnestheticLocal Anesthetic

A local anesthetic is an agent that A local anesthetic is an agent that interrupts pain impulses in a specific interrupts pain impulses in a specific region of the body without a loss of region of the body without a loss of patient consciousness. Normally, the patient consciousness. Normally, the process is process is completely reversible-completely reversible--the -the agent does not produce any residual agent does not produce any residual effect on the nerve fiber. effect on the nerve fiber.

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HistoryHistory The first local anesthetic was Cocaine which The first local anesthetic was Cocaine which

was isolated from coca leaves by Albert was isolated from coca leaves by Albert Niemann in Germany in the 1860s. The very Niemann in Germany in the 1860s. The very first clinical use of Cocaine was in 1884 by first clinical use of Cocaine was in 1884 by Sigmund Freud who used it to wean a patient Sigmund Freud who used it to wean a patient from morphine addiction.  It was Freud and from morphine addiction.  It was Freud and his colleague Karl Kollar who first noticed its his colleague Karl Kollar who first noticed its anesthetic effect.  Kollar first introduced it to anesthetic effect.  Kollar first introduced it to clinical ophthalmology as a topical ocular clinical ophthalmology as a topical ocular anesthetic.  Also in 1884, Dr. William Stewart anesthetic.  Also in 1884, Dr. William Stewart Halsted was the first to describe the injection Halsted was the first to describe the injection of cocaine into a sensory nerve trunk to of cocaine into a sensory nerve trunk to create surgical anesthesia. create surgical anesthesia.

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ChemistryChemistryAll local anesthetics are weak bases, classified as tertiary amines. 

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Esters: Esters: These include cocaine, procaine, These include cocaine, procaine,

tetracaine, and chloroprocaine. tetracaine, and chloroprocaine. They are hydrolyzed in plasma by They are hydrolyzed in plasma by

pseudo-cholinesterase. One of the pseudo-cholinesterase. One of the by-products of metabolism is by-products of metabolism is paraaminobenzoic acid, the common paraaminobenzoic acid, the common cause of allergic reactions seen with cause of allergic reactions seen with these agentsthese agents

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Amides: Amides: These include lidocaine, mepivicaine, These include lidocaine, mepivicaine,

prilocaine, bupivacaine, and prilocaine, bupivacaine, and etidocaine. etidocaine.

They are metabolized in the liver to They are metabolized in the liver to inactive agents. True allergic inactive agents. True allergic reactions are rare (especially with reactions are rare (especially with lidocaine) lidocaine)

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Mechanism of ActionMechanism of Action

Local anesthetics work to block nerve Local anesthetics work to block nerve conduction by reducing the influx of sodium conduction by reducing the influx of sodium ions into the nerve cytoplasm.ions into the nerve cytoplasm.

Sodium ions cannot flow into the neuron, Sodium ions cannot flow into the neuron, thus the potassium ions cannot flow out, thus the potassium ions cannot flow out, thereby inhibiting the depolarization of the thereby inhibiting the depolarization of the nerve.  nerve. 

If this process can be inhibited for just a If this process can be inhibited for just a few Nodes of Ranvier along the way, then few Nodes of Ranvier along the way, then nerve impulses generated downstream nerve impulses generated downstream from the blocked nodes cannot propagate from the blocked nodes cannot propagate to the ganglion. to the ganglion.

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Mechanism of actionMechanism of action

local anesthetics bind local anesthetics bind directlydirectly to the to the intracellularintracellular voltage-dependent voltage-dependent sodium channelssodium channels

Block primarily Block primarily openopen and and inactiveinactive sodium channels, at specific sites sodium channels, at specific sites within the channelwithin the channel

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Mechanism of actionMechanism of action

1) slow rate of depolarization1) slow rate of depolarization 2) reduce height of action potential2) reduce height of action potential 3) reduce rate of rise of action potential3) reduce rate of rise of action potential 4) slow axonal conduction 4) slow axonal conduction 5) ultimately prevent propagation of action 5) ultimately prevent propagation of action

potentialpotential 6)6) do not do not alter resting membrane potential alter resting membrane potential 7) increase threshold potential7) increase threshold potential

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Factors affecting Factors affecting local anesthetic actionlocal anesthetic action

Effect of pHEffect of pH charged (cationic) form binds to charged (cationic) form binds to

receptor site uncharged form receptor site uncharged form penetrates membrane ,efficacy of penetrates membrane ,efficacy of drug can be changed by altering drug can be changed by altering extracellular or intracellular pH extracellular or intracellular pH

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Effect of lipophilicity Effect of lipophilicity ANESTHETIC POTENCYANESTHETIC POTENCY

Lipid solubility appears to be the Lipid solubility appears to be the primaryprimary determinant of intrinsic anesthetic potency. determinant of intrinsic anesthetic potency. Chemical compounds which are highly Chemical compounds which are highly lipophilic tend to penetrate the nerve lipophilic tend to penetrate the nerve membrane more easily, such that less membrane more easily, such that less molecules are required for conduction molecules are required for conduction blockade resulting in enhanced potency. blockade resulting in enhanced potency.

more lipophilic agents are more potent more lipophilic agents are more potent as local anestheticsas local anesthetics

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Effect of protein binding -Effect of protein binding - increased increased binding increases duration of actionbinding increases duration of action

Effect of diffusibility - Effect of diffusibility - increased increased diffusibility = decreased time of diffusibility = decreased time of onsetonset

Effect of vasodilator activity - Effect of vasodilator activity - greater greater vasodilator activity = decreased vasodilator activity = decreased potency and decreased duration of potency and decreased duration of actionaction

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FIBER SIZE AND FUNCTION FIBER SIZE AND FUNCTION αα: (dia 12-20um; cond vel 70-120m/s) : (dia 12-20um; cond vel 70-120m/s)

largest, afferent to and efferent from largest, afferent to and efferent from muscles and joints. Actions: motor muscles and joints. Actions: motor function, proprioception, reflex activity. function, proprioception, reflex activity.

ββ: (dia 5-12um; 30-70m/s) large as A-: (dia 5-12um; 30-70m/s) large as A-alpha, afferent to and efferent from alpha, afferent to and efferent from muscles and joints. Actions: motor muscles and joints. Actions: motor proprioception, touch, pressure, touch and proprioception, touch, pressure, touch and pressure. pressure.

γγ: (dia 3-6um; 15-30m/s) muscle spindle : (dia 3-6um; 15-30m/s) muscle spindle tone. tone.

δδ: (dia 2-5um; 12-30m/s) thinnest, pain : (dia 2-5um; 12-30m/s) thinnest, pain and temperature. Signal tissue damage. and temperature. Signal tissue damage.

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B fibers: (dia – 2-5um) Myelinated : (dia – 2-5um) Myelinated preganglionic autonomic. Innervate preganglionic autonomic. Innervate vascular smooth muscle. Though vascular smooth muscle. Though myelinated, they are more readily myelinated, they are more readily blocked by LA than C fibers. blocked by LA than C fibers.

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C fibers: (dia 0.4-1.2 um) : (dia 0.4-1.2 um) Nonmyelinated, very small nerves. Nonmyelinated, very small nerves. Smallest nerve fibers, slow Smallest nerve fibers, slow transmission. Transmit dull pain and transmission. Transmit dull pain and temperature, post-ganglionic temperature, post-ganglionic autonomic.autonomic.* Both A-d and C fibers transmit pain * Both A-d and C fibers transmit pain and are blocked by the same and are blocked by the same concentration of LA. concentration of LA.

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Susceptibility to block by local Susceptibility to block by local anesthetics of types of nerve fibers anesthetics of types of nerve fibers

In general, small nerve fibers are In general, small nerve fibers are more susceptible than large fibers; more susceptible than large fibers; however, however, – the type of fiberthe type of fiber– degree of myelinationdegree of myelination– fiber length and fiber length and – frequency- dependence are also frequency- dependence are also

important in determining susceptibilityimportant in determining susceptibility

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Order of sensory function block Order of sensory function block

1. pain1. pain 2. cold2. cold 3. warmth3. warmth 4. touch4. touch 5. deep pressure 5. deep pressure 6. motor6. motor

Recovery in reverse order

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TOXICITIES OF LOCAL TOXICITIES OF LOCAL ANESTHETICSANESTHETICS

Essentially all systemic toxic reactions Essentially all systemic toxic reactions associated with local anesthetics are associated with local anesthetics are the result of over-dosage leading to the result of over-dosage leading to high blood levels of the agent given. high blood levels of the agent given. Therefore, to avoid a systemic toxic Therefore, to avoid a systemic toxic reaction to a local anesthetic, the reaction to a local anesthetic, the smallest amount of the most dilute smallest amount of the most dilute solution that effectively blocks pain solution that effectively blocks pain should be administered.should be administered.

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HypersensitivityHypersensitivity. Some patients are . Some patients are hypersensitive (allergic) to some local hypersensitive (allergic) to some local anesthetics. Although such allergies are anesthetics. Although such allergies are very rare, a careful patient history should very rare, a careful patient history should be taken in an attempt to identify the be taken in an attempt to identify the presence of an allergy. There are two presence of an allergy. There are two basic types of local anesthetics (the amide basic types of local anesthetics (the amide type and the ester type). A patient who is type and the ester type). A patient who is allergic to one type may or may not be allergic to one type may or may not be allergic to the other type. allergic to the other type.

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Central Nervous System Central Nervous System Toxicities. Toxicities.

Local anesthetics, if absorbed Local anesthetics, if absorbed systematically in excessive amounts, systematically in excessive amounts, can cause central nervous system can cause central nervous system (CNS) excitement or, if absorbed in (CNS) excitement or, if absorbed in even higher amounts, can cause CNS even higher amounts, can cause CNS depression. depression.

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CNS toxicity contCNS toxicity cont

Excitement. Tremors, shivering, and Excitement. Tremors, shivering, and convulsions characterize the CNS convulsions characterize the CNS excitement.excitement.

Depression. The CNS depression is Depression. The CNS depression is characterized by respiratory characterized by respiratory depression and, if enough drug is depression and, if enough drug is absorbed, respiratory arrest.absorbed, respiratory arrest.

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Cardiovascular ToxicitiesCardiovascular Toxicities. Local . Local anesthetics if absorbed systematically in anesthetics if absorbed systematically in excessive amounts can cause depression of excessive amounts can cause depression of the cardiovascular system. the cardiovascular system.

Peripheral vascular action arteriolar dilation Peripheral vascular action arteriolar dilation (except (except cocainecocaine which is vasoconstrictive which is vasoconstrictive

Hypotension and a certain type of Hypotension and a certain type of abnormal heartbeat (atrioventricular block) abnormal heartbeat (atrioventricular block) characterize such depression. These may characterize such depression. These may ultimately result in both cardiac and ultimately result in both cardiac and respiratory arrest. respiratory arrest.

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Signs of toxicity occur on a continuum. Signs of toxicity occur on a continuum. From early to late stages of toxicity, From early to late stages of toxicity, these signs are: circum-oral and tongue these signs are: circum-oral and tongue numbness, lightheadedness, tinnitus, numbness, lightheadedness, tinnitus, visual disturbances, muscular twitching, visual disturbances, muscular twitching, convulsions, unconsciousness, coma, convulsions, unconsciousness, coma, respiratory arrest, then cardiovascular respiratory arrest, then cardiovascular collapse. collapse.

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Types of Local AnesthesiaTypes of Local Anesthesia

Local InfiltrationLocal Infiltration (Local (Local Anesthesia)Anesthesia). Local infiltration occurs . Local infiltration occurs when the nerve endings in the skin when the nerve endings in the skin and subcutaneous tissues are blocked and subcutaneous tissues are blocked by direct contact with a local by direct contact with a local anesthetic, which is injected into the anesthetic, which is injected into the tissue. Local infiltration is used tissue. Local infiltration is used primarily for surgical procedures primarily for surgical procedures involving a small area of tissue (for involving a small area of tissue (for example, suturing a cut). example, suturing a cut).

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Topical BlockTopical Block. A topical block is . A topical block is accomplished by applying the anesthetic accomplished by applying the anesthetic agent to mucous membrane surfaces and in agent to mucous membrane surfaces and in that way blocking the nerve terminals in the that way blocking the nerve terminals in the mucosa. This technique is often used during mucosa. This technique is often used during examination procedures involving the examination procedures involving the respiratory tract. The anesthetic agent is respiratory tract. The anesthetic agent is rapidly absorbed into the bloodstream. For rapidly absorbed into the bloodstream. For topical application (that is, to the skin), the topical application (that is, to the skin), the local anesthetic is always used without local anesthetic is always used without epinephrine. The topical block easily epinephrine. The topical block easily anesthetizes the surface of the cornea (of the anesthetizes the surface of the cornea (of the eye) and the oral mucosa. eye) and the oral mucosa.

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Surface AnesthesiaSurface Anesthesia. This type of . This type of anesthesia is accomplished by the anesthesia is accomplished by the application of a local anesthetic to application of a local anesthetic to skin or mucous membranes. Surface skin or mucous membranes. Surface anesthesia is used to relieve itching, anesthesia is used to relieve itching, burning, and surface pain (for burning, and surface pain (for example, as seen in minor sunburns). example, as seen in minor sunburns).

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Nerve BlockNerve Block. In this type of . In this type of anesthesia, a local anesthetic is anesthesia, a local anesthetic is injected around a nerve that leads to injected around a nerve that leads to the operative site. Usually more the operative site. Usually more concentrated forms of local concentrated forms of local anesthetic solutions are used for this anesthetic solutions are used for this type of anesthesia. type of anesthesia.

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Peridural AnesthesiaPeridural Anesthesia. . This type of This type of anesthesia is accomplished by anesthesia is accomplished by injecting a local anesthetic into the injecting a local anesthetic into the peridural space. peridural space.

The peridural space is one of the The peridural space is one of the coverings of the spinal cord. coverings of the spinal cord.

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Spinal AnesthesiaSpinal Anesthesia. . In spinal In spinal anesthesia, the local anesthetic is anesthesia, the local anesthetic is injected into the subarachnoid space injected into the subarachnoid space of the spinal cord of the spinal cord

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VasoconstrictorsVasoconstrictors

Vasoconstrictors decrease the rate of Vasoconstrictors decrease the rate of vascular absorption which allows more vascular absorption which allows more anesthetic to reach the nerve membrane anesthetic to reach the nerve membrane and improves the depth of anesthesia. and improves the depth of anesthesia.

There is variable response between LA and There is variable response between LA and the location of injection as to whether the location of injection as to whether vasoconstrictors increase duration of vasoconstrictors increase duration of action. 1:200,000 epinephrine appears to action. 1:200,000 epinephrine appears to be the best vasoconstrictor. be the best vasoconstrictor.

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