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LocalAnesthetics

Dr. Hiwa K. Saaed, PhD Pharmacology & Toxicology

College of Pharmacy, University of Sulaimani

2019-2020

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Local anesthetics (LAs)

LAs are drugs that:- Reversibly block impulse conduction along nerve axons

& other excitable membranes that utilize Na+ ion channels that is required for an action potential generation.

- Thus, block nerve conduction of sensory impulses from the source of stimulation to the brain and, in higher concentrations, motor impulses from the periphery to the CNS.

- used to prevent or relieve pain in specific regions of the body without loss of consciousness

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Delivery techniques include:• Topical,• Infiltration, • Peripheral nerve blocks, • Neuraxial (spinal, epidural, or

caudal) blocks.

Most sensitive nerves are: Small, Unmyelinated nerve fibers for pain, Temperature, and Autonomic Nerves

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Local anesthetics (LAs)

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History of local anesthetics

3000 B.C.: cocaine 1905: procaine1932: Tetracaine1943: Lidocaine1957: Mepivacaine1960: Prilocaine1963: Bupivacaine1972: Etidocaine1996: Ropivacaine1999: Levobupivacaine

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First local anesthetic introduced into medical practice, cocaine, was isolated by Niemann in 1860

Procaine was synthesized by Einhorn in 1905 and became the dominant local anesthetic for the next 50 years

Lidocaine was synthesized in 1943 by Löfgren

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Chemistry of LAs

Most local anesthetic drugs are esters or amides of simple benzene derivatives.

Structurally, LAs consists of three parts :

1. A lipophilic ‘hydrophobic’ aromatic group.

2. An intermediate chain (ester or amide).

3. A hydrophilic an ionizable group (usually a

tertiary amine group).

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Esters usually have a shorter duration of action because ester links are more prone to hydrolysis than amide link

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Classification of LAs

Short:Procaine Chlorprocaine

Intermediate:Lidocaine, mepivacaine Prilocaine

Long acting:Tetracaine, Bupivacaine, Etidocaine Ropivacaine.

Classes: The rule of “i”Amides

LidocaineBupivacaineLevobupivacaineRopivacaineMepivacaineEtidocainePrilocaine

EstersProcaineChloroprocaineTetracaineBenzocaineCocaine

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The choice of LA for a specific procedure is usually based on the duration of procedure required

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Local anesthetics

• Local anesthetics are weak bases. • The pKa for most local anesthetics is in the range of 8-9

(Except benzocaine).• the larger percentage in body fluids at physiologic pH will

be the charged, cationic form.• The ratio between the cationic and uncharged forms of

these drugs is determined by the Henderson-Hasselbalchequation: Log cation (charged)/ uncharged= pKa - pH

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Effect of pH:

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Effectiveness of Local anesthetics are affected by pH of the application site; altering extracellular or intracellular pH • Charged (cationic) form binds to receptor site • uncharged form penetrates membrane

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Local anesthetics; Effect of lipophilicity

• LAs bind to receptor near the intracellular end of the channel. It is not readily accessible from the external side of the cell membrane.

• The uncharged form is more lipophilic and thus more rapidly diffuses through the membrane.

• However, the charged form has higher affinity for the receptor site of the sodium channel, because it cannot readily exit from closed channels.

• Therefore, LA are much less effective when they are injected into infected tissues because a larger % of the LA is ionized in an environment with a low extracellular pH and can not diffused across the membrane.

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Systemic absorption

Local anaesthetics can also affect sodium channels in other parts of the body, such as the conduction system of the heart. This can lead to an abnormal heartbeat; thus, systemic distribution of local anaesthetics is best kept to a minimum.

Local anesthetics are removed from depot site mainly by absorption into blood.

Systemic absorption is determined by several factors, including:• Dosage• Site of injection • Local blood flow: highly or poorly perfused• Use vasoconstrictors (e.g., epinephrine)• Drug tissue binding• Physicochemical properties of the drug itself

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Effect of epinephrine on local anesthetics

Addition of vasoconstrictor drugs such as epinephrine • reduces absorption of local anesthetics by decreasing blood flow

(important for intermediate & short duration of action), thus prolonging anesthetic effect and reducing systemic toxicity.

• Vasoconstrictors are less effective in prolonging anesthetic action of the more lipid-soluble, long-acting drugs (eg, bupivacaine and ropivacaine), – because these molecules are highly tissue-bound

• In spinal anesthesia, – Epinephrine also reduce sensory neuron firing via α2 receptors, which inhibit release of substance-P (neurokinin-1).

– Clonidine (α2 agonist) augment LA effect– Dexmedetomidine: pure α2 agonist – prolongs the local anesthetic effect by up to 50%.Epinephrine is included in many local anesthetic preparations. Know

your patient’s health status!

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Pharmacokinetics

Distribution• Amide are widely distributed & sequestered in fat.• Ester short plasma t1/2 ; No enough time for distribution

Metabolism and excretion• Amide: in the liver by CYP450• Ester: in plasma butyrylcholinesterase

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LAs mechanism of action

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Local anesthetics reversibly bind to the voltage-gated Na+ channel, block Na+ influx, and thus block action potential and nerve conduction.

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Membrane Potential and neurotransmission

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The excitable membrane of neuronal axons maintains a transmembrane potential of -90 to -60 mv.The transmembrane ionic gradients are maintained by the Na+/K+ ATPase (Na+ pump).During excitation the Na+

channels open, a fast inward Na+

current quickly depolarizes the membrane toward the Na+

equilibrium potential +40mv.

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Membrane Potential and neurotransmission

As a result of depolarization: - the Na+ channels close (inactivate) - & K+ channels open→ outward flow of K+

repolarizes the membrane toward the K+

equilibrium potential about -95mv

As a result of repolarization the Na+

channels returns to the rested state.

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Effects of Ca+2 & K+ on LAs

• Elevated extracellular Ca+2 [↑ surface potential on the membrane potential → resting state (which favors the low-affinity rested state)] partially antagonized the action of LA.

• Increase of extracellular K+ depolarizes the membrane potential & favors the inactivated state → enhance the effect of LA

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Actions on Nerves

• Since LAs are capable of blocking all nerves, their actions are not usually limited to the desired loss of sensation.

• Although motor paralysis may at times be desired, it may also limit the ability of the patient to cooperate, e.g., during obstetric delivery.

• During spinal anesthesia, motor paralysis may impair respiratory activity & AN blockade may lead to hypotension

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Effect of fiber diameter

Local anesthetics preferably block small, unmyelinated fibers that conduct pain, temperature, and autonomic nerves. For the same diameter, myelinated nerves will be blocked before unmyelinated nerves. For this reason, the preganglionic B fibers are blocked before the smaller unmyelinated C fibers involved in pain transmission.

- The smaller B preganglionic autonomic & C (pain) fiber are blocked 1st.

- The small type A delta (sensations) are blocked next.- Motor function is blocked last.

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Relative size and susceptibility to block of types of nerve fibers

1. pain, 2. cold, 3. warmth, 4. touch, 5. deep pressure & 6. motorRecovery is in reverse order

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Time & voltage-dependent fashion

The effect of a drug is more marked in rapidly firing axons than in resting fibers. Because LAs block the channel in a time & voltage-dependent fashion.

• Channels in the rested state (-ve mps) have a low affinity for LAs • Channels in the activated (open state) and inactivated (+ve mps) have a

high affinity for Las.

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Effect of firing frequency (state dependent mechanism)

• Nerves with higher firing frequency, more positive membrane potential, & with longer depolarization (duration) are more sensitive to local anesthetic block

• Sensory fibers especially pain fibers, have a high firing rate & relatively long action potential duration (up to 5 ms)

• Motor fibers fire at a slower rate & have a shorter AP (<0.5 ms)

• In nerve bundles, fibers that are located circumferentially are affected first by local anesthetics

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Effect on other excitable membranes

• LAs have weak NM blocking-little clinical importance.

• Cardiac cell membrane; • antiarrhythmia at concentration lower than those

required to produce nerve block • Arrhythmogenic: and all can cause arrhythmias in high

enough concentration.

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Clinical pharmacology of LAs

Local anesthetics can provide highly effective analgesia in well-defined regions of the body. The usual routes of administration include:

• topical application (eg, nasal mucosa, wound [incision site] margins) • injection in the vicinity of peripheral nerve endings (perineural

infiltration) and major nerve trunks (blocks), • injection into the epidural or subarachnoid spaces surrounding the

spinal cord. • Intravenous regional anesthesia (so-called Bier block) is used for

short surgical procedures (< 60 minutes) involving the upper and/or lower extremities.

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Clinical pharmacology of LAs

• The onset of LAs is sometimes accelerated by the use of solutions saturated with CO2 (carbonated) → intracellular acidosis → intracellular accumulation of the cationic form of LA.

• Repeated injection of LAs during epidural anesthesia → tachyphylaxis because of extracellular acidosis.

• Pregnancy appears to increase susceptibility to LAs

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Toxicity and side effects

A. CNS: -low dose: sleepiness, light headedness, visual and auditory disturbances, restlessness, circumoral & tongue numbness.-high dose (stimulatory effects): nystagmus, muscular twitching, finally tonic-clonic convulsions, followed by CNS depression→ death may occur.Convulsion because of cortical inhibitory pathways → unopposed activity of excitatory components.

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Convulsion prevention

Convulsion prevented by:*administering smallest dose of LA *premedication with BDZ (diazepam)↓LA toxicity by: Prevent hypoxemia (hypercapnia) & acidosis by hyperventilation → ↑blood pH → ↓ E.C K+ → hyperpolarization → resting state → ↓LA toxicitySeizure Rx: • thiopental 1-2mg/kg• Diazepam 0.1 mg/kg• succinylcholine for muscular manifestation.

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Toxicity and side effects

B. PNS (neurotoxicity)C. Cardiovascular system (CVS): direct & indirect effect Direct: all LAs are vasodilators (except cocaine) and also decrease the strength of cardiac contraction→ both effects → hypotension Indirect: ANS, cocaine blockade of NE reuptake →

- vasoconstriction → ischemia → ulceration of mucous membrane & damage nasal septum .

- HTN- Precipitate cardiac arrhythmia

NB. Bupivacaine is more cardiotoxic → CV collapse, after accidental I.V

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Toxicity and side effects

D. Blood: prilocaine (large dose; 10mg/kg) → accumulation of metabolite an oxidating agent, convert Hb to metHb → cyanotic

RX: methylene blue or ascorbic acid I.V to rapidly convert metHb → Hb.

E. Allergy: the ester type LA are metabolized to PABA derivative responsible for allergic reaction in a small % of population.

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