Local anesthetics

Objectives Recall how an action potential is generated and propagated Classify local anesthtics Describe the machanism of action, pharmacokinetics and toxic effects of local anesthetics Describe the different techniqes of use of LA Describe the risks and benefits of using vasoconstrictors with LA

Overview Local anesthetics produce a transient and reversible loss of sensation (analgesia) in a circumscribed region of the body without loss of consciousness. Normally, the process is completely reversible.

ANESTHETICS

Local anesthetics - esters or amides –a lipophilic aromatic group –to a hydrophilic, ionizable amine. Most are weak bases

Classification of LA Esters Benzocaine Procaine/Procaine Proparacaine

Classification of LA Amide Bupivacaine Levobupivacaine Lidocaine/LignocaineLidocaineLignocaine Mepivacaine

Ionized

Local anesthetics gain access to the inner axonal membrane by 1.traversing sodium channels while they are more often in an open configuration 2.passage directly through the plasma membrane

Block inititation and propagation of action potential

Sympathetic block (vasodilatation) Loss of pain and temperature sensation Loss of proprioception Loss of touch and pressure sensation Loss of motor function Sequence of clinical anesthesia

Potency = lipid solubility Higher solubility = can use a lower concentration and reduce potential for toxicity Anesthetic Potency

DURATION OF ACTION Duration = protein binding Bupivacaine 95% Lidocaine 65%

Pharmacokinetics Effective within 5 min Duration of action – h Activity is Ph dependent Increased action in acidic ph

CLEARANCE ESTERS hydrolysis via cholinesterase AMIDES metabolism via hepatic enzymes

LA Infiltration anesthesia Regional anesthesia Surface anesthesia

LA Infiltration anesthesia Regional anesthesia Surface anesthesia

Gegional anaesthesia Nerve block Intravenous Extradural Intrathecal block/ spinal anaesthesia

Nerve block Inject a drug around the nerve Anaesthetise a region

Intravenous 0.5-1% lidocaine without adrenaline

Extradural/epidural Thoracic, lumbar, sacral Act on nerve roots No hypotention

Spinal anesthesia Sympathetic nerve block hypotension

LA Infiltration anesthesia Regional anesthesia Surface anesthesia

On intact skin – eutectic mixture of bases of prilocaine (EMLA) Slow absorption

Prolongation of action Add vasoconstrictor – adrenaline Can use a larger dose Not to – fingers, toes, nose, penis

Adverse effects LA’s cause some vasodilatation at site LA toxicity related to rate of absorption via blood flow

Systemic Toxicity Blockage of voltaged-gated Na channel affects action potential propagation throughout the body Potential is present for systemic toxicity

Effects of local anesthetics Excitation – anxiety, agitation, restlessness Convulsions Reduced myocardial contractility Vasodilatation