Local anesthetics Objectives Recall how an action potential

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/ • Proparacaine

Classification of LA Amide • Bupivacaine • Levobupivacaine • Lidocaine/Lignocaine • 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

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

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

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

Pharmacokinetics • Effective within 5 min • Duration of action – 1 -1. 5 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

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