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 LAAmide • 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

•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 – 1-1.5 h

• Activity is Ph dependent

• Increased action in acidic ph

CLEARANCE

• ESTERShydrolysis via cholinesterase

• AMIDESmetabolism 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