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
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