local anesthetics
TRANSCRIPT
Local anesthetics
DR.GAYATRI MISHRAASST.PROFESSOR ANAESTHESIOLOGY
&CRITICAL CARE
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