Local Anaesthetic Agents

Local Anaesthetic Agents

·        Reversible inhibition of conduction in nerve fibres and endings

·        Classification:

o  Esters of benzoic acid: cocaine, tetracaine

o  Esters of para-aminobenzoic acid: chloroprocaine, procaine

o  Amides: e.g. lignocaine and bupivacaine (Marcain)

·        Bind to internal opening of Na channel, preventing threshold and progression of action potential

·        Pharmacokinetics depends on:

o  Mass movement of drug around and away from nerve

o  Diffusion into axon: best if unionised (i.e. weak base) 

o  Absorption is determined by route of administration, site of administration (e.g. vascularity), and presence of vasoconstrictors (some anaesthetics themselves have vaso-constricting/dilating properties) 

o  Metabolism: esters hydrolysed by plasma cholinesterase, amides by liver

·        Adverse effects:

o  CNS: first cause excitation due to suppression of inhibitory neurons 

o   CVS: negative inotropy, depression of conduction, reduction in automaticity (® sinus bradycardia) 

o  Vasoconstriction – decrease rate of absorption, ¯bleeding. Never use adrenaline in digital extremities ® ischaemia 

o  Allergy: extremely rare

o  Fainting

·        Treatment of overdose of local anaesthetic:

o  O2

o  Diazepam/thiopentone for convulsions

o  Other resuscitation: airway, ventilation, elevate legs, IV fluids, atropine for bradycardia

·        Clinical uses:

o  Topical anaesthesia: slow.  Good for cannulating kids but takes an hour

o  Infiltration: e.g. around suturing.  Don‟t inject through wound edge of unsterile wound

o  Nerve block: large area of analgesia, fewer injections, smaller doses 

o  Extradural: between dura mater and periosteum of vertebral canal. Also blocks autonomic efferent nerves ® vasodilation ® hypotension. If it goes into subarachnoid space ® total spinal: respiratory paralysis, hypotension – treat with IPPV O2, IV fluids and vasopressors 

o  Spinal/subarachnoid anaesthesia: direct into CSF.  More potent, more pronounced motor block

o  Intravenous regional anaesthesia: tourniquet inflated to 100mmHg above systolic blood pressure