14. Local Anaesthetics Flashcards
How do LAs work
Stop nerve conduction by blocking voltage-gated Na channels
LA efficacy depends on (2)
Concentration of solution
Type of nerve being anaesthetised
Nerve susceptibility to block by LA (4)
A-delta
C-fibres
A-beta
A-alpha
Mechanism of LA action (4)
LA binds to site in Na channel
LA blocks channel, preventing Na influx
This blocks AP generation and propagation
Block persists so long as sufficient number of Na channels are blocked
Composition of LAs (3)
Organic molecules
Presented as hydrochloride (B.HCl)
Partly dissolved
Organic molecule composition of LA (3)
Aromatic region (hydrophobic)
Ester/amide bond
Basic amine side chain (hydrophilic)
LA dissolution features (2)
Active in ionised form
Crosses membrane in un-ionised form
Are smaller/larger axons more susceptible to LA and why
Small - fewer Na channels
Where are Na and K channels concentrated in myelinated axons
Nodes of Ranvier
LA blocks AP by
Acting on several nodes of Ranvier along the axon
Types of LA (2)
Esters
Amides
Examples of ester LAs (3)
Benzocaine
Cocaine
Procaine
Examples of amide LAs (6)
Lignocaine (lidocaine) Prilocaine Articaine Bupivacaine Ropivacaine Mepivacaine
Are LAs vasodilators or vasoconstrictors
Vasodilators so contain vasoconstrictors
Function of vasoconstrictors in LA (2)
Reduce blood flow
Increase duration of action
Local action of adrenaline
Vasoconstriction (on a-receptors)
Systemic effect of adrenaline (3)
Lowers TPR (b > a)
Increases CO
Little/no effect on MABP
Local effect on noradrenaline
Vasoconstriction (more effective on a-receptors than b-receptors)
Systemic effect of noradrenaline (3)
Increases TPR (a > b) Increases CO Raises MABP (can result in fall in BP - overcompensation)
How are ester type LAs broken down
By tissue esterases (brief action)
How are amide type LAs broken down
By liver amidases (longer duration of actioN0
Modes of LA administration (6)
Topical (surface application) Injection Local infiltration Regional nerve block Nerve root block (spinal, epidural) Intravenous
Max. safe dose of adrenaline
500ug
Adrenaline in one lidocaine cartridge (2%, 1:80,000, 2.2ml)
27.5ug
MAx. safe dose of lidocaine
Approx. 4mg/kg body weight
