local anaesthetic Flashcards
how do local anaesthetics stop nerve conduction?
blocking voltage-gated Na channels
what is the goal when injective LA?
to produce anaesthesia of receptor/axon at level before spinal dorsal horn
what are the CT layers in a peripheral nerve?
epineurium
perineurium
endoneurium
what is the order of block of fibres?
A delta, C, A beta, A alpha
what is the mechanism of action?
LA binds to site in Na channel
blocks channel preventing Na influx
blocks AP generation & propagation
block persists so long as a sufficient no. Na channels blocked
what is a problem with the mechanism of action?
doesnt differentiate which Na channels blocked- so other excitable tissue eg heart muscle can be blocked causing bradycardia and hypotension
describe LA components/arrangement
3 organic molecules -aromatic region (hydrophobic) -ester/amide bond -basic amine side chain (hydrophilic) B.HCL -base & hydrochloride arrangement -allows to be hydrophilic needs to be hydrophobic as if not soluble wouldn't inject/move around tissues/enter membrane
how does B.HCL enter the membrane and diffuse?
broken down into B.H + Cl
-pharmacologically active, non-diffusible
broken down into B. + H
diffusible, non-active
needs to be ionised when in intra-cellular space
why are small diameter axons more susceptible to la block?
small has less Na channels
more channels = more LA to act on all channels
what is safety factor?
the local currents are strong enough to flow past the blocked region, and to regenerate the AP at the next node of ranvier
-to block the AP, LA needs to act on several nodes of ranvier along the axon
what are the components of LA?
base as hydrochloride -to increase solubility 2-4% solution sodium metabisulphide -reducing agent preservative fungicide vasoconstrictor
what esters are used?
benzocaine
what amides are used?
lignocaine prilocaine -alternative articaine bupivacaine -longer lasting due to greater affinity to fatty tissues, more toxic
why are vasoconstrictors added?
most LA are vasodilators
increased blood flow will increase wash out of LA
to increase duration of action
what vasoconstrictors are used?
adrenaline
felypressin
-synthetic vasopressin
how do vasoconstrictors work?
act on receptors on vascular SM -adrenoreceptors alpha-vasoconstriction beta2-vasodilation beta1-cardiac muscle -ADH receptors -vasopressin
describe the effects of adrenaline as a vasoconstrictor
equally effective on alpha and beta fibres
systemically lowers TPR
increases cardiac output
little/no effect on mean arterial BP
describe the effects of noradrenaline as a vasoconstrictor?
more effects on alpha than beta receptors systemically increases TPR increases cardiac output increases mean arterial BP not used
how is LA inactivated?
washout from tissues by blood supply ester broken down by tissue esterases -action brief amide broken down by liver amidases -longer action
what are the modes of administration?
topical injection local infiltration regional nerve block nerve root block (epidural) intravenous
what preparations of lignocaine are available?
2% lignocaine HCL
2% lignocaine HCL + 1:80,000 adrenaline
what preparations of prilocaine are available?
4% prilocaine HCL
3% prilocaine HCL + felypressin (0.03u/ml)
how is x% solution found?
x mass/volume
what is the max dose of lignocaine?
4mg per kg body weight
what is the max dose adrenaline?
500ug
how much adrenaline is in a cartridge?
27.5 ug
