local anaesthetic

Question 1

how do local anaesthetics stop nerve conduction?

Answer 1

blocking voltage-gated Na channels

Question 2

what is the goal when injective LA?

Answer 2

to produce anaesthesia of receptor/axon at level before spinal dorsal horn

Question 3

what are the CT layers in a peripheral nerve?

Answer 3

epineurium
perineurium
endoneurium

Question 4

what is the order of block of fibres?

Answer 4

A delta, C, A beta, A alpha

Question 5

what is the mechanism of action?

Answer 5

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

Question 6

what is a problem with the mechanism of action?

Answer 6

doesnt differentiate which Na channels blocked- so other excitable tissue eg heart muscle can be blocked causing bradycardia and hypotension

Question 7

describe LA components/arrangement

Answer 7

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

Question 8

how does B.HCL enter the membrane and diffuse?

Answer 8

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

Question 9

why are small diameter axons more susceptible to la block?

Answer 9

small has less Na channels

more channels = more LA to act on all channels

Question 10

what is safety factor?

Answer 10

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

Question 11

what are the components of LA?

Answer 11

base as hydrochloride
-to increase solubility
2-4% solution
sodium metabisulphide
-reducing agent
preservative
fungicide
vasoconstrictor

Question 12

what esters are used?

Answer 12

benzocaine

Question 13

what amides are used?

Answer 13

lignocaine
prilocaine
-alternative
articaine
bupivacaine
-longer lasting due to greater affinity to fatty tissues, more toxic

Question 14

why are vasoconstrictors added?

Answer 14

most LA are vasodilators
increased blood flow will increase wash out of LA
to increase duration of action

Question 15

what vasoconstrictors are used?

Answer 15

adrenaline
felypressin
-synthetic vasopressin

Question 16

how do vasoconstrictors work?

Answer 16

act on receptors on vascular SM
-adrenoreceptors
alpha-vasoconstriction
beta2-vasodilation
beta1-cardiac muscle
-ADH receptors
-vasopressin

Question 17

describe the effects of adrenaline as a vasoconstrictor

Answer 17

equally effective on alpha and beta fibres
systemically lowers TPR
increases cardiac output
little/no effect on mean arterial BP

Question 18

describe the effects of noradrenaline as a vasoconstrictor?

Answer 18

more effects on alpha than beta receptors
systemically increases TPR
increases cardiac output
increases mean arterial BP
not used

Question 19

how is LA inactivated?

Answer 19

washout from tissues by blood supply
ester broken down by tissue esterases
-action brief
amide broken down by liver amidases
-longer action

Question 20

what are the modes of administration?

Answer 20

topical
injection
local infiltration
regional nerve block
nerve root block (epidural)
intravenous

Question 21

what preparations of lignocaine are available?

Answer 21

2% lignocaine HCL

2% lignocaine HCL + 1:80,000 adrenaline

Question 22

what preparations of prilocaine are available?

Answer 22

4% prilocaine HCL

3% prilocaine HCL + felypressin (0.03u/ml)

Question 23

how is x% solution found?

Answer 23

x mass/volume

Question 24

what is the max dose of lignocaine?

Answer 24

4mg per kg body weight

Question 25

what is the max dose adrenaline?

Answer 25

500ug

Question 26

how much adrenaline is in a cartridge?

Answer 26

27.5 ug