Local Anaesthetics

Question 1

how do local anaesthetics work?

Answer 1

stop nerve conduction by blocking the voltage-gated Na+ channels

Question 2

what is the order of nerve block?

Answer 2

A delta - C - A beta - A alpha

Question 3

what is the mechanism of action of LA?

Answer 3

binds to site in Na channel, blocks channel and prevents Na influx, block AP generation and propagation, block persists as long as a sufficient number of Na channels are blocked

Question 4

what are the 3 components of LA?

Answer 4

aromatic region (hydrophobic) to enter membrane, ester or amide bond, amine side chain (hydrophilic) to be soluble in water

Question 5

what form is LA active in?

Answer 5

the ionised form

Question 6

what form does LA cross the membrane in?

Answer 6

un-ionised form

Question 7

explain what happens when LA changes between ionised and unionised forms

Answer 7

to pass through the membrane LA needs to be unionised so it removes H ion, once it reaches the intracellular space it needs to be ionised again to work so it joins back with the H ion

Question 8

what happens if you inject into an acidic area with lots of H ions

Answer 8

it will take longer for the LA to work as there are lots of H ions causing problems with dissociation

Question 9

why are smaller diameter axons more susceptible to LA block?

Answer 9

sodium channels are evenly distributed for the space they have so you need more LA to act on all channels in the thicker nerve, whereas there is not as many in the smaller nerve

Question 10

where does LA work on myelinated axons?

Answer 10

at the nodes of Ranvier (needs to block several at the one time)

Question 11

what are the ester anaesthetics?

Answer 11

cocaine, procaine, benzocaine

Question 12

what are the amide anaesthetics?

Answer 12

lignocaine, prilocaine, articaine

Question 13

what is used to increase the duration of action of LA?

Answer 13

vasoconstrictors

Question 14

what do vasoconstrictors act on?

Answer 14

receptors on vascular smooth muscle

Question 15

what are the types of receptors that vasoconstrictors work on

Answer 15

adrenoreceptors - alpha (vasoconstriction), beta2 (vasodilation), beta 1 (cardiac) 
ADH receptors (vasopressin)

Question 16

what does adrenaline act on locally?

Answer 16

vasoconstrictor action on alpha receptors

Question 17

what is the systemic effect of adrenaline

Answer 17

lowers total peripheral resistance (greater effect on beta than alpha receptors) and increases cardiac output

Question 18

what does noradrenaline work on?

Answer 18

alpha receptors

Question 19

what is the systemic effect of noradrenaline?

Answer 19

increases TPR (greater effect on alpha), increases cardiac output AND mean BP

Question 20

how is LA inactivated

Answer 20

washout from tissues by blood, presence of vasoconstrictor agent, tissue esterases, liver amidases

Question 21

what are the modes of administration?

Answer 21

topical, injection, local infiltration, regional nerve block, nerve root block, intravenous

Question 22

what are the possible preparations for dental injections of lignocaine

Answer 22

2% lignocaine plain OR 2% lignocaine with 1:80,000 adrenaline

Question 23

what are the possible preparations for dental injections of prilocaine

Answer 23

4% prilocaine plain OR 3% prilocaline with felypressin

Question 24

how much prilocaine is in a 2ml cartridge of 3% prilocain?

Answer 24

60mg

Question 25

what is the equation for working out solution?

Answer 25

X% solution = X mass/volume

Question 26

what is the maximum safe does of lignocaine?

Answer 26

4.4mg per kg