Local Anaesthesia

Question 1

what is the MOA of local anaesthetics?

Answer 1

block the voltage-dependant sodium channels that depolarise the neuron

Question 2

name 4 factors that influence local anaesthetic action

Answer 2

1. local concentration of the LA
2. the size of the nerve fibre
3. nerve myelination
4. length of nerve exposed to the LA

Question 3

out of pain, sensory and motor fibres, whcih fibres do LAs block first?

Answer 3

pain fibres (myelinated A-delta and non-myelinataed C fibres)

Question 4

what effect does hypokalaemia and hypercalcaemia have on LA?

Answer 4

it antagonizes the blockade.

Question 5

what is the potency of a LA related to?

Answer 5

Its lipid solubility.

a lipophilic aromatic group is what enables the molecule to cross the nerve membrane.

Question 6

what are the implications of a LA having a high pKa in practice?

Answer 6

it means it will be more ionised at physiological pH so its speed of onset will be slower but it will also re-ionise to a greater extent within the cell and produce mroe effective blockade.

Question 7

what does the term use-dependence mean

Answer 7

the more channels are open, the greater the block becomes

Question 8

name 5 clinical uses for local anaesthetics

Answer 8

1. local infiltration e.g. for suturing
2. topical
3. nerve blocks (local and regional)
4. epidural
5. spinal

Question 9

what is the most commonly used LA?

Answer 9

lidocaine

Question 10

is an epidural given inside or outside of the dura mater?

Answer 10

outside

Question 11

what is an epidural anaesthetic often used in combination with?

Answer 11

opiate

Question 12

what space is a spinal aneasthesia given and between which vertebrae usually?

Answer 12

the subarachnoid space.

between L3 and L4 usually

Question 13

what does the spread of the LA within the subarachnoid space depend on in a spinal anaesthetic?

Answer 13

1. the density of the solution

2. the posture of the person during the first 10-15 mins while the solution flows up or down the subarachnoid space.

Question 14

what does IV regional anaesthesia involve?

Answer 14

injection of a dilute solution of LA into a limb after application of a tourniquet

Question 15

what is IV regional anaesthesia used for?

Answer 15

reduction of fractures or removal of ganglia etc.

Question 16

why should the tourniquet not be released too early when a IV regional anaesthetic is used?

Answer 16

risk of systemic toxicity

Question 17

why are vasoconstrictors (e.g. adrenaline) sometimes added to LA solution for injection?

Answer 17

most LAs cause vasodilation at the site of injection whcih will enhance their removal. vasoconstrictors are added to prevent this.

Question 18

name 2 local unwanted effects of LAs

Answer 18

irritatation and inflammation, ischaemia from use of vasoconstrictor agents

Question 19

name 4 unwanted cardiovascular effects of using LAs

Answer 19

myocardial depression,
vasodilatation,
hypotension,
arrhythmias

Question 20

name 5 unwanted CNS effects from LAs

Answer 20

agitation, 
confusion,
tremors,
convulstions,
respiratory depression