Local Anaesthetics

Question 1

LA (a) blocks (b) when applied to a (c) area of the body

HINT:

(a) reversibly/irreversibly?
(b)
(c) large/restricted?

Answer 1

(a) reversibly
(b) nerve conduction
(c) restricted

LA reversibly blocks nerve conduction when applied to a restricted area of the body

Question 2

Which type of neurone detects pain?

Answer 2

nociceptors - sensory neurone

Question 3

Which chemical structures from LA?

Answer 3

1. aromatic ring
2. amine group
3. linkage

Question 4

What property does the aromatic ring provide?

Answer 4

makes LA lipid soluble

Question 5

Which structural group can be ionised if it picks up H+?

Answer 5

amine group

Question 6

What is the significance of the linkage group?

Answer 6

site of metabolism:

enzymes breaks down LA at the linkage group

Question 7

What type of chemical group is the linkage?

Answer 7

Can be esters or amides

Question 8

Will a LA with an ester linkage group break down faster or slower than a LA with an amide linkage group?

Answer 8

quicker

Question 9

Which of the following have an ester linkage?

procaine, lidocaine (lignocaine), prilocaine, bupivacaine

Answer 9

procaine

lidocaine, prilocaine + bupivacaine have an amide linkage

Question 10

Which of the following have a long duration?

procaine, lidocaine (lignocaine), prilocaine, bupivacaine

Answer 10

bupivacaine

Question 11

Which of the following have a medium duration?

procaine, lidocaine (lignocaine), prilocaine, bupivacaine

Answer 11

lidocaine

prilocaine

Question 12

Which of the following have a short duration?

procaine, lidocaine (lignocaine), prilocaine, bupivacaine

Answer 12

procaine

Question 13

LA are (weak)/(strong) (acids)/(bases)

What determines its ionisation state?

Answer 13

weak bases

pH determines ionisation state

Question 14

If the environment is alkaline, will there be more unionised/ionised LA molecules

Answer 14

more unionised

Question 15

Which is the more common state of LA molecules at a physiological pH?

Answer 15

unionised + ionised bother present

BUT ionised > unionised

Question 16

Which LA molecules are lipid soluble?

ie, ionised/unionised

Answer 16

unionised molecules are lipid soluble

Question 17

How does LA stop detection of pain?

Answer 17

1. anaesthetic present outside the neurone
2. unionised LA molecules pass across neurone membrane + enter axon
3. unionised LA molecules inside the neurone become ionised
4. ionised LA molecules block open Na+ voltage gated channels
5. = AP can’t be generated/propagated + signal isn’t transmitted

Question 18

LA gives rise to use dependant block. What does this mean?

Answer 18

ionised LA molecules block open Na+ channels

channels only open if stimulus is present to generate AP

—> more painful stimulus = more open channels
= blocked by ionised LA

DEGREE OF BLOCK PROPORTIONAL TO RATE OF STIMULATION

Question 19

Why does acidity reduce effectiveness of LA?

eg, during inflammation/infection

Answer 19

increased acidity = more LA molecules ionised

= few unionised molecules enter the axon

= fewer molecules available to block to Na+ channels

Question 20

If a patient does have an infection, should the procedure be done?

Answer 20

better to give antibiotics to remove the infection/inflammation and increase effectiveness of LA

Question 21

Which nerves are more sensitive to LA? Wider/narrower?

Answer 21

narrower

C type neurones + A delta neurones = nociceptors

motor neurones = wider neurones

Question 22

Are C type neurones myelinated?

Answer 22

no

Question 23

Where is topic anaesthesia administered? How effective is it?

Answer 23

Applied to the skin

Not very effective
- skin has multiple layers = hard for LA to diffuse across

Question 24

Where is infiltration anaesthesia administered?

When is it administered?

Answer 24

injected into skin

eg, when getting stitches/tooth extraction

Question 25

Which type of anaesthetic targets large mixed nerves?

Answer 25

nerve block - anaesthetic affects all the nerves included = covers a large area

Question 26

Which type of anaesthetic bathes nerve roots exiting the spinal cord to numb the lower part of the body?

Answer 26

epidural - injected into epidural space (within vertebrae but outside spinal cord)

Question 27

Where is spinal anaesthesia administered?

Answer 27

anaesthetic goes into cerebrospinal fluid in the subarachnoid space - affects any nerve running thorugh that space

Question 28

What is the non-specific side effect of LA? What can cause them?

Answer 28

hyper-sensitivity reaction preservatives in anaesthetic that prevent bacterial growth + keep solution sterile

Question 29

What is a non-specific side effect?

Answer 29

side effect not caused by the anaesthetic itself caused by something else in the solution

Question 30

What is the consequence of anaesthetic being injected at high doses into vessels?

Answer 30

LA can circulate + effect other tissues eg, can cross blood-brain barrier eg, can decrease blood pressure by reducing contractility of CVS + dilating blood vessels

Question 31

Which LA is the most cardiotoxic?

Answer 31

bupivacaine

Question 32

Which other drugs are administered with LA?

Answer 32

vasoconstrictors

Question 33

Why are vasoconstrictors administered with LA? HINT: 2 effects of VC + 2 wider advantages

Answer 33

vasoconstriction of blood vessels in the region LA is injected = reduced blood flow = LA better localised/less likely to circulate ``` =: 1. reduces unwanted effects 2. increases duration of action - reduced rate that anaesthetic is metabolised in blood = can give a lower dose = reduces chances of side effects ```

Question 34

Why does caution need to be taken with administering vasoconstrictors?

Answer 34

if LA is being injected in an extremity (eg, finger/toe). reducing blood flow can be problematic