Local Anaesthetics Flashcards
LA (a) blocks (b) when applied to a (c) area of the body
HINT:
(a) reversibly/irreversibly?
(b)
(c) large/restricted?
(a) reversibly
(b) nerve conduction
(c) restricted
LA reversibly blocks nerve conduction when applied to a restricted area of the body
Which type of neurone detects pain?
nociceptors - sensory neurone
Which chemical structures from LA?
- aromatic ring
- amine group
- linkage
What property does the aromatic ring provide?
makes LA lipid soluble
Which structural group can be ionised if it picks up H+?
amine group
What is the significance of the linkage group?
site of metabolism:
enzymes breaks down LA at the linkage group
What type of chemical group is the linkage?
Can be esters or amides
Will a LA with an ester linkage group break down faster or slower than a LA with an amide linkage group?
quicker
Which of the following have an ester linkage?
procaine, lidocaine (lignocaine), prilocaine, bupivacaine
procaine
lidocaine, prilocaine + bupivacaine have an amide linkage
Which of the following have a long duration?
procaine, lidocaine (lignocaine), prilocaine, bupivacaine
bupivacaine
Which of the following have a medium duration?
procaine, lidocaine (lignocaine), prilocaine, bupivacaine
lidocaine
prilocaine
Which of the following have a short duration?
procaine, lidocaine (lignocaine), prilocaine, bupivacaine
procaine
LA are (weak)/(strong) (acids)/(bases)
What determines its ionisation state?
weak bases
pH determines ionisation state
If the environment is alkaline, will there be more unionised/ionised LA molecules
more unionised
Which is the more common state of LA molecules at a physiological pH?
unionised + ionised bother present
BUT ionised > unionised
Which LA molecules are lipid soluble?
ie, ionised/unionised
unionised molecules are lipid soluble
How does LA stop detection of pain?
- anaesthetic present outside the neurone
- unionised LA molecules pass across neurone membrane + enter axon
- unionised LA molecules inside the neurone become ionised
- ionised LA molecules block open Na+ voltage gated channels
- = AP can’t be generated/propagated + signal isn’t transmitted
LA gives rise to use dependant block. What does this mean?
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
Why does acidity reduce effectiveness of LA?
eg, during inflammation/infection
increased acidity = more LA molecules ionised
= few unionised molecules enter the axon
= fewer molecules available to block to Na+ channels
If a patient does have an infection, should the procedure be done?
better to give antibiotics to remove the infection/inflammation and increase effectiveness of LA
Which nerves are more sensitive to LA? Wider/narrower?
narrower
C type neurones + A delta neurones = nociceptors
motor neurones = wider neurones
Are C type neurones myelinated?
no
Where is topic anaesthesia administered? How effective is it?
Applied to the skin
Not very effective
- skin has multiple layers = hard for LA to diffuse across
Where is infiltration anaesthesia administered?
When is it administered?
injected into skin
eg, when getting stitches/tooth extraction
Which type of anaesthetic targets large mixed nerves?
nerve block
- anaesthetic affects all the nerves included = covers a large area
Which type of anaesthetic bathes nerve roots exiting the spinal cord to numb the lower part of the body?
epidural
- injected into epidural space (within vertebrae but outside spinal cord)
Where is spinal anaesthesia administered?
anaesthetic goes into cerebrospinal fluid in the subarachnoid space
- affects any nerve running thorugh that space
What is the non-specific side effect of LA? What can cause them?
hyper-sensitivity reaction
preservatives in anaesthetic that prevent bacterial growth + keep solution sterile
What is a non-specific side effect?
side effect not caused by the anaesthetic itself
caused by something else in the solution
What is the consequence of anaesthetic being injected at high doses into vessels?
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
Which LA is the most cardiotoxic?
bupivacaine
Which other drugs are administered with LA?
vasoconstrictors
Why are vasoconstrictors administered with LA?
HINT: 2 effects of VC + 2 wider advantages
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
Why does caution need to be taken with administering vasoconstrictors?
if LA is being injected in an extremity (eg, finger/toe). reducing blood flow can be problematic
