local anaesthetics Flashcards

1
Q

how do local anaesthetics work

A

reversible blocking the generation and conduction of APs via blocking voltage gated sodium channels

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2
Q

what is mechanism of local anaesthetics

A

they plug sodium channel from the inside

block develops faster and is greater when nerve is conducting APs at high frequency

reason for this is that inactivated sodium channels have higher affinity for local anaesthetics than resting ones, also when nerve depolarises it drives LAs into the sodium channel

this process is called use-dependency block

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3
Q

how does LA structure affect membrane permeability and how they are administered

A

most LAs are weak bases, charged forms cannot penetrate axon membrane

uncharged forms diffuse across

once inside 80-90% become charged at intracellular pH

they are administered as water soluable hydrochlorides

after injection the tertiary amine base is liberated by alkaline pH of tissue fluids

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4
Q

what is the most significant property of LAs in determining potency

A

lipid solubility

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5
Q

what affects duration of block

A

capacity of LA to bind plasma and tissue proteins

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6
Q

how does pKa of the LA affect speed of block

A

pKa= pH which substance is 50% ionic

lower pKa greater fraction exists in uncharged form, so faster blockade

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7
Q

how does size of fibre effect block

A

thin fibres are blocked more easily than thicker ones

myelinated ones are blocked more readily than non myelinated ones

small diameter C and Adelta pain fibre, fire at high frequency so size and rate means they are very easily blocked and more selectively blocked than Aalpha fibres

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8
Q

in terms of sensation, what is blocked first-last

A

degree of block of sensations: pain>cold>warm>touch>pressure>motorneurones

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9
Q

how does structure affect how LA stability and metabolism

A

amino ester LAs: relatively unstable in solution, rapidly hydrolysed by plasma cholinesterase and other esterase’s

amino amide LAs: stable in solution, slowly metabolised in liver by hepatic amidases

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10
Q

what are CNS side affects of LAs in low concs

A

tinnitus, numbness of tongue, blurred vision, drowsiness

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11
Q

what are CNS side affects of LAs in large concs?

A

agitation with hyperactivity, occasional convulsions followed by CNS depression and respiratory depression

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12
Q

what are CV effects of LAs

A

vasodilation and slowing of heart rate leading to hypotension

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13
Q

what metabolite of LAs is associated with hypersensitivity reactions

A

pABA, metabolite of amino ester LAs

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14
Q

what are methods of administration of LAs

A

topical application for surface anaesthsia e.g lidocaine or benzocaine

drug injected into tissue for inflitration anaesthesia e.g lidocaine and prilocaine

injected LAs are usually supplemented with vasoconstrictors to delay absorption which prolongs duration of action and reduces risk of systematic toxicity, lowers dose required

examples of vasoconstrictor adjuncts are (nor)adrenaline and vasopressin

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15
Q

how would you anaesthetise an area served by a specific nerve

A

nerve block anaesthesia: drug injection close to nerve trunk

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16
Q

how is spinal anaesthesia administered, what is it used for

A

injection into subarachnoid space between L2-5, used for major surgery where patient still need to be awake, e.g C section

17
Q

how is epidural anaesthesia administered

A

injection into epidural space (outside dura mater) which causes direct action on nerve roots and spinal cord, used in obstetrics