Local anaesthetics Flashcards
When are local anaesthetics used?
when loss of consciousness is neither needed or desirable
What are the two structures of local anaesthetics?
either esters (-COO) or amides (-NHCO)
What are the amide containing drugs with their action and onset?
lidocaine - medium acting, rapid onset
prilocaine - medium acting, no vasodilation
bupivacaine- long acting, slow onset
articaine - short acting, rapid onset
What are the ester containing drugs with their action and onset?
tetracaine - long acting, very slow onset
chloropaine - medium acting
benzocaine - atypical action
What is the mechanism of action of LAs?
work by reversibly blocking voltage-gated Na+ channels to prevent membrane from being depolarised
What is the structure of peripheral nerves?
consists of fibres of different function, diameter and insulation
What is the order of loss?
pain
temperature
proprioception
skeletal muscle tone
What does the probability that a LA will block an impulse rely on?
- diameter of the fibre
- myelination status
- length of nerve exposed to drug
- length of time exposed to drug
- concentration of drug
When can voltage gated sodium channels not be blocked?
in the resting state
How do LAs block voltage-gated sodium channels?
they block the channels when they are ionised via an intracellular binding site
What is the pKa?
the pH at which 50% of the drug is ionised
What is the pKa of most LAs?
between 8 and 9
What is duration of action dictated by?
rate of removal
How can LAs be administered?
- surface
- infiltration (injection)
- nerve block
- intravenous regional
- extradural aka epidural
- subarachnoid
What is an example of an eutectic mixture of LAs?
EMLA
- mixture of lignocaine and prilocaine
- dermal anaesthesia
- melting point of mix lower that that of individual chemicals therefore higher concentration can be used
