Local Anaesthesia Flashcards
(25 cards)
what are the 2 types of anaesthetics?
general and local
what are the 3 types of local anaesthesia?
regional anaesthesia - loss of sensation to a region or part of the body
local infiltration - via cuts or skin incisions
topical - applied over the skin or on the eye
what is a local anaesthetic?
a drug which reversibly prevents transmission of the nerve impulse in the region to which it is applied, without affecting consciousness
block nerve conduction along the pathway to the CNS
what is the mechanism of action of local anaesthetics?
they block voltage gated sodium channels, and prevent sodium ions from entering the axon when it gets depolarised
why are local anaesthetics never injected into the nerves?
the needle might cut some axons and cause permanent nerve damage. local anaesthetics are injected around nerves, need to get past the epineurium, perineurium and endoneurium
how do local anaesthetics enter the axons?
they have to cross the PM of the neutron, because they block sodium channels from the inside. to do this, they cannot be ionised. but the ionised form of the local anaesthetic binds to the sodium channels.
what are the characteristics of an ideal local anaesthetic?
- reversible
- good therapeutic index
- quick onset
- suitable duration
- no local irritation/allergy or side effects
- applicable by all rules
- cheap, stable and soluble
what is the structural classification of local anaesthetics?
esters and amides
esters don’t have an ‘i’ before the caine, amides do
name some commonly used amides
prilocaine
lidocaine
etidocaine
bupivacaine
name some commonly used esters
procaine
cocaine
benzocaine
chlroroprocaine
what is our body’s pH?
7.4
when will a local anaesthetic have a fast onset?
when its pKa is closest to 7.4
what happens when pKa = pH?
the ionised and non ionised forms of the local anaesthetic are equal
what is the pH of pus?
6.9 (therefore local anaesthetics do not work as well in inflammed tissue or where there is pus)
what is the relationship between protein binding and duration of action?
higher the protein binding, longer the duration of action
what does the protein binding depend upon?
intermediate chain
what are the structural characteristics of local anaesthetics?
aromatic group
amine/ester group
intermediate chain
what is potency?
dose requires to produce the desired effect
what does potency depend upon?
lipid solubility, mosre lipid soluble drug penetrates the cell membrane easily and a smaller amount needs to be given to produce an effect
what are the factors affecting the ability of the drug to block neuronal conduction?
type of nerve fibre - larger it is, slower the onset
location of the nerve fibres - is it outside or inside the mantle?
why are vast contractors given with local anaesthetics?
- prolong action
- reduce plasma levels
- greater anaesthesia
- reduced operative haemorrhage
what are the contraindications of vasoconstrictors?
body parts that are supplied with end vessels - eg. fingers and toes, penis, ear lobule, alas of nose
because it causes hypoxia and irreversible damage
which are the 2 commonly used vasoconstrictors?
adrenaline, felypressin
what are the adverse effects of local anaesthetics?
hypersensitivity
methaemoglobinaemia - main toxic effect of prilocane - oxidation of iron in Hb, unable to carry oxygen
