Local Anaesthetic Flashcards
define a local anaesthetic
reversible anaesthesia and analgesia by temporarily blocking nerve conduction
define an action potential
reversal of cell membrane potential that is the mechanism by which electrical messages are conducted along nerves
Threshold potential
-55mV
will depolarise if stimulus strong enough
MOA
block voltage-gated calcium channels hence depolarisation cannot occur, and does not reach CNS
alpha nerve fibre
motor
AB nerve fibre
touch and pressure
A-cos
Proprioreception
As nerve fibre
C-fibres
pain and temp
other C fibre role
vasodilation
structure of LA
aromatic ring, lipophilic with intermediate chain to amine, hydrophilic ( hence an ester and an amide )
examples of ester LAs
cocaine
tetracaine
examples of amide LAs
lignocaine
bupivacaine
ropivicaine
levobupivicaine
prilocaine
are LAs acids or bases?
weak bases
- can be ionised in water = acid
- must be un-ionsied to cross cell membrane and then is ionised
( pKa is when its 50/50 )
Lignocaine pKA
7.9
Bupivacaine pKA
8.1
why pKA relevant?
affects speed of onset of drug
what will result in a more rapid onset of action?
- More un-ionised form
- closer the pKA is to the tissue pKA ( 7.4 )
components of LA activity and what factors affect each?
> potency
- lipid sol
- vasoconstriction = decreases rate of absorption hence improve
> onset of action
- pKA
- lipid
- alkalisation
> duration
- protein binding
-lipid
- analgesic = increases duration
- vasoconstriction
can you use full doses?
NO - additives only
Lignocaine max dose
3mg/kg or 7mg/kg with adrenaline
Max dose of bupivacaine
2mg/kg with or without adrenaline
signs of overdose in LAs
CNS:
- paraesthesia
- tinnitus
- blurred vision
- confusion
a. excitation = twitches
b. depression = LOC, resp, coma
CVS:
a. excitation = hypertension and tachycardia
b. depression = hypotension and Brady
c. terminal = cardiac ischemia and cardiac arrest
mx for LA toxicity
- stop injection
- ABCs
- hyperventilate pt
- Antidote: intralipid
20% bolus 1.5ml/kg
+ Infusion
