Local anaesthetics Flashcards
What are the three subtypes of local anaesthesia (and what do they mean)?
regional anaesthesia (loss of sensation to a region or part of body) local infiltration (cut, skin incision) topical (eye, skin)
Name some non-reversible pharmacological methods of local anaesthesia?
phenol, ethanol, radiofrequency, surgical (kills the nerve)
Define local anaesthetic
a drug which REVERSIBLY prevents transmission of the nerve impulse in the REGION to which it is applied without affecting CONSCIOUSNESS
How do local anaesthetics prevent conduction of action potential?
blocking sodium channels
Where is local anaesthetic injected?
AROUND the nerves (so must pass through the perineurium and endoneurium before having effect, hence the delay in action)
Where does the local anaesthetic bind?
on the sodium channel INSIDE the cell (therefore must pass through cell membrane)
Why can local anaesthetic pass through the cell membrane?
They are not ionised.
What are the ideal characteristics of local anaesthetic?
- reversible
- good therapeutic index
- quick onset
- suitable duration
- no local irritation
- no side effects
- no potential to induce allergy
- applicable by all routes
- cheap, stable, soluble
What does the onset of action depend on?
pKa (when pKa=pH, quickest onset)
What is the pKa?
pH at which the ionised and non-ionised forms of local anaestheticas are equal
Why doesn’t local anaesthetic work as well in areas of inflammation or where there is pus?
the pH of pus is much lower than the rest of the body (about 6.9) so the pKa and pH will be too different for action and there will be much more of the non-ionised form
What does duration of action depend upon?
protein binding (more binding, longer duration of action)
What is protein binding dependant on?
the length of the intermediate chain joining the aromatic and amide/ester group
Define potency
dose required to produce a desired effect
What does potency of LA depend on?
lipid solubility (more lipid soluble will be more potent)
What does ability to block neuronal conduction depend upon?
type of nerve fibre (larger the fibre, slower the onset)
location of nerve fibre (deeper within nerves will take longer to be blocked)
what are the advantages of using vasoconstrictors?
- prolong action
- reduce plasma levels (less risk of CNS effects)
- ‘greater anaesthesia’ or reduced dose
- reduced operative haemorrhage
What are the contraindications of vasoconstrictors?
body parts that are supplied by end-vessels (fingers, toes, penis, ear lobe, alae of nose)
How does adrenaline act as a vasoconstrictor?
stimulation of alpha adrenoreceptors constrict blood vessels
Does adrenaline decrease the safe dose of anaesthetics?
NO
Which type of LA are more likely to induce a hypersensitivity reaction?
ester LAs
What is the main toxic effect of prilocaine?
methaemoglobinaemia (it oxidises ferropus to ferric ions –> hypoxia)
What is the treatment of methaemoglobinaemia?
methylene blue
What is the treatment for LA toxicity?
STOP INJECTING
- maintain airway
- give oxygen
- confirm/establish IV access
- control seizures
- consider drawing blood for analysis
- give intravenous lipid emulsion
What is the toxic dose of lidocaine (with and without adrenaline)?
3 mg/kg
with adrenaline 7 mg/kg
