Local Anaesthetics Flashcards
What is general anaesthesia?
total loss of sensation
What is local anaesthesia? (3)
- regional
- local infiltration - cuts, skin incisions
- Topical - eye/skin
What are examples of non-pharmacological local anaesthesia methods? (3)
- cold
- pressure
- hypoxia
What are the 2 types of pharmacological local anaesthesia methods?
- not reversible
2. reversible
What are examples of not reversible pharmacological local anaesthesia? (4)
- Phenol
- Ethanol
- Radiofrequency
- Surgical
What is a local anaesthetic?
a drug
What does a local anaesthetic do? (2)
- reversibly prevents transmission of the nerve impulse in the region to which it is applied
- doesn’t affect consciousness
Where can a local anaesthetic block a neuron?
Anywhere along the pathway
How is an axon broken down (what components)? (4)
- epineurium
- perineurium
- endoneurium
- schwann cell
When does the local anaesthetic cross the membrane?
Only when ionised
What is an ideal local anaesthetic? (11)
- reversible
- good therapeutic index - ED50 and LD50
- Quick onset
- Suitable duration
- no local irritation even on repeated application
- no side effects
- no potential to induce allergy
- applicable by all rules
- cheap
- stable
- soluble
What is the structure of local anaesthetics?
- esters - Procaine
- amides - Lidocaine
RULE
- no “i” = ESTER
- “i” = AMIDE
What was the first local anaesthetic?
cocaine
What is the characteristics of local anaesthetics? (3)
- onset
- duration
- differential blockade
What does onset of action depend on?
pKa
In terms of pH what contributes to a quicker onset of the local anaesthetic?
pKa = pH
- Ionised and non-ionised forms of local anaesthetic is equal
If nearer to 7.35-7.45 the quicker the onset
In terms of pH what leads to a delayed onset of the local anaesthetic?
pKa > pH
ionised form > unionised form
What is the pH of pus?
6.9
What does duration of action depend on?
- Protein binding
- the more protein binding the longer the duration
Bupivacaine - the longest
- Length of the intermediate chain joining the aromatic and amine groups
What is potency?
dose required to produce desired effect
What does potency depend on?
lipid solubility
- the more lipid soluble drug penetrates the cell membrane more
- hence a smaller amount is required to produce a given effect
What is differential block?
ability to block neuronal conduction
What does the ability to block neuronal conduction depend on? (2)
- type of nerve fibre
- larger the fibre the slower the onset - Location of nerve fibre
- outside or in the mantle
What are the types of nerve fibres?
- A
- alpha - proprioception and somatomotor
- beta - touch and pressure
- gamma - motor to muscle spindle
- delta - pain, cold and touch
