Week 10: Local Anaesthetics Flashcards
What are the 3 types of local anaesthesia?
- Regional anaesthesia –> loss of sensation to a region or part of body
- Local infiltration –> anaesthetic is injected where the injury is
- Topical –> eye, skin (no injection
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
What are the non-pharmacological methods of local anesthesia?
- cold temperature (neuroconduction is blocked if temp falls below 8 degrees)
- pressure
- hypoxia
What are the 2 types of pharmalogical local anaesthesias?
- reversible e.g local anaesthetics block nerve conduction for a variable period of time
- irreversible e.g ethanol, surgical –> once you use these, the nerve conduction is blocked forever
How does an injury cause pain?
- At the site of injury, the Na+ channels open and Na+ moves inside the cell so the membrane depolarises and generates an action potential
- 1st order neuron takes information from site of injury to spinal cord
- 2nd order neuron takes information from spinal cord to thalamus
- 3rd order neuron takes information from the thalamus to the somatosensory cortex
Where is the severity of pain analysed?
the somatosensory cortex
How do local anaesthetics work?
- local anaesthetics block the alpha subunit of the voltage-gated sodium channel so the impulse cannot cross the areas
- so the brain is not receiving information about the pain so you cannot feel it
What is the endoneurium?
each nerve fibre (axon) is surrounded by a layer of connective tissue called the endoneurium
What does the perineurium surround?
- bundles of nerves are called vesicles
- each vesicle is surrounded by perineurium
Many vesicles join together to form a nerve covered by what?
epineurium
Where are LAs injected?
- LAs are injected outside the nerves and then they diffuse through epineurium, perineurium and endoneurium to act on the sodium channels on nerve axons.
- LAs are never injected into the nerves as this will cause damage to the nerves
What two forms does local anaesthetic exist in
- ionised/protonated
- unionised/ unprotinated
Which form of LA can:
- Cross the cell membrane
- Can bind to the sodium channel
- Unionised
2. Protonated
If there is an injury of the hand, explain how the information about pain reaches the somtosensory cortex
- the information will be carried from the site of injury to the spinal cord by the median nerve
- The information will synapse at the grey horn
- then travel in the lateral spinothalamic tract to the thalamus
- then to the somatosnesory cortex
What is the ideal local anaesthetic?
- reversible
- good therapeutic index
- quick onset
- suitable duration
- no local irritation
- no side effects
- no potential to induce allergy
- cheap, stable and soluble
What was the first local anaesthetic used?
cocaine
What are the 3 parts of every LA?
- the lipophilic part
- the intermediate chain
- the hydrophillic part
the intermediate chain connects the lipophilic and hydrophilic parts
What determines the type of anaesthetic?
- the bond that connects the intermediate chain with the benzene ring
- determines whether the LA is an ester or an amide
How can you tell which LA is an amide and which is an ester?
- all LAs end in ‘caine’
- if there is an i earlier in the word, the LA is an amide
- if there is no i in it its an ester
What does the onset of action of an LA depend on?
- Depends on the ability of the LA to go into the nerve cells and block the Na+ channel from the inside
- unionised form of the drug crosses the membrane
- so more of the unionised form the faster the action
When does pKa = pH?
unionised form = ionised forms of local anaesthetics
When is pKa > pH
when unionised form > ionised form
What is the relationship between pKa and body pH?
further away the pKa of local anaesthetic from body pH the lesser the unionised form, therefore the slower in onset
Why is an LA less effective in inflamed tissue?
inflammation results in local acidosis which decreases the surrounding pH of tissue
