Local Anaesthetics Flashcards
How are the different effects of cocaine produced?
Local anaesthetic- blocking sodium channels
Psychotropic- blockage of monoamine reuptake
What was the first pure local anaesthetic with no psychotropic effects called?
Amylocaine
Describe the features of type A alpha spinal nerve fibres.
Heavily myelinated
12-20um in diameter
Proprioception motor function
Describe the features of type A beta spinal nerve fibres.
Heavily myelinated
5-12um in diameter
Touch and pressure response function
Describe the features of type A gamma spinal nerve fibres.
Heavily myelinated
3-6um diameter
Muscle spindle function
Describe the features of type A delta spinal nerve fibres.
Heavily myelinated
2-5um diameter
Pain and temperature response function
Describe the features of type B spinal nerve fibres.
Light myelination
<3um diameter
Pre-ganglionic ANS function
Describe the features of type C sensory spinal nerve fibres.
Not myelinated
0.3-1.2um diameter
Pain response function
Describe the features of type C sympathetic spinal nerve fibres.
Not myelinated
0.2-1.3um diameter
Post-ganglionic function
What are polymodal spinal nerve fibres?
Able to respond to more than one type of stimulus
What is the major structural difference between motor and sensory neurones in the spinal cord?
Motor neurones enter the spinal column via ventral roots
Sensory neurones enter the spinal column via dorsal routes
Briefly describe the structure of dorsal route ganglion sensory neurones.
Cell bodies have a single axon that splits
One branch goes to periphery, other into the spinal cord
Briefly describe the structure of nociceptors.
Peripheral axon terminal is a bare nerve ending possessing receptors for noxious stimuli
May also be stimulated by protons and ATP released by damaged tissue
Describe the action of chilli peppers on nociceptors in the mouth.
Contain capsaicin which activates VR1 receptors
Normal function of these is to respond to high temperatures
Not water soluble therefore water does not cool mouth after spicy
What is the mechanism of action local anaesthetics?
Block sodium channels to prevent formation of action potentials and blocking nociception
Describe the structure of voltage gated sodium channels.
Alpha subunit is a long chain with 24 membrane spanning domains
Divided into 4 blocks of 6 transmembrane domains
Each pseudosubunit forms a structure with a charged 4th TM domain and membrane dipping domain
Describe how the structure of local anaesthetics aids their function.
Amine group allows local anaesthetics to become protonated
LA blocks sodium channels from within the cell therefore uncharged form must be present to allow passage through membrane
Describe risk of use dependence with local anaesthetics.
Local anaesthetics block sodium channels more strongly in their open or activated states giving rise to use dependence
If LA dissolves through the membrane uncharged and then becomes protonated when reaching the channel binding site it does not require the channel to be open therefore does not show use dependence
What is topical anaesthesia?
LA formulated as cream or spray and applied directly to skin surface
Allows blockage of signal close to, or at, the peripheral nerve terminal
When is topical anaesthesia used?
Pain relief for minor conditions i.e. ulcers, haemorrhoids
What is infiltration anaesthesia?
LA injected into tissue around area to be numbed
Usually affects only the more distal parts of the nerves (distal branches)
When may a vasoconstrictor be used in local anaesthesia? Give an example.
To prolong duration of infiltration anaesthesia or nerve block
i.e. adrenaline
When is infiltration anaesthesia used?
Dentistry
What is a nerve block?
LA injected around a spinal nerve trunk, everything distal to this point will be numbed
Needle is often guided into place with the aid of imaging
Why may a nerve block be used?
Can give better pain relief than infiltration or topical anaesthesia
What is IV regional anaesthesia?
Pressure cuff used to cut off blood supply to a limb
LA is then injected and the cuff removed after 20 minutes
Why is there a delay between administering IV regional anaesthesia and restoring blood supply?
Allows anaesthesia to develop and reduce concentrations of LA in the blood
What is spinal anaesthesia?
Injection into the subarachnoid space just above the spinal cord causing pain sensation to be lost from all regions supplied by nerves below the level of injection
Where can spinal anaesthesia be used?
Injection only into the lower spine due to the size of the subarachnoid space
Describe the pros and cons of spinal and epidural anaesthesia.
Spinal is less versatile than epidural
Epidural shows less motor block
Onset of spinal is faster and less anaesthetic is required
Placement of epidural needle is easier than spinal so can be used anywhere along the spinal column
What is epidural anaesthesia?
Injection into epidural space just below the vertebral column, usually via a catheter to facilitate further administration
How does the effect of epidural anaesthesia vary with dose?
Low dose epidurals are often used for analgesia alone
Higher doses can be used for surgical analgesia where the patient needs to remain awake
What are the 3 main properties of spinal nerve fibres that affect sensitivity?
Degree of myelination
Fibre diameter
Position within the nerve
Why is there an allergy risk with LAs?
Ester linked drugs can be metabolised to para-aminobenzoic acid which can produce allergy
Describe the risk of brain toxicity with LAs?
Neuronal activity depressed at low doses
As concentrations increase, activity of inhibitory neurones suppressed more than excitatory ones, can result in convulsions
Describe the risk of CV toxicity with LAs?
Slow heart rate and reduce blood pressure
Can cause arrhythmia
Vasodilators with the exception of cocaine
When may a LA be used in CV emergency?
Reduce heart rate in ventricular fibrillation
Which LA has a duration of action of 1-3 hours?
Lidocaine
Which LA has a duration of action of 30-60 minutes?
Tetracaine
Which LA has a duration of action of 2-5 hours?
Bupivacaine
Which LA is not safe in obstetrics?
Prilocaine
