Local Anaesthetics Flashcards
Where does LA stop the signal in the nerve pathway?
before it reaches spinal dorsal horn/ trigeminal nucleaus
at the receptor/ axon (1st order neurons)
What are the layers of peripheral nerves?
endoneurium
perineurium
epineurium
What is the order of block in diifferent fibres from first to last?
Ad, C, Ab, Aa
smallest. thinly myelinated/not myelinated blocked first
Function of each fibre?
Aα fibers (Group I, myelinated):
- Proprioceptors (muscle spindles) - sense body position/movement
- Motor neurons to skeletal muscle
Aβ fibers (Group II, myelinated):
- Mechanoreceptors - detect touch, pressure, vibration
Aδ fibers (Group III, thinly myelinated):
- Polymodal nociceptors - detect noxious/painful stimuli
- Mechanoreceptors
- Thermoreceptors for cold
- Chemoreceptors for taste
C fibers (Group IV, unmyelinated):
- Nociceptors - detect noxious/painful stimuli
- Thermoreceptors for hot temperatures
- Chemoreceptors
- Postganglionic autonomic fibers
What is the function of a-gamma?
motor (muscle spindles)
myelinated
How does LA work?
- LA binds to a site in the Na+ channel
- LA blocks the channel and prevents Na+ influx
- This blocks action potential generation and propagation
- Block persists so long as a sufficient number of Na+ channels are blocked
How can LA affect the heart?
Local anaesthetics block Na+ channels in other excitable tissue, e.g. heart muscle
* LA can cause bradycardia and hypotension
What are the three chemical components of LA?
– Aromatic region (hydrophobic) (pass membrane)
– Ester or amide bond
– Basic amine side chain (hydrophilic) (soluble)
What form of LA is active?
ionised form
Why is it in un-ionised form?
can cross membrane only in un-ionised form
How does the LA present as?
hydrochloride (B.HCL)
What happens if the tissue is acidic?
H+ dissociation of B.HCL takes longer
In regards to the H-gate, what do LA drugs do?
they close the H-gate extending the refractory period indefintely
Where are ion channels concentrated?
nodes of ranvier
What must happen to block the AP?
the LA needs to act on several nodes of Ranvier along the axon
What is the LA preparation made of?
- LA base present as hydrochloride, to increase solubility in aqueous solution 2 - 4% solutions
- Reducing agent (sodium metabisulphide)
- Preservative(s) and fungicide
- ± vasoconstrictor
What is an example of a topical anaesthetic ester used today?
benzocaine
What injectable amides are used today?
lidocaine
prilocaine
articane
bupivacaine
What are the vasoconstrictors used?
adrenaline
felypressin (synthetic vasopressin)
How do vasoconstrictors act on beta and alpha receptors?
– a receptors: vasoconstriction
– b2 receptors: vasodilation
– b1 receptors:
– Cardiac muscle:
» Positive chronotropic effect (^ rate)
» Positive inotropic effect (^force)
Where does vasopressin work?
ADH receptors
What does adrenaline do systemically?
Systemically, it lowers TPR (b > a)
* Adrenaline increases Cardiac Output
* Overall, adrenaline has little or no effect on mean arterial BP
What does noradrenaline do systemically?
- Noradrenaline (NA) is more effective on a than on b receptors
- Systemically, it increases TPR (a > b)
- NA increases Cardiac Output
- Overall, NA raises mean arterial BP
Breaking down of Esters VS Amides
- Ester local anesthetics: tissue esterases in blood (brief action)
- Amide local anesthetics: liver amidases cytochrome P-450) (long action)
What are the main modes of administration?
topical
local infiltration
regional nerve block
What is the preparation for lidocaine with VC?
2% lignocaine HCl + 1:80,000 adrenaline
What is the preparation for prilocaine with VC?
3% prilocaine HCl + felypressin (0.03U/ml)
How are percentage solutions calculated?
X% solution = X mass / volume
* e.g. 3% Prilocaine HCl solution
* 3%=3g/100ml
* = 30mg / 1ml
What is the max dose of lidocaine?
4.4mg per kg body weight
What is the max dose of adrenaline?
500 micrograms
