Lecture 15 - Local anaesthetics Flashcards
How does local anaesthetics act?
by blocking conduction in nerve fibres
how is the anaesthesia achieved by local anaesthetics?
without loss of consciousness or depressed activity of CNS
- loss of sensation
- muscle paralysis - depending on type of fibre involved, route of application
What is the normal use of local anaesthetics?
block conduction in pain fibres
- for diagnostic purposes
permit minor surgery
antiarrhythmic treatment
What are the two chemical types of local anaesthetics?
esters and amides
give some examples of esters (local anaesthetics)
procaine
chloroprocaine
oxybuprocain
tetracaine
benzocaine
give some examples of amides (local anaesthetics=
lidocaine
prilocaine
mepivacaine
bupivacaine
ropivacaine
What is the mechanism of action concerning LAs
blocks action potential generation by blocking Na+ channels
LAs act in their cationic form (they have to be weak bases), but must reach their site of action by penetrating the nerve sheath and axonal membrane as unionised species
What is the affinity to the LA and the effect of the channel when it is in refractory status?
Affinity: small
Effect: inhibition of the opening of sodium ion channels
What is the affinity to an ion channel and the effect of it when it is in closed status?
Affinity is large
the effect is: inhibition of reaopening - major effect
What is the affinity and the effect of the ion channel when it is in open status?
affinity is large
effect is closing of Na ion channels - minor effect
what is the affinity and the effect of the ion channel when it is in inactivated status?
large affinity
the effect is a prolonged refractory period - major effect
In what order is LAs blocking the conduction (axons)?
non-myelinated axons
small myelinated
large mylinated
What type trnasmittion is blocked first with LAs?
noiciceptice and sympathetic transmission
When the nervous function dissapears, in what order is the different function lost?
- pain
- warmth
- touch
- deep pressure
- motorfunction
In what order is nervous function returning?
- motor function
- deep pressure
- touch
- wramt
- pain
Describe the nerve fibre: Aa, Ab
- myelin
- diameter
- speed of conduction
- function
- sesnitivity to lidocaine
- yes
- 6-22
- 10-85
- motoric and proprioceptive
- +,++
Describe the nerve fibre: A-gamma
- myelin
- diameter
- speed of conduction
- function
- sesnitivity to lidocaine
- yes
- 3-6
- 15-35
- muscle tone
- ++
Describe the nerve fibre: Aδ
- myelin
- diameter
- speed of conduction
- function
- sesnitivity to lidocaine
- yes
- 1-4
- 5-25
- rapid pain and temperature
- +++
Describe the nerve fibre: B
- myelin
- diameter
- speed of conduction
- function
- sesnitivity to lidocaine
- yes
- <3
- 3-15
- vasmotor, visceromotor, sudomotor, pilomotor
- ++++
Describe the nerve fibre: C (sympathetic)
- myelin
- diameter
- speed of conduction
- function
- sensitivity to lidocaine
- no
- 0,3-1,3
- 0,7-1,3
- vasomotor, visceromotor, sudomotor, pilomotor
- ++++
Describe the nerve fibre: C dorsal horn
- myelin
- diameter
- speed of conduction
- function
- sensitivity to lidocaine
- no
- 0,4-1,2
- 0,1-2,0
- slow pain and temperature
- ++++
What are kinetics?
study of reaction rate and how they are affected
What are the kinetics of absorption?
site of application
dose
if its a vasoconstrictive drug
features of LAs
How long will the effect of a LA last?
until the concentration falls below the critical level due totheir absorption from the site of application
How are ester-type LAs metabolised?
they are hydrolysed primarly by plasma esterase and degraded by hepatic metabolism
decomposition in tissue and blood circualtion
How is the metabolism of amide LAs?
degraded by hepatic inactivation (liver degradation)
Which chemical type of LAs are the safest?
esters
How are the metabolites of LAs excreted?
through the kidneys
what is the order of clearnace of amides?
prilocaine>lidocaine>mepi-/ropicacaine>bupivacaine
Esters: procaine
- relative potency
- onset of action
- duration of action
- non-ionised (%)
- protein binding (%)
- lipid solubility (%)
- 1
- slow
- 45-60
- 3
- 6
- 0,6
Amides: lidocaine
- relative potency
- onset of action
- duration of action
- non-ionised (%)
- protein binding (%)
- lipid solubility (%)
- 2
- rapid
- 60-120
- 25
- 70
- 2,9
Esters: tetracaine
- relative potency
- onset of action
- duration of action
- non-ionised (%)
- protein binding (%)
- lipid solubility (%)
- 8
- slow
- 60-180
- 7
- 76
- 80
Amides: mepivacaine
- relative potency
- onset of action
- duration of action
- non-ionised (%)
- protein binding (%)
- lipid solubility (%)
- 1-5
- meidum
- 90-180
- 39
- 77
- 1
Amides: bupivacaine
- relative potency
- onset of action
- duration of action
- non-ionised (%)
- protein binding (%)
- lipid solubility (%)
- 8
- medium
- 180-480
- 15
- 95
- 28
What the effect of the protein % of LAs?
its the limiting factor
- cannot get through BBB
How are the side effects of LAs?
relatively free from hramful side effects
Rate the LAs toxicity in increased order
- mepivacaine
- procaine
- lidocaine
- tetracaine, bupivacaine
What are the most common side effects of LAs?
- allergy
- asthmatic seizure
- local tissue irritation
- systemic effects - elevated plasma conc
- CNS effecs
- first mild sensory inhibitiom
- agitation
- confusion
- tremors progressing to convulsions
- respiratory depression
- cardivascular effects
- myocardial depression
- vasodilation
- fall in blood pressure
- CNS effecs
- effect of adjuvant - adrenaline
When can absorption of LA cause toxic effects?
when the rate of absorption is faster than the rate of elimination
how can the rate of absorption of LAs be reduced?
with vasoconstrictors
What are the uses of LAs??
topical anasthesia
local infiltration
perineural injection
intra-articular use
intravenous regional anaesthesia (IVRA)
epidural and spinal (subarachnoid) block
Where can LAs be used topically?
applied to
the cornea
mucous membranes
skin
What is the intension of adding drugs to teh cornea, mucous membranes or skin?
cause loss of sensation by paralyzing sensory nerve ending
when lidocain or prilocain is added to the skin, how long time does it usually take for the anasthesia to work?
60min
What form is LAs in when it is applied topically?
cream or gels
what indications are topically LAs used for?
ulcers
opthalmology
oral
rectal
pharyngeal painful diseases before examination
burnings
sun irritation
inflammation of outer, middle ear
intact skin surface
What is the common method of adminitrating LA?
Local infiltration
how do we administer the LAs with local infiltration?
numerous SC inj. of small volumes
what isthe indications of local infiltration?
minor surgery
dentistry (may be ineffective)
castration of males (with sedative, anaesthetics)
subcutaneous regional anaesthesia
what happens whn LAs are injected along a line?
it will block the conduction in the nerves that passes through the tissue
all regions supplied by the distal section of these nerves will be anaesthetised
How is LAs perineural injection performed?
its a peripheral nerve block
the anaesthetics is injected in the immediate vicinity of peripheral nerves
What are the indications of perneural injections?
dentistry
minor surgery - head, legs, hoofs
what other blocks than peripheral nerve blocks cna be performed with LAs?
paravertebral blocks
intercostal blocks
brachial plexus blocks
How are LAs used intra-articular?
rarely
its used for diagnostic reasons or for therapeutic indication - operating the affected joint
How is LAs administered as an intravenous regional anaesthesia?
it should be injected IV dital to a torniquet
only less tocic agents should be administeres - prilocain
used in cattle digit operations
How is LAs administered when its used as an spinal block?
subarachnoid
lowe dose
single application
what is the disadvantage with administering LAs into the spine?
spinal cord injury can occur
What are the advantages of using LAs in the epidural space or the subarachnoid space?
large areas of the body can be anaesthetised with small amount of drugs
what are the indications of epidural/spinal administration of LAs
surgery of perneal region
anal
peri-anal region
obsteric operations
urology
Whare is the place of injection of epidural anasthesia in horses and cattle?
first or second intercoccigeal spaces ( caudal anaesthesia=
what happens if the epidural is administered higher than indicated?
limb paralysis
where is the place of injection of epidural anaestheisa in dog, sheep and pig?
at the lumbosacral space
lumbral anaesthesia
how much lidocain 1% is administered in
- large animals
- dogs, pigs
- 60-80ml
- according to body size
- 40cm= 2ml
- 60cm=5ml
- 80cm=8ml
- 100cm=10,75ml
Posology of certain local anaesthetic
doses in % of solution
procain
tetracaine
lidocaine
bupivac
mepivac