L.A 1 Flashcards
Define LA
Local anaesthetics are
agents which abolishes the general sensation of pain, touch, temperature and pressure over a specified anatomic area
where it is injected or applied without loss of consciousness
action is always REVERSABLE.
Properties of the “ideal” local
anesthetic
not be irritating
should not cause any permanent alteration
systemic toxicity should be low
effective regardless of whether it
is injected into the tissue or applied locally
Short onset, long duration
should not cause any allergic
should be readily soluble
should be stable in solution
should be easily sterilized.
Advantages of L.A over G.A
- Safety.
- Ease of administration.
- Cooperation of the patient.
- Unlimited operation time.
- Reduced bleeding
- When the patient is unfit for a G.A
- Economical.
Disadvantages of L.A
- Difficulty to obtain L.A in children and mentally retarded patients.
- Unstable for major operations where muscle relaxation is required.
- Possibility of failure.
- Result in postoperative numbness of lower lip for few hours which may be objectional to the patients.
Pain receptors
Nociceptors have two different types of axons.
1. Aδ fiber axons.
myelinated 20 meters/second
- C fiber ,
non-myelination 2 meters/second.
Cells of the Nervous System
– Dendrites.
– Soma/ cell body.
– Axon.
– Presynaptic terminals.
A nerve impulse is
electrical message transmitted along the axon ,does not travel directly but is regenerated at points
resting potential refers to
the state of the neuron prior to the sending of a nerve impulse.
action potential (nerve impulse) is
a self- regenerating wave of electrochemeical activity in response to stimuli
At rest, the membrane maintains an
electrical polarization or a difference in the electrical charge of two locations.
The inside of the membrane is slightly
negative with respect to the outside.
(approximately -70 millivolts)
Depolarization refers to
decreasing the polarization towards zero.
threshold potential is
the membrane potential to which a membrane must be depolarized to initiate an action potential.
Depolarization stage , Membrane potential rises from- to
-70 mV to +40mV
Repolarization stage , The membrane potential fall to
fall to -80 mV
(called undershoot)
sodium-potassium pump later restores
All-or-none law
states that the amplitude
and velocity of an action potential are
independent of the intensity of the stimulus that initiated it.
(Action potentials are equal in intensity and speed within a given neuron)
Refractory period
during which time the neuron resists another action potential.
absolute refractory period : - first part can not produce an action potential.
relative refractory period :- take a stronger than usual stimulus to trigger an action potential.
Multiple sclerosis is
disease in which the
myelin sheath is destroyed and associated with poor muscle coordination.
Theories of action of L.A
- Acetylcholine theory.
- Calcium displacement theory.
- Surface charge (repulsion) theory.
- Membrane expansion theory.
- Specific receptor theory.
Mechanism of action
- Displacement of calcium ions from the sodium channel receptor site,
which permits … - Binding of the local anesthetic molecule to this receptor site, which
produces … - Blockade of the sodium channel, and a …
- Decrease in sodium conductance, which leads to …
- Depression of the rate of electrical depolarization, and …
- Failure to achieve the threshold potential level, along with …
- Lack of development of propagated action potentials, which is called …
- Conduction blockade.
Pka of amides
ranges from 7.6 to 8.1
The pH of the tissue becomes relevant in conditions of
infection or inflammation, in
which the natural pH may be more acidic.
acidity results in a greater proportion of the ionized (charged) form of the anaesthetic, which can not diffuse across cell membrane, thereby delaying or preventing the onset of action.
Factors Affecting Local Anesthetic
PKa
Lipid solubility
Protein binding
Tissue diffusibility
Vasodilating activity
Classification of Local anesthetics Based on chemical structure
Esters :
BENZOIC ACID ex Butacaine Cocaine
PABA ex Procaine
amides :
Artricaine
Bupivacaine
Etidocaine
Lidocaine
Mepivacaine
Prilocaine
Classification of Local anesthetics
Based on duration of action
Ultra short
Pulpal = < 10 min
Soft tissue = 30 – 45 min
procaine
Short
Pulpal = < 30 min
Soft tissue = 60 – 120 min
Lidocaine Mepivaciane
Medium
Pulpal = 45 – 90 min
Soft tissue = 120 – 240 min
Lidocaine Hcl 2% + epinephrine
Long
Pulpal =90 – 180 min
Soft tissue = 240 – 540 min
Bupivacaine + epinephrine
Patients with atypical form of pseudocholinesterase should not
receive
ester type of L.A
( to prevent systemic toxicity)
– Articaine =
– Prilocaine =
liver + blood
liver + lung
Except for cocaine, all other L.A produce
peripheral vasodilation
Systemic actions of L.A on cvs
1.8 - 5 = antidysrhythmic
5 - 10 = ecg alterations , myocardial depression, peripheral vasodilation
10+ = massive peripheral vasodilation, intensive myocardial depression , cardiac arrest
Systemic actions of L.A on CNS
0.5 - 4.0 = anticonvulsant
4.5 - 7.0 = CNS depression
7.5 - 10 = CNS depression + tonic- clonic seizures
10+ = generalized CNS depression + respiratory depression
