Local Anesthetics Flashcards
What is the goal of LA administration?
Goal is to reduce the sensation of pain from a painful stimuli.
Do LA produce reversible or irreversible conduction block?
Reversible.
Which two anatomical qualities of a nerve have the greatest effect on the conduction speed?
The size and the degree of mylenation.
The ability for a LA to produce conduction blockage is dependent on two things:
- Physiochemical properties of the local anesthetic.
2. Anatomy of the nerve being blocked
Where are the nodes of Ranvier located?
The space between myelin sheaths.
What is saltatory conduction?
Electrical conduction the produces a jumping action along a nerve.
Named by the metal/non-metal reaction which is often called a salt (saltatory)
Do mylenated or unmylenated nerves produce a “jumping” effect of nerve impulse conduction?
Mylenated
What is faster for nerve conduction- a pushing mechanism for action potential or jumping mechanism?
Jumping via mylenated nerve fibers.
Nerve fibers are classified into three groups: what are they?
A
B
C
A Fibers are broken down into 4 sub-groups: what are they?
Alpha
Beta
Gamma
Delta
Which A fiber sub-group conducts mostly pain and temperature sensation?
A-Delta fibers (and A Beta Fibers)
Which A fiber sub-group conducts mostly touch and motor function?
A-Alpha fibers
What is proprioception?
Touch sensation
What type of fibers are autonomic nerves?
B Fibers
Between A, B, and C fibers, which conduct the fastest?
A>B>C
Which type of pain impulse is conducted by C-Fibers?
Slow, chronic, dull, aching type of pain.
Which type of fibers are the only ones that are not mylenated?
C-Fibers
Which A-fiber subgroup conducts mostly muscle spindles and reflex?
A-Gamma
Higher or lower dosages of LA produce different responses. What are they?
Lower dose=Autonomic fibers.
Moderate doses=Sensory (heat, cold, pressure, pain)
Higher dose= Motor
MOA of all LA
Reversibly block sodium channels
Where are the receptors located that LA act on?
Intracellular side of the nerve cell membrane.
What physiochemical property effects how well a LA enters the nerve fiber?
It’s level of lipophilicity.
What is required of the LA to bind to the receptor?
Ionization
How does a LA become ionized inside the nerve cell?
It is more acidic inside the nerve cell. For stability, the LA becomes more acidic by kicking off hydrogen once inside the nerve cell. This causes it to be able to bind to the receptor
What condition can cause the LA to have no or little effect?
In the presence of local infection, LA has difficulty working because the body tissues it is injected into is more acidic. This means it never gets to enter the nerve cell (Where the receptors are located).
What three chemical characteristics do all LA have?
- An unsaturated aromatic ring system.
- An intermediate carbon group.
- A tertiary amine
What does the unsaturated aromatic ring system provide of the LA?
Big Lipophilic body
What are the two major differences between ester and amide linkages of LA?
- Rate/Type of metabolism.
2. Allergic potential/side effects
Which of these is ester and which are amides?.
Procaine, chloroprocaine, etidocaine, bupivacaine, cocaine, mepivacaine, lidocaine, etidocaine, tetracaine, ropivacaine, levobupivacaine
Esters:
Procaine, chloroprocaine, tetracaine, cocaine.
Amides:
Lidocaine, mepivacaine, prilocaine, bupivacaine, levobupivacaine, ropivacaine, etidocaine.
An LA that is more lipophilic, does it have a longer duration or shorter duration of effect?
Longer duration of effect
Does an area of higher vascularity have shorter or longer duration of effect?
Shorter
What does the addition of epinephrine provide for LA administration?
Can cause roughly a 25% increase in duration of action by constricting blood vessels.
It also decrease possible toxic side effects
Why would you see less toxic side effects with epinephrine additive in LA?
It is being released into the blood stream more slowly and steadily.
What is the only LA that provides vasoconstriction?
Cocaine
What should you do if a patient was allergic to a LA?
Ask them which LA was used and use a different type (ester vs amide)
Which two LAs are associated with increased allergic reaction related to PABA metabolite?
Procaine and benzocaine
What is the metabolite associated with allergic reaction?
PABA- p-aminobenzoic acid
How are amides metabolized?
Via liver P450 enzymes
How are esters metabolized?
Via pseudocholinesterases (found in blood)
What patient diagnosis could increase risk of toxicity of amide LA?
Liver dysfunction or failure
Metabolite of prilocaine and benzocaine convert hemoglobin to what?
Methemoglobin
How does methemoglobinemia do in the body?
It converts Fe3+ to Fe2+ which limits its ability to bind to O2.
What is the treatment for methemaglobinemia?
IV Methylene blue
What patient population has increased clinical significance of methemoglobinemia compared to a healthy patient?
Severely anemic.
Heart failure
What three things can be added to LAs during administration to increase safety, quality, intensity, and duration and rate of onset of anesthesia?
Opioids- pain management capabilities.
Sodium bicarbonate- for injecting into more acidic environment.
Epinephrine- longer duration, better receptor binding.
What is LAST?
Local Anesthetic Systemic Toxicity
What are the two main reasons for LAST?
- Inadvertant intravascular injection.
2. Administration of excessive dose
CV effects of LAST?
Initially with lesser toxic levels you will see HTN and tachycardia.
With higher toxic levels, bradycardia and eventuall asystole.
CNS effects of LAST
Initially patient talks incoherently or is confused. Higher toxicity can be seizures.
Why do all LAs depress myocardial automaticity?
Because they are Sodium Channel Blockers
Which LA has highest incidence of cardiovascular toxicity?
Bupivacaine
What is the main treatment LA toxicity?
Lipid Rescue.
What is the recommended dose/administration of Lipid Rescue?
Recommended 20% intralipid administration.
What is Transient Neurologic Symptoms (TNS)?
13% incidence with lidocaine.
It is not permanent (up to 10 days).
Characterized by tingling, burning, aching in lower extremities and buttocks.
Which has higher prevalence of allergic reactions? Esters or Amides?
Esters
What does EMLA stand for?
Eutectic Mixture of Local Anesthetics
How long does it take for satisfactory dermal analgesia to occur with EMLA?
Approx 1 hour
What is the onset of most injectable LAs?
Under 15mins
Overdosage of Cocaine is treated how?
Administration of both Beta Blocker and Alpha Blocker
What is the fastest type of all the nerve fibers?
A-Alpha
What is the second fastest of all the nerve fibers?
A-Beta
