ANESTHETICS
By
Dr. Faraza Javaid
Local Anesthetics
Drugs used to provide local anesthesia are also used
to achieve regional anesthesia. Regional anesthesia
is an injection of a local anesthetics around nerves
so that the area supplied by these nerves will not
send pain signals to the brain. They anesthetized
area is usually larger than the area affected by local
anesthesia.
Mechanism of action
▶
Mechanisms:
Nonionized form crosses axonal membrane
- From within, ionized form blocks the inactivated Na+
-
channel
Slows recovery and prevents propagation of action potentials
When local anesthetics are applied to the area that they numb,
they need to get to the axon by crossing to the membrane. Only
non ionized forms R-NH2 can enter the axon through the
membrane. Upon entering the axon, only the ionized forms can
block the sodium channels which causes depolarization of the
sodium channel consequently numbing the area.
Ionized form RNH3+
Classification of Local Anesthetics
based on chemical group
▶-
Esters:
Procaine, Cocaine, Benzocaine are
metabolized by plasma and tissue esterases
▶-
Amides:
Lidocaine, Bupivacaine, Mepivacaine
are metabolized by liver amides
May also be classified into
a.
b.
c.
Short acting – Cocaine, Procaine
Intermediate acting – Lidocaine,
Mepivacaine, Dibucaine, Prilocaine
Long acting – Tetracaine,
Bupivacaine, Etidocaine
Metabolism
▶Esters
are metabolized by the plasma
by enzymes CHOLINEesterase
▶Amides are metabolized by liver by
the enzyme hepatic amidases
ALLERGIC REACTIONS
Patient reports of allergic reactions to local anesthetics are
fairly common
True allergy to an amide local anesthetic is exceedingly rare,
while the ester procaine is more allergenic and has largely
been removed from the market.
Allergy to one ester rules out use of another ester, because
the
allergenic
component
is
the
metabolite
paraaminobenzoic acid, produced by all esters.
A patient may be allergic to other compounds in the local
anesthetic, such as preservatives in multidose vials.
Local anesthetic systemic toxicity
(LAST)
The signs, symptoms, and timing of local anesthetic
systemic toxicity (LAST) are unpredictable.
One must consider the diagnosis in any patient with
altered mental status, seizures, or cardiovascular
instability following injection of local anesthetic.
Treatment for LAST may include seizure suppression,
airway management, and cardiopulmonary support.
Clinical note
Don’t give amides to patients with poor
liver function. Eg. patient with any form of
hepatitis! WHY?
The amides will pile up due inability of the
liver to metabolize. Hence elicit a
complete history in your patient. In cases
where patients have liver problem; your
local anesthesia should be an ester.
▶
How do we know which cocaine
derivative is an amides or ester?
▶The
trick:
the letter i
▶ in almost all cases if an i precedes the
“caine” sound, the drug is an amide but if not,
it is an ester.
Amides
Esters
▶ Articaine
▶ Butacaine
▶ Bupivacaine
▶ Cocaine
▶ Dibucaine
▶ Ethyl
▶ Etidocaine
▶ Lidocaine
▶ Mepivacaine
▶ Prilocaine
▶Roppivacaine
aminobenzoate (benzocaine)
▶ Hexylcaine
▶ Tertracaine
▶ Cloroprocaine
▶ Procaine
▶ propoxycaine
Limiting local anesthetics
▶ All
local anesthetics should be co-administered
with alpha-1 agonist. E.g. phenylephrine,
metoximine. WHY?
Reason;
The alpha-1 agonist will cause a powerful
vasoconstriction and limit the access of
surrounding tissues to the local anesthetics.
The
Cocaine saga
▶I
am the only local anesthetics that does
not need an alpha-1 agonist.
WHY?
▶ANS: I am a NEP re-uptake blocker.
NOTE:
it causes NEP build up in the synapsis. From the synapsis NEP
binds to the alpha-1 receptors and causes vasoconstriction in the
surrounding tissues. Therefore you don’t need an alpha-1 agonist after
administration of cocaine.
Anesthetic Adjuncts
Adjuncts are a critical part of the practice of
anesthesia and include drugs that affect
gastrointestinal (GI) motility, PONV, anxiety, and
analgesia. Adjuncts are used in collaboration to
help make the anesthetic experience safe and
pleasant.
Thank you
Any question?