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P A R T 4
Drugs acting on the central and peripheral nervous systems
Local anaesthetics are classified as esters or amides.
The agent of choice depends on the method of admini
stration, the length of time for which the area is to be
anaesthetised and consideration of potential adverse
effects. Esters include benzocaine (
Applicaine, Cepacol
)
and amethocaine (generic). Amides include bupiv-
acaine (
Marcain
), cinchocaine (
Proctosedyl, Rectinol,
Scheriproct
), levobupivacaine (
Chirocaine
), lignocaine
(
Xylocaine
), mepivacaine (
Scandonest
), prilocaine
(
Citanest
) and ropivacaine (
Naropin
).
Therapeutic actions and indications
Local anaesthetics work by causing a temporary inter-
ruption in the production and conduction of nerve
impulses. They affect the permeability of nerve mem-
branes to sodium ions, which normally infuse into the
cell in response to stimulation. By preventing the sodium
ions from entering the nerve, they stop the nerve from
depolarising. A particular section of the nerve cannot
be stimulated, and nerve impulses directed towards that
section are lost when they reach that area.
The way in which a local anaesthetic is adminis-
tered helps to increase its effectiveness by delivering it
directly to the area that is causing or will cause the pain,
thereby decreasing systemic absorption and related toxic
effects (Figure 27.2). Local anaesthetics are indicated for
infiltration anaesthesia, peripheral nerve block, spinal
anaesthesia and the relief of local pain.
Pharmacokinetics
The ester local anaesthetics are broken down imme-
diately in the plasma by enzymes known as
plasma
esterases
. The amide local anaesthetics are metabolised
more slowly in the liver, and serum levels of these drugs
(i.e. active metabolites) can increase and lead to toxicity.
Contraindications and cautions
The local anaesthetics are contraindicated with any of
the following conditions: history of allergy to any one
of these agents or to parabens
to avoid hypersensitivity
reactions
; heart block,
which could be greatly exacer-
bated with systemic absorption
; shock,
which could
alter the local delivery and absorption of these drugs
;
and decreased plasma esterases,
which could result in
toxic levels of the ester-type local anaesthetics.
They should be used during pregnancy and breast-
feeding only if the benefit outweighs any potential risk
to the fetus or neonate that could occur if the drug is
inadvertently absorbed systemically
because of the sup-
pressive effects on nerves.
Adverse effects
The adverse effects of these drugs may be related to their
local blocking of sensation (e.g. skin breakdown, self-
injury, biting oneself). Loss of skin integrity is always a
problem if the person is unable to move, and care must
Without local anaesthetic
To brain and
conscious perception
of pain
Pain impulses
in sensory nerve
Spinal cord
Wound
With local anaesthetic
No pain
perceived
Infiltration
anaesthesia
A
No pain
impulses
Spinal cord
Wound
No pain
perceived
Peripheral
nerve block
B
Wound
No
pain
impulses
Pain
impulses
FIGURE 27.2
Mechanism of action of local anaesthetics.
Top.
An injury produces pain impulses (action potentials) that are conducted and
transmitted in an area of the brain in which pain is perceived.
A.
Conduction of the pain impulse has been blocked by infiltration anaesthetics
at the site of the injury.
B.
A nerve block at some distance from the injury. Local anaesthetics block the movement of sodium into the nerve and
prevent nerve depolarisation, stopping the transmission of the pain impulse.
