Healthcare and Medicine Reference
In-Depth Information
agent will therefore be shorter. Vasopressors, such as epinephrine
and felypressin, are commercially added to some preparations to
prolong the duration of action. Because systemic absorption is
reduced, this may also increase the maximum safe dose of local
anaesthetic for a given patient (Table 4.1). Vasoconstrictors should
be avoided in the extremities, particularly the digits and the penis,
because of the risk of ischaemia.
a longer duration of action. The properties of the commonly used
agents are listed in Table 4.1.
Most amide local anaesthetics cause local vasodilatation. Cocaine
vasoconstricts, and is used in nasal surgery for analgesia and to
reduce blood loss.
In the United Kingdom, the most commonly used agents are
lidocaine, which has a relatively fast onset and brief duration of
action; and bupivacaine and its derivative levobupivacaine, which
have a slightly slower onset and longer duration.
Infected tissues are acidic, such that local anaesthetics will tend
to be ionised and cross nerve membranes more slowly, and are
therefore less effective.
Side-effects and treatment of toxicity
At high dose, all local anaesthetics cause central nervous system
(CNS) and cardiovascular effects. The CNS effects are initially excit-
atory, with depression occurring at higher plasma concentrations.
Initial effects include light-headedness or dizziness, and numb-
ness or tingling around the mouth. As the plasma concentration
rises, confusion, drowsiness and hypotension may ensue. With
severe toxicity, convulsions, coma, respiratory arrest and cardio-
vascular collapse may develop. It is important to remember that,
while toxicity is a spectrum, inadvertent intravenous administra-
tion can cause a patient to rapidly deterioriate to cardiorespiratory
arrest.
Treatment of local anaesthetic toxicity is largely supportive, along
an ABCDE format. Anticonvulsant drugs (benzodiazepines), and
urgent critical care assistance for airway and ventilatory support
may be required. Recently, lipid emulsions such as Intralipid® have
been advocated (seek specialist advice). These lipid emulsions are
of particular potential benefi t in bupivacaine toxicity resulting in
cardiac compromise.
Prilocaine may cause methaemoglobinaemia, which should
be considered for treatment with methylene blue. Cocaine may
occasionally cause coronary artery spasm and acute myocardial
ischaemia. Expert help should be sought immediately if either of
these rare complications are suspected.
Additives
Local anaesthetics are cleared from the site of action in the blood-
stream. In more vascular areas, the duration of action of a given
Interstitial
space
Na +
LA + HCl
Axonal
membrane
LAH +
Sodium channel
Cytoplasm
Cl -
LAH +
+
LA + HCl
Safe use of local anaesthetics
Naturally, a history of adverse reaction to local anaesthetic agents
should be sought.
Four things are crucial:
to have secure intravenous access
Figure 4.1 Local anaesthetics are weak bases and usually prepared as
hydrochlorides (LA
HCl). At the pH of the interstitial space (7.4) they
exist largely in this unionised form, which can cross the lipophilic axonal
membrane with ease. Once in the cytoplasm (pH around 7.1), equilibrium
shifts in favour of the ionised form (LAH + , and Cl - ). The ionised LAH +
blocks voltage-gated sodium channels from inside the cell, preventing the
transmission of an action potential and thus blocking the nerve.
+
1
2
to know the maximum safe dose of the agent you are using
Table 4.1 Properties of commonly used local anaesthetic agents.
Local anaesthetic
pKa
Onset
Protein binding (%)
Duration
Maximum dose (per kg ideal body weight)
Lidocaine
7.9
Rapid
64
Intermediate
4 mg/kg (7 mg/kg with epinephrine)
Bupivacaine
8.1
Intermediate
96
Long
2 mg/kg (3 mg/kg with epinephrine)
Prilocaine
7.9
Rapid
55
Intermediate
6 mg/kg (9 mg/kg with epinephrine/octapressin)
Ropivacaine: less cardiotoxic, slightly less
potent than bupivacaine
8.1
Intermediate
95
Long
3 mg/kg
Levobupivacaine
(s-enantiomer of bupivacaine): less
cardiotoxic, ? reduced motor block
8.1
Intermediate
97
Long
3 mg/kg
Cocaine (ester): causes vasoconstriction,
topical only (eyes/mucous membranes)
8.7
Slow
98
Long
3 mg/kg
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