Small Animal Clinical Pharmacology - CYF MEDICAL DISTRIBUTION

CHAPTER 5 ANESTHETIC AGENTS
Table 5.3 Uses of local anesthetics
Drug Topical Infiltration Peripheral nerve block Intravenous regional analgesia Epidural
Procaine + +
Proxymetacaine +
Lidocaine + + + + +
Prilocaine +
Mepivacaine + + +
Bupivacaine + + +
Ropivacaine + + +
doses may be lower in cats and doses should not exceed
4 mg/kg lidocaine, 2.5 mg/kg mepivacaine or 1.5 mg/kg
bupivacaine. Toxicity is additive and if combinations of
local anesthetics are used the dose of individual drugs
should be reduced accordingly.
A
Aromatic group
O
C
O
Amine side-chain
Infiltrative block
● 0.5–2% lidocaine or 0.125–0.5% bupivacaine or
1.0–2.0% mepivacaine
● Solutions containing adrenaline (epinephrine)
should not be used to desensitize extremities
● Dilution with 0.9% sodium chloride is
recommended to minimize the overall dose
Peripheral nerve blocks
● 0.5–2.0% lidocaine or 0.125–0.5% bupivacaine or
1.0–2.0% mepivacaine
● 0.25–2.0 mL per site
Intra-articular analgesia
● Up to 1 mL/4.5 kg of 2% lidocaine or 0.5%
bupivacaine
● Mepivacaine may cause less tissue irritation and
has also been recommended for intra-articular use
Interpleural blockade
● 1–2 mg/kg 0.25% bupivacaine in dogs (0.5 mg/kg
in cats)
Intravenous regional analgesia in dogs
(Bier block)
● 2.5–5 mg/kg lidocaine without adrenaline
(epinephrine) is injected into a superficial vein
distal to a tourniquet
● The tourniquet must be removed within 60–90 min
● Bupivacaine is more cardiotoxic than lidocaine and
should not be used
Lumbosacral epidural
● In dogs 1 mL/4.5 kg of 2% lidocaine or 0.5%
bupivacaine
● In cats 1 mL/4.5 kg of 2% lidocaine or 1 mL/7 kg
of 0.5% bupivacaine
B
Aromatic group
NH
● The total volume of drug injected epidurally should
not exceed 6 mL in dogs or 1.5 mL in cats
Pharmacokinetics
O
C
Amine side-chain
Fig. 5.5 Basic chemical structure of (A) an ester-linked
and (B) an amide-linked local anesthetic.
Most local anesthetics share a common chemical structure
comprising a lipophilic aromatic ring linked to a
hydrophilic amine side chain by an ester or amide bond
(see Fig. 5.5). The amphiphilic nature of the molecule
is important in conferring both lipid- and water-soluble
characteristics. The linkage also has an impact on the
biotransformation of the drug and local anesthetics can
be classified as being ester or amide linked.
Most local anesthetics are weak bases and are largely
ionized at physiological pH. Only unionized drug is
sufficiently lipid soluble to diffuse through the axon
membrane to reach the binding site within the ion
channel. Therefore a drug that is less ionized at physiological
pH will have a faster onset than a highly ionized
drug (see Table 5.4). This explains the slow onset time
of bupivacaine (20–30 min) compared to lidocaine (10–
15 min). Once the drug gains access to the channel it is
the ionized form that binds most avidly to the receptor.
The degree of ionization can be influenced by the pH of
the tissues. Inflammation tends to lower pH and this can
increase ionization sufficiently to interfere with drug
activity.
The lipid solubility of a local anesthetic is correlated
to potency: the more lipid soluble the agent, the greater
the potency. Protein binding is variable (see Table 5.4).
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