Selection of the Anesthetic Solution
Sound selection of a local anesthetic depends on the planned surgery,
the expected duration of postoperative pain, and the pharmacologic characteristics
of the drugs.
TABLE 45-5 -- Pediatric doses and clinical characteristics of commonly used local anesthetics
|
Local Anesthetic
*
|
Usual Concentration (%) |
Usual Doses (mg/kg) |
Maximum Dose, Plain
†
(mg/kg) |
Maximum Dose with Epinephrine
†
(mg/kg) |
Latency (min) |
Duration of Effects (hr) |
|
Aminoesters |
|
|
|
|
|
|
|
Procaine |
1–2 |
7 |
10 |
10 |
10–15 |
0.3–1 |
|
Chloroprocaine |
2–3 |
7 |
10 |
10 |
7–15 |
0.5–1 |
|
Aminoamides |
|
|
|
|
|
|
|
Lidocaine |
0.5–2 |
5 |
7.5 |
10 |
5–15 |
0.75–2 |
|
Prilocaine
‡
|
0.5–1.5 |
5 |
7 |
10 |
15–25 |
0.75–2 |
|
Mepivacaine |
0.5–1.5 |
5–7 |
8 |
10 |
5–15 |
1–1.25 |
|
Bupivacaine |
0.25–0.5 |
2 |
2.5 |
3 |
15–30 |
2.5–6 |
|
Levobupivacaine |
0.25–0.5 |
3 |
4 |
4 |
15–30 |
2.5–6 |
|
Ropivacaine |
0.2–10 |
3 |
3.5 |
NA |
7–20 |
2.5–5 |
*These
data are not applicable to spinal anesthesia, intravenous regional anesthesia, and
local anesthesia.
†Maximum
doses are controversial and irrelevant because toxicity is caused by the free, unbound
form and not the total dose given or the plasma peak concentration. The doses mentioned
are safe when given as single injections in children older than 6 months, whereas
they may or may not be safe after multiple previous injections (or continuous infusion),
especially during the postoperative course; this is particularly true for long-lasting
local anesthetics (i.e., bupivacaine, levobupivacaine, and ropivacaine).
‡There
is danger of severe methemoglobinemia in infants, even with low therapeutic doses.
The most suitable local anesthetics and their recommended doses are detailed in Table
45-5
. Levobupivacaine has become available, and pediatric data are still
scarce, but being a pure (S)-enantiomer of bupivacaine, its indications should be
the same as those of bupivacaine but with less danger of systemic toxicity. Ropivacaine
has elicited a number of pediatric publications, and this agent has many advantages
over racemic bupivacaine, especially in a 0.2% concentration for continuous techniques
(for single-injection techniques, standard bupivacaine remains an interesting agent,
despite its greater potential of cardiac toxicity).[13]
[16]
[17]
[18]
[19]
[20]
[43]
[44]
[45]
[46]
[47]
The use of mixtures of local anesthetics has
gained acceptance for single-shot procedures but remains controversial.[113]
Expected advantages of such combinations include compensation against relative limitations
of each agent (especially in regard to latency until completion of sensory block)
and reduction of the hazards of systemic toxicity by producing low and separated
peak blood concentrations of each drug. However, it should be kept in mind that
the toxicity of mixtures of local anesthetics is additive.[114]
Warming the anesthetic solution before injection reduces pain on infiltration.[115]
Addition of epinephrine to the local anesthetic solution is mandatory
for pharmacokinetic reasons, such as reduction of vascular uptake, increased duration
of effects, and early detection of an intravascular injection. However, epinephrine
can produce ischemic disorders and gangrene when injected in areas supplied by terminal
arteries. Its use is contraindicated for penile, peribulbar, pudendal, and digital
nerve blocks. Its addition is questionable for local anesthetics with strong intrinsic
vasoconstrictive properties such as ropivacaine. Opioids, especially morphine, provide
prolonged pain relief when administered along the neuraxis, but their complication
rates are high and require appropriate monitoring of respiratory parameters for the
24 hours postoperatively. This addition should be restricted to major surgical
procedures in hospitalized patients only. Clonidine is a valuable additive to neuraxial
and peripheral nerve blocks, and ketamine, still under evaluation, has probably a
brilliant future because it seems to provide a longer and better analgesia when added
to local anesthetics. Commonly used additives and their pediatric doses are listed
in Table 45-3
.
