Central Neural Blockade (also
see Chapter 43
)
Any of the local anesthetics may be used for epidural anesthesia
( Table 14-7
), although procaine
and tetracaine are rarely used because of their long onset times. Drugs with intermediate
potency produce surgical anesthesia of 1 to 2 hours' duration, whereas the long-acting
drugs usually produce 3 to 4 hours of anesthesia. The duration of short- and intermediate-acting
drugs is significantly
TABLE 14-8 -- Spinal anesthesia
*
|
Drug |
Usual Concentration (%) |
Usual Volume (mL) |
Total Dose (mg) |
Baricity |
Glucose Concentration (%) |
Usual Duration (min) |
|
Procaine |
10.0 |
1–2 |
100–200 |
Hyperbaric |
5.0 |
30–60 |
|
Lidocaine |
1.5, 5.0 |
1–2 |
30–100 |
Hyperbaric |
7.5 |
30–90 |
|
Mepivacaine |
4 |
1–2 |
40–80 |
Hyperbaric |
9.0 |
30–90 |
|
Tetracaine |
0.25–1.0 |
1–4 |
5–20 |
Hyperbaric |
5.0 |
75–150 |
|
0.25 |
2–6 |
5–20 |
Hypobaric |
0 |
75–150 |
|
1.0 |
1–2 |
5–20 |
Isobaric |
0 |
75–150 |
|
Dibucaine |
0.25 |
1–2 |
2.5–5.0 |
Hyperbaric |
5.0 |
75–180 |
|
0.5 |
1–2 |
5–10 |
Hypobaric |
0 |
75–180 |
|
0.06 |
5–20 |
3–12 |
Isobaric |
0 |
75–180 |
|
Bupivacaine |
0.5 |
3–4 |
15–20 |
Isobaric |
0 |
75–150 |
|
0.75 |
2–3 |
15–22.5 |
Hyperbaric |
8.25 |
75–150 |
|
Doses listed refer to 70-kg adults. Doses should be modified
as detailed in Chapter 44
and Chapter 45
for children,
during pregnancy, and for elderly subjects. |
*Also see Chapter
43
.
prolonged by the addition of epinephrine (1:200,000), but the long-acting drugs benefit
little from its addition. The onset of lumbar epidural anesthesia occurs within
5 to 15 minutes after the administration of chloroprocaine, lidocaine, mepivacaine,
and prilocaine. Bupivacaine has a slower onset of action.
Bupivacaine at 0.125% produces adequate analgesia with only mild
motor deficits. Such solutions of bupivacaine are useful for labor epidural analgesia
and postoperative analgesia. Bupivacaine at 0.25% may be used for more intense analgesia
(particularly during combined epidural-light general anesthesia) with moderate degrees
of motor blockade. At concentrations of 0.5% to 0.75%, bupivacaine is associated
with a more profound degree of motor block, which makes these solutions most suitable
for major surgical procedures, particularly when epidural anesthesia is not combined
with general anesthesia. Etidocaine produces adequate sensory analgesia and profound,
long-lasting motor block and is primarily useful for surgical procedures in which
muscle relaxation is required.
Drugs that can be used for subarachnoid administration are shown
in Table 14-8
. Tetracaine
is available both as crystals and as a 1% solution, which may be diluted with 10%
glucose to obtain a 0.5% hyperbaric solution.
Hypobaric solutions of tetracaine (tetracaine in sterile water)
may be used for specific operative situations, such as anorectal or hip surgery.
Isobaric tetracaine, obtained by mixing 1% tetracaine with cerebrospinal fluid or
normal saline, is useful for lower limb surgical procedures. Bupivacaine is widely
used as a spinal anesthetic, either as a hyperbaric solution at a concentration of
0.75% with 8.25% dextrose or by using the nearly isobaric 0.5% solution.
Intrathecal bupivacaine has an anesthetic profile similar to that
of tetracaine.[82]
[83]
However, differences do exist between the two drugs. Although two-segment regression
of anesthesia is similar for bupivacaine and tetracaine, the total duration of sensory
anesthesia is significantly longer after the subarachnoid administration of tetracaine.
The depth and duration of motor blockade are also greater with tetracaine than with
bupivacaine. On the other hand, bupivacaine has been reported in some studies to
be associated with less hypotension. In addition, the frequency of tourniquet pain
in the lower limbs during certain orthopedic surgical procedures has been reported
to be significantly reduced when bupivacaine instead of tetracaine is used for spinal
anesthesia.[84]
[85]
Whereas tetracaine and bupivacaine are considered to be drugs
of long duration, lidocaine provides a short duration of spinal anesthesia. The
onset of spinal anesthesia is extremely rapid with a drug such as lidocaine. The
addition of vasoconstrictors may prolong the duration of spinal anesthesia; for example,
the addition of 0.2 to 0.3 mg of epinephrine to tetracaine solutions will produce
a 50% or greater increase in duration. The duration of spinal anesthesia produced
by tetracaine can also be increased to a similar extent by adding 1 to 5 mg of phenylephrine.
The addition of epinephrine to bupivacaine or lidocaine may not significantly prolong
the duration of spinal anesthesia in thoracic segments,[86]
[87]
although the total duration of anesthesia (e.g.,
in the lumbosacral roots) will be significantly increased. Even though lidocaine
has long been used for spinal anesthesia as a 5% solution, recent studies of local
anesthetic neurotoxicity have led some to question this practice; this issue is discussed
later in the chapter in the section on neurotoxicity of local anesthetics.
 |
