Epidural Anesthesia
When flexibility is necessary (e.g., for a potentially prolonged
cesarean section), a catheter technique is optimal, and an epidural technique is
often chosen. In addition, women with an indwelling epidural catheter for labor
who require cesarean section usually receive the operative anesthetic through that
catheter. In high-risk parturients, epidurals are often placed early so that they
will be available for emergency cesarean section, thus illustrating the importance
of continual assessment of epidural catheters and their effectiveness during labor.
The ideal local anesthetic should provide rapid onset of sensory
block with an appropriate duration of action. Commonly used agents include 2-chloroprocaine,
lidocaine, and bupivacaine. When compared with spinal anesthesia, very large doses
of local anesthetic are used to achieve adequate levels for cesarean section. Epidural
catheters can migrate, and therefore even negative aspiration of the catheter does
not absolutely rule out intrathecal or intravascular placement. Because large volumes
of potentially toxic local anesthetic are administered via an epidural for cesarean
section, several measures can be used
to reduce the risk of local anesthetic toxicity. First, the catheter should be aspirated
before use and an appropriate test dose administered. Second, the anesthetic should
be administered in fractionated doses. Last, safer drugs (such as chloroprocaine
and lidocaine) or the newer amide local anesthetics (such as ropivacaine and levobupivacaine)
should be considered.
As with spinal anesthesia, adjuncts can be used to improve the
quality of the block. Because local anesthetics are all weak bases and are commercially
prepared in an acidic solution, they are ionized and do not readily cross lipid membranes.
The addition of small quantities of bicarbonate increases the pH of the solution
and the proportion of un-ionized local anesthetic; this practice shortens the onset
time.[178]
The addition of bicarbonate to lidocaine
and chloroprocaine clearly shortens their onset; however, not all studies have demonstrated
this effect with bupivacaine.[179]
In addition,
bupivacaine may precipitate when alkalinized, so much smaller doses (0.1 mEq/20 mL
local anesthetic) should be used.
Commercially prepared solutions of local anesthetics with epinephrine
have a lower pH, so they may have a slower onset time. The addition of epinephrine
to the local anesthetic immediately before injection may decrease the time of onset
of the block for lidocaine,[180]
but this effect
does not seem to occur with bupivacaine.[181]
Intraoperative conditions under epidural anesthesia may be improved
if fentanyl (50 to 100 µg)[182]
or sufentanil
(10 to 20 µg)[183]
is added to the local anesthetic.
Clonidine has also been used as an additive to epidural local
anesthetics, but it has been associated with sedation, bradycardia, and hypotension.
[184]
