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Newer Amide Local Anesthetics: Introduction
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The use of regional anesthesia has been increasing not only in
obstetrics, where it is the predominant anesthetic technique used, but also
during surgery and for acute postoperative pain management. This has been
partly due to the safety of regional anesthesia because of better injection
techniques and equipment, increased attention to detecting (preventing)
misplaced injection, greater vigilance/monitoring, and the introduction of
newer long-acting amide local anesthetics. The increasing demand for
regional anesthesia is nowhere more true than in obstetric anesthesia, where
local anesthetics have become the most frequently administered drugs for
obstetric pain relief or cesarean delivery. When injected epidurally or
intrathecally, local anesthetics provide effective labor analgesia that is
superior to that of systemic opioids and without the attendant risks of
maternal sedation and neonatal depression. Regional anesthesia is now the
most frequently used technique for cesarean section delivery in the
U.S.1
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Historical Perspective
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Why the need for new amide local anesthetics? The answer is partly
related to the history of bupivacaine use, particularly in North America and
Europe. After its introduction into clinical practice, bupivacaine quickly
became very popular for several reasons, particularly for use in obstetrics.
It has a longer duration of action than 2-chloroprocaine and lidocaine and
thus requires less frequent supplemental doses, a feature that is less
important now with the widespread use of continuous epidural infusion
techniques. More important and in contrast to other local anesthetics,
bupivacaine has a motor-sparing effect; it produces less motor block for a
comparable degree of sensory analgesia. This is particularly true at the low
concentrations used for labor epidural analgesia and acute postoperative
pain management. Furthermore, bupivacaine has excellent compatibility with
neuraxial opioids, and this allows for concentrations as low as 0.03% and
0.04% bupivacaine to be used successfully so that many patients are
pain-free and even able to ambulate during labor or with regional analgesia
after surgery. Less motor block also improves expulsive efforts during the
second stage of labor and may reduce the need for an instrumental vaginal or
abdominal delivery.2 The ability of bupivacaine to provide
good sensory analgesia with little motor block is essential for management
of postoperative, during which early mobilization may decrease the risk of
deep venous thrombosis and result in better respiratory mechanics.
Nonetheless, despite its many advantages, there have been some concerns
regarding bupivacaine, particularly in obstetric anesthesia.
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Clinical experience has been that cardiac arrest after unexpected
intravascular injection of clinical doses of local anesthetics could be
prevented by prompt oxygenation, ventilation, and, if necessary,
cardiovascular support. ...