Epidural nerve blockade

Central neural blockade. The anesthetic is injected within the spaces surrounding the spinal cord10 (Fig. 12-2). Specifically, the term epidural nerve blockade refers to injection of the drug into the epidural space—that is, the space between the bony vertebral column and the dura mater. A variation of epidural administration known as a caudal block is sometimes performed by injecting the local anesthetic into the lumbar epidural space via the sacral hiatus (see Fig. 12-2). Spinal nerve blockade refers to injection within the subarachnoid space— that is, the space between the arachnoid membrane and the pia mater. Spinal blockade is also referred to as intrathecal anesthesia because the drug is injected within the tissue sheaths surrounding the spinal cord (intrathecal means within a sheath see Chapter 2). 

Amide-type agents include articaine, lidocaine, bupivacaine, prilocaine, mepivacain and ropiva-caine. These are metabolized in the liver by microsomal enzymes with amidase activity. The amide group is preferred for parenteral and local use. If by accident rapidly administered intravascularly these agents, especially bupivacaine but also lidocaine, can produce serious and potentially lethal adverse effects including convulsions and cardiac arrest. They can more easily accumulate after multiple administrations. Intravenous lidocaine is sometimes used for regional anesthesia, for infiltration procedures, for the induction of nerve blockade and for epidural anesthesia. However, it is also used as an antiarrhythmic. Bupivacaine is a long-acting local anesthetic used for peripheral nerve blocks and epidural anesthesia. 

For continuous epidural analgesia, levobupivacaine may be administered in combination with fentanyl or clonidine. Levobupivacaine has been approved in European Union states for ilio-inguinal and iliohypogastric nerve blockade in children. (See also bupivacaine, above.)... 

The effects on muscle relaxation are dose-dependent. When administered as an epidural or to produce peripheral nerve blockade, use of low-dose bupivacaine will produce partial relaxation. Progressively higher doses produce more effect, and high doses (e.g.. use of a 75 percent solution) will produce complete muscle relaxation. 

Ropivacaine follows linear pharmacokinetics and the maximum plasma concentration is proportional to the dose. In general, the systemic concentration is dependent on multiple factors, including the route of administration, vascularity of the administered site, total dose and concentration given, and the patient s medical condition. Ropivacaine is primarily used for epidural and peripheral nerve blockade. From the epidural space, ropivacaine shows complete and biphasic absorption, half-life of the two phases being 14 7 minutes and 4.2 . 9 h. 

However, these results contradict long-accepted in vitro studies on mammalian peripheral nerve fiber axon diameter. These studies suggest that larger myelinated A fibers which conduct with higher velocity may be more susceptible to peripheral nerve blockade with local anesthetics than smaller unmyehnated C fibers which conduct more slowly [12,13]. Jaffe and Rowe in 1996 [14] actually found no difference in sensitivity to lidocaine based on axon diameter. Many mechanisms in addition to axon diameter have been proposed for lidocaine s differential neural blockade, such as differences in Na channel block (state-de-pendent vs. frequency-dependent) between different nerve fiber types. Further research is needed to elucidate the underlying mechanism as well as the discrepancy between the differential blockade witnessed in epidural and peripheral nerve blocks. 

Local anesthetics are used for topical anesthesia, local infiltration, peripheral nerve block, paravertebral anesthesia, intravenous block also known as regional anesthesia, epidural block, and spinal i.e. subarachnoid blockade. The local anesthetics may be divided into two main groups, the esters and the amide-type agents. 

Regional block, a form of anesthesia that includes spinal and epidural anesthesia, involves injection near a nerve or nerve plexus proximal to the surgical site. It provides excellent anesthesia for a variety of procedures. Brachial plexus block is commonly used for the upper extremity. Individual blocks of the sciatic, femoral, and obturator nerves can be used for the lower extremity. An amount that is close to the maximally tolerated dose is required to produce blockade of a major extremity. 

Local anaesthetics can be applied topically, deposited around peripheral nerves, or infiltrated into tissues. Central neural blockade can be produced by injection into the subarachnoid or epidural spaces. Less common uses are for intravenous regional anaesthesia and attenuation of cardiovascular responses to tracheal intubation. The membrane-stabilising effect of local anaesthetics has been utilised in the treatment of myocardial arrhythmias. 

Schematic diagram of the typical sites of injection of local anesthetics in and around the spinal canal. When local anesthetics are injected extradurally, it is known as epidural (or caudal) blockade. Injections around peripheral nerves are known as perineural blocks (eg, paravertebral block). Finally, injection into the subarachnoid space (ie, cerebrospinal fluid), is known as spinal blockade.

The choice of local anesthetic for infiltration, peripheral nerve blocks, and central neuraxis (spinal/epidural) blockade is usually based on the duration of action required. Procaine and chloroprocaine are short-acting lidocaine, mepivacaine, and prilocaine have an intermediate duration of action and tetracaine, bupivacaine, levobupivacaine, and ropivacaine are long-... 

Etidocaine is a long-acting derivative of lidocaine but is far more potent. It is effective for infiltration anesthesia, peripheral nerve block, and epidural and caudal blockade. 

Local anesthetics are used in the clinical situation to interrupt the nociceptive process at one or more points between the peripheral, high-threshold nociceptor and the cerebral cortex, by blocking transduction by infiltration at the site of injury or incision by preventing transmission in afferent myelinated A 8 and unmyelinated C fibers by blockade of peripheral nerves or nerve plexuses or by epidural injection. 

The concentration of local anesthetic used determines the type of nerve fibers blocked The highest concentrations are used when sympathetic, somatic sensory, and somatic motor blockade are required. Intermediate concentrations allow somatic sensory anesthesia without muscle relaxation. Low concentrations will block only preganglionic sympathetic fibers. A significant difference between epidural and spinal anesthesia is that the dose of local anesthetic used can produce high concentrations in blood following absorption from the epidural space. 

Surgical acute pain bupivacaine is indicated as a local anesthetic and used for infiltration, peripheral nerve block, nerve plexus blockade, epidural, and intrathecal (spinal) anesthesia and analgesia. It can be used as a single-shot injection or administered through a catheter for prolonged anesthesia and/or analgesia. 

Lidocaine s differential blockade has been examined with peripheral and neuraxial administration [ 10-11 ]. Peripheral nerve libers seem to respond differently to lidocaine than dorsal nerve roots. Epidural lidocaine seems to block small unmyelinated C fibers (supplying temperature sensation) more effectively than 281 larger myelinated A fibers (A-beta supplying touch. 

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