Central Neuraxial Nerve Blocks
Spinal and Epidural Anesthesia: Spinal and epidural blocks are also referred as conduction anesthesia or regional anesthesia. As opposed to general anesthesia, these blocks provide surgical anesthesia at the selected site. Spinal and epidural blocks provide excellent anesthesia for procedures of the lower extremities as well as low abdominal and pelvic surgery. Spinal and epidural blocks are also used to provide obstetrical and post-operative analgesia. Mechanism of action Spinal anesthesia is produced by the introduction of local anesthetics into the subarachnoid space. Motor, sensory and sympathetic innervation are interrupted by spinal blockade. This is seen as a “differential” blockade because of the anatomy of the spinal nerves. The unmyelinated small diameter nerve fibers are close to the surface and are the first to be blocked whereas as the large- diameter myelinated fibers are located deep in the nerve. Thus the sympathetic blockade is usually seen first. The level of sympathetic blockade is several dermatomal levels higher than the sensory blockade, whereas the level of motor blockade is several dermatomal levels lower than the sensory blockade. Epidural anesthesia is produced by injection of local anesthetics into the epidural space. The local anesthetics are thought to block the spinal nerve roots as well as the spinal cord by diffusion into the subarachnoid space.
Level and Duration of Action:
Baricity of the local anesthetic solution, volume of solution injected, contour of the spinal canal and patient position all contribute to the level of spinal blockade. Duration of action is dependent on the local anesthetic chosen and presence or absence of vasoconstrictors. The level of epidural blockade is dependent on the volume of local anesthetic injected. The duration of action is dependent on the local anesthetic chosen. The duration of epidural blockade however can be extended by the placement of an indwelling catheter at the time the epidural space is identified. The catheter allows for the administration of more local anesthetic as needed.
Spinal blockade is usually performed with the patient in either a sitting or lateral decubitus position. The lumbar region is sterilely prepped and draped. A skin wheal is made with local anesthetic over the selected lumbar interspace. A spinal needle is then introduced in the midline of the lumbar interspace. Spinal needles are available in a wide variety of sizes but 22-gauge, 24- gauge and 25-gauge needles are commonly used. As the needle is advanced it passes through the supraspinous ligament, the interspinous ligament, the ligamentum flavum and finally the dura mater. As the needle passes through the dura mater a characteristic “pop” is felt. Confirmation that the needle tip is in the subarachnoid space is made by the appearance of clear cerebrospinal fluid (CSF) at the hub of the needle. A syringe with local anesthetic is then attached to the spinal needle and the local anesthetic injected. The technique of identifying the epidural space is similar, but more technically difficult. The epidural space is a potential space which is located between the ligamentum flavum and the dura mater. An 18-gauge needle is introduced in the midline of a selected interspace and advanced until the tip is lodged in the ligamentum flavum. The stylette is then removed and a needle filled with air or saline is attached to the hub. Constant pressure is then exerted on the plunger of the syringe as the needle is advanced. When the epidural space is entered, a characteristic “loss of resistance” is felt at the syringe. At this point a small gauge catheter may be placed to allow for repeat injections of local anesthetic if the surgical procedure is to be prolonged. Placement of an epidural catheter may also be done to provide postoperative analgesia. See Appendix VI.
Patient refusal, localized infection at site of insertion, coagulopathy, hypovolemia, bacteremia and pre-existing neurological disorders are all contraindications to regional anesthesia.
Hypotension, postdural puncture headache, high spinal anesthesia (causing bradycardia, dyspnea, apnea and unconsciousness), urinary retention , infection, systemic local anesthetic toxicity by direct intravascular injection and rarely neurologic impairment are all associated with spinal blockade. Complications of epidural blockade are similar. Postdural puncture headache is postural in nature. It typically appears 24 hours after a spinal anesthetic. The headache is aggravated by the upright position and relieved in the supine position. The overall incidence is between 5-10% and greatest in young female patients. The incidence increases with the size of the puncture and the number of punctures in the dura. The headache can be managed conservatively with analgesics, fluids and bed rest. However if the headache persists, an epidural blood patch may be useful.
Peripheral Nerve Blocks
Peripheral nerve blocks are used to provide anesthesia, postoperative analgesia and to diagnose and treat chronic pain syndromes.
Cervical plexus block can be used to provide anesthesia for patients undergoing carotid endarterectomy or hemithyroidectomy. The transverse processes of C2-4 are identified and local anesthetic injected. Complications of this block include local anesthetic toxicity because of direct intravascular injection and hoarseness due to blockade of the recurrent laryngeal nerve. A Horner’s syndrome (ptosis, miosis, enophthalmos and anhydrosis) may also be seen.
Brachial plexus blockade provides excellent anesthesia for surgery of the upper extremity. The plexus can be blocked via an interscalene approach, a supra- and infra-clavicular approaches as well as an axillary approach. The site of the surgery determines the approach. Distal nerve blocks of the upper extremity include median, ulnar and radial nerve blocks. Anesthesia of the upper extremity can also be provided by an intravenous regional technique or Bier block. The extremity is completely exsanguinated and the blood volume replaced by a dilute solution of local anesthetic. The local anesthetic is kept in place by the use of a tourniquet. Blockade occurs immediately and lasts until the tourniquet is released.
Lower extremity nerve blocks include sciatic nerve block, femoral nerve block, lateral femoral cutaneous nerve block and obturator nerve block. The blocks can be combined to provide complete anesthesia of the lower extremity.
Celiac plexus block is used to provide pain relief from abdominal carcinomas usually pancreatic carcinoma. Stellate ganglion blockade is used to treat reflex sympathetic dystrophy (RSD) of the upper extremity. Lumbar sympathetic blockade is used also used in the diagnosis and treatment of RSD.