Neuraxial Anesthesia

Neuraxial anesthesia including spinal, epidural, and caudal anesthesia are commonly referred to as conduction or regional anesthesia. Spinal anesthesia is produced by injection of local anesthetic solutions into the lumbar subarachnoid space. Epidural anesthesia results when local anesthetic solutions are placed into the epidural space, often at the lumbar and thoracic levels. Caudal anesthesia is produced by injection of local anesthetics into the epidural space by introducing a needle through the sacral hiatus. Each neuraxial technique may be performed either as single injection or as a continuous catheter technique. Caudal anesthesia is more commonly performed in the pediatric population. Preoperative preparation does not differ from that for general anesthesia. Efficacy of duration, level obtained, and intensity of neuraxial block may be enhanced with the addition of adjuncts (e.g., epinephrine, opioids) to the local anesthetic solutions and by patient position immediately subsequent to local anesthetic injection.

The use of thromboprophylaxis in the perioperative period may conflict with neuraxial techniques in which maintaining hemostasis integrity is essential. A host of anticoagulants have been approved, both oral and parenteral, for use in a wide range of disorders. Neuraxial anesthesia, often used for orthopedic surgery (both for anesthesia and analgesia), should be performed with an optimal coagulation status. When thromboprophylaxis is needed, it remains necessary to keep in mind the pharmacology of the drug(s) used and to follow time interval recommendations in order to minimize the risk of related bleeding complications (Table 20.5).

Mechanism of Action

Neuraxial anesthesia results in selective anesthesia for the surgical site. Local anesthetics injected into the proper space will result in sodium channel blockade, with nerve transmission interruption along nerve roots. Blockade of neural transmission in the posterior nerve root fibers interrupts somatic and visceral sensation, whereas blockade of anterior nerve root fibers prevents efferent motor and autonomic outflow. The level and onset of neuraxial anesthesia obtained should be documented accordingly: (1) sympathetic nervous system is usually blocked first and tested by ability to discriminate temperature change and observation of hemodynamic effects, (2) level of sensory anesthesia is evaluated by ability to discriminate for sharpness, and (3) skeletal muscle strength is evaluated by testing the reduction in foot and knee movement power.

Sensory and Motor Blockade

Local anesthetics acting on the nerve roots block painful stimuli and produce profound skeletal muscle relaxation to the dermatome level achieved.

Autonomic Blockade

Interruption of efferent autonomic transmission at the spinal nerve roots can produce sympathetic and some parasympathetic blockade.

Indications

As primary anesthesia and/or postoperative pain relief (analgesia).

Clinical Uses Surgical

Lower abdominal and pelvic surgery, including inguinal, urogenital, and rectal procedures. Lower extremity, caesarean section, and lumbar spinal surgery may also be performed under spinal anesthesia. Upper abdominal (e.g., cholecystectomy) and lower thoracic procedures may be performed with a spinal or an epidural, although it may be difficult to achieve a sensory level adequate for patient comfort while avoiding the complications of a high block.

Supplemental and/or Postoperative Analgesia

All of the surgeries indicated above along with selected thoracic and cardiac procedures.

Technique Preparation

Mask and cap, sterile gloves, gauze pads, chloroprep, prefabricated spinal or epidural kit.

Needles

Typically a 17 g Tuohy for epidural/caudal and 22-27 g pencil point for single shot spinal.

Agents

Chloroprocaine, lidocaine, ropivacaine, bupivacaine, tetracaine.

Surface Anatomy and Landmarks Spinal Anesthesia

Spinal anesthesia can be performed with the patient in a sitting or a lateral decubitus position. Principal landmarks include the spinous processes (midline) and the iliac crests (Fig. 20.1). A line connecting the highest point on each of iliac crests corresponds to the L4 vertebrae (midline). Interspace above the line represents L3-L4 and below the line indicates L4-L5. Since the spinal cord ends at L1-L2 in adults, interspaces of L3-L4 or L4-L5 are most commonly selected as the needle insertion site. The interspaces (palpable depression) between the spinous processes above and below the level to be used are palpated; the skin is then

Figure 20.1 Spinal and epidural anesthesia/analgesia. Dashed lines in the midline indicate the palpable spinous processes. Right and left lateral arcs represent the iliac crests. Line #1 is the L4 lumbar interspace.

Figure 20.1 Spinal and epidural anesthesia/analgesia. Dashed lines in the midline indicate the palpable spinous processes. Right and left lateral arcs represent the iliac crests. Line #1 is the L4 lumbar interspace.

t prepped in sterile fashion and anesthetized by infiltration at the selected site. A spinal needle is introduced in the midline, remaining perpendicular in all directions to the patients, back (parallel to the spinous processes). The following structures are encountered in route to the subarachnoid space as the needle is deliberately and slowly inserted - skin, subcutaneous tissue, supraspinous ligament, interspinous ligament, epidural space, dura, then arachnoid mater, and finally the subarachnoid space, confirmed by appearance of CSF. Needle is stabilized on the patient's back and then connected to syringe containing local anesthetic; after confirming free flow of CSF (no blood), the syringe contents are injected.

Complications

If blood-tinged CSF continues to flow through the spinal needle, it should be removed and inserted at a different interspace. Additional issues include hypotension, bradycardia, postspinal headache, high spinal, nausea, urinary retention, backache, hypoventilation, and adverse neurologic consequences (rare).

Epidural Anesthesia

Epidural anesthesia is instituted with the patient in sitting or lateral decubitus position. Skin prep of the back is performed with antiseptic solution and the Tuohy (or another type) needle is inserted through a local anesthetic skin wheal placed at a selected interspace (lumbar or thoracic) using the landmarks described above for midline spinal anesthesia (Fig. 20.1). A method for identifying the epidural space during placement of epidural anesthesia is a sudden loss of resistance encountered (reflecting negative pressure) as the Tuohy needle penetrates through the ligamentum flavum. A technique to identify the epidural space is to connect a glass syringe with a freely movable plunger to the Tuohy needle. With either slow and continuous pressure or small incremental movements advance the needle attached to the syringe into and through the ligaments until a loss of resistance is detected as pressure on the plunger of the syringe is exerted. The syringe is then withdrawn from the positioned and stable Tuohy needle to check for evidence of CSF or blood flow. A test dose of 3 ml of local anesthetic solution containing epinephrine (1:200,000) is injected through the needle (for single shot), unless a catheter is to be placed through the needle, in which case the test dose may be injected through the catheter. A sterile plastic catheter is advanced 2-4 cm into the epidural space permitting repeated and/or continuous local anesthetic injection. Evidence of intravenous injection and/or subarachnoid blockade is observed following test dose injection. After a negative test dose, the needle is removed, epidural catheter is secured, and incremental boluses of local anesthetic can be administered.

Complications

Complications are similar to those described for spinal anesthesia, but with the added potential for accidental dural puncture and local anesthetic toxicity. Formation of an epidural hematoma is a rare, but potentially devastating concern.

Caudal Epidural Anesthesia

Caudal anesthesia can be performed with the patient in a lateral, prone, or jackknife position. The sacral hiatus is palpated and identified between the sacral cornua that are positioned about 5 cm cranial to the coccyx tip. A skin wheal with 1% lidocaine is raised between the sacral cornua following antiseptic skin preparation. The needle is positioned 90° to the skin and advanced through the sacrococcygeal ligament contacting the sacrum resulting in a "pop" sensation. The needle is withdrawn slightly as the perpendicular needle angle is reduced to 20-30° to the skin, directed cephalad, and then advanced about 2 cm into the caudal canal. The stylet is withdrawn to check for CSF or blood flow, and confirmation of proper needle placement can be made with a 5 ml injection of air through the needle (skin crepitation will be felt if needle placed erroneously into subcutaneous tissue). A test dose of 3 ml of local anesthetic solution containing epinephrine (1:200,000) is injected and observed for signs of subarachnoid or IV injection (similar to epidural anesthesia). A caudal catheter can be placed in a similar fashion as described for epidural anesthesia using a 17-gauge Tuohy needle.

Complications

Infection, subarachnoid injection, increased failure rate (up to 10%) secondary to abnormalities of anatomy within the caudal canal.

Pitfalls and Pearls Pitfalls

Relative and absolute contraindications of neuraxial blockade:

• Patient refusal

• Deformities of the back and spine

• Coagulation status during the perioperative period (concern for hematoma development and neurologic symptoms)

• Severe hypovolemia

• Elevated intracranial pressure

• Sepsis or infection at the site of intended injection

• Severe stenotic valve heart disease or ventricular outflow obstruction

Pearls

• If bone is contacted superficially, a midline needle is likely hitting the lower spinous process.

• Contact with bone at a deeper depth usually indicates that the needle is in the midline and hitting the upper spinous process or directed lateral to the midline and hitting a lamina.

• Loss of resistance in the epidural space can be confirmed with loss of resistance to air/saline, or both.

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