As with any anesthetic procedure, the risks and benefits of neuraxial regional anesthesia must be discussed with the patient. Potential risks are shown in Table 13.1.

Spinal anesthesia is primarily indicated for lower abdominal surgery, the perineum, and lower extremities. Epidural anesthesia is primarily indicated for lower abdominal surgery, thoracic surgery, surgery on the lower extremities, and labor. Epidurals can have sacral nerve root “sparing” and may not be optimal for surgery involving this area. Contraindications to neuraxial anesthesia are listed in Table 13.2.

The most common medication given for regional anesthesia is a local anesthetic. Local anesthetic that has been injected directly into the subarachnoid space (spinal) or that has diffused into the subarachnoid space from the epidural space (epidural) bathes the nerve root and inhibits synaptic transmission of action potentials. The effect of local anesthetics on nerve fibers varies according to the size of the nerve fiber, myelination and the concentration of the local anesthetic (also see Chap. 6, Pharmacology of Local Anesthetics). Differential blockade (the order of effects among the different nerve types) typically results in sympathetic blockade (often accompanied by change in temperature sensitivity), followed by sensory blockade (pain, light touch), and finally motor blockade (paralysis). A well-placed neuraxial anesthetic can provide total anesthesia for a variety of surgical procedures.

There are a number of other medications that can be used for both spinal and epidural anesthesia. Opioids, alpha-2-receptor agonists (e.g., clonidine), and vasoconstrictors (e.g., epinephrine, phenylephrine) have all been given with the effect of enhancing the quality or the duration of the block. Epinephrine can prolong the duration of spinal anesthesia by decreasing the rate of absorption of the local anesthetic.

Epidural anesthesia allows the delivery of medication either continuously or intermittently into the epidural space for up to several days after the surgical procedure. Sitting is the most common position in which an epidural is performed. Benefits of the sitting position include better identification of the midline and more flexion of the vertebral column. As the spine is flexed, it helps to open the space between spinous processes, allowing more room for the epidural needle to enter. An epidural may also be performed with the patient in the lateral position. This increases patient comfort, especially pregnant patients in active labor. However, the midline may be more difficult to identify.

The risks and benefits must be discussed with the patient and informed consent obtained. Standard monitors should be applied including blood pressure, ECG, and pulse oximetry. The patient may be sedated with an intravenous opioid or benzodiazepine. The desired interspace is identified and the patient’s skin is prepared with antiseptic solution. An epidural kit is typically used, which includes a 17- or 18-G Tuohy needle and a 19- or 20-G catheter.

A midline or paramedian approach can be used. After infiltration of skin with local anesthetic, the epidural needle is advanced through the skin, subcutaneous tissue, the supraspinous ligament, the interspinous ligament, and finally into the ligamentum flavum. Identification of the epidural space may be found with a loss of resistance technique or a hanging-drop technique.

With the loss of resistance technique , a syringe containing saline or air is attached to the epidural needle. As the needle is slowly advanced, the anesthesiologist places pressure on the syringe. The positive pressure encountered in the supraspinous ligament, interspinous ligament and ligamentum flavum prevents the plunger of the syringe from depressing (see Fig. 13.4). As the needle advances past the ligamentum flavum, a distinct loss of positive pressure is felt, as the plunger gives way and the saline or air is injected into the epidural space. A small catheter can then be threaded into the epidural space, usually 3–5 cm past the needle tip. Once the catheter is placed, a syringe containing a “test dose” of lidocaine with epinephrine 1:200,000 is attached. The catheter is aspirated first, to ensure no blood or cerebrospinal fluid (CSF) can be withdrawn. The test dose, typically 3 mL, is injected rapidly through the epidural catheter. The epinephrine serves as a surrogate marker to ensure the catheter has not threaded into a blood vessel (if positive, one would expect to see an increase in heart rate). The test dose also helps to determine if the catheter is in the subarachnoid space (spinal). If there are no sensory or motor changes within 3 min, the catheter is most likely not in the subarachnoid space.

With the hanging-drop technique , a small drop of saline is placed at the hub of the needle. As the needle passes through the positive pressure structures stated above, the drop of saline will remain at the hub of the needle. Once the needle contacts and passes through the ligamentum flavum, the drop of saline is retracted back into the needle as the negative pressure of the epidural space is encountered.

Similar local anesthetics can be used for both epidural and spinal anesthesia. Chloroprocaine and lidocaine are fast onset medications with a short duration of action, while bupivacaine and ropivacaine have a slower onset and longer duration. Unlike spinal anesthesia, the level of anesthesia in an epidural is not influenced by baricity or position of the patient immediately after injection (see below).

The amount of local anesthetic required to produce surgical anesthesia with an epidural is significantly more than with a spinal, as the local anesthetic must traverse more layers to act on the nerve roots. The addition of epinephrine can prolong the effect of local anesthetic by decreasing vascular uptake, allowing more time for the medication to act on the nerve roots. Opioids, such as morphine or fentanyl, can also be added to an epidural. They help to enhance the quality of the epidural as well as provide postoperative pain control.

As with general anesthesia, prior to starting a spinal patient monitors should be applied (blood pressure, pulse oximeter, and ECG). Supplemental oxygen is often administered. Intravenous access also must be established. In most situations, the patient may be sedated with an intravenous opioid such as fentanyl and/or a benzodiazepine such as midazolam. Patient comfort will help in both positioning and anxiolysis while performing the spinal. As with an epidural, a spinal may be placed in either the sitting or lateral position.

As stated above, the spinal cord typically ends at the level of L1 in adults and L3 in children. Placing the spinal needle below the level of L3 provides an additional margin of safety, by decreasing the likelihood of any spinal cord penetration. The iliac crest has been traditionally used as an anatomic landmark corresponding with an L4–L5 interspace (see Fig. 13.2).

There are two main techniques for performing a spinal anesthetic: midline and paramedian. With each technique, the patient is positioned optimally for both physician and patient, the desired interspace is identified, and the skin is cleaned and prepared with antiseptic solution. Local anesthesia is infiltrated in the skin and subcutaneous tissues to improve patient comfort. With the midline approach , the spinal needle is first introduced into the skin between the upper and lower spinous processes at the desired interspace. After passing through the skin, the needle continues to pass through subcutaneous tissue, the supraspinous ligament, the interspinous ligament, the ligamentum flavum, and finally advancing through the epidural space into the subarachnoid space (Fig. 13.4). Often a distinct “pop” is felt by the anesthesiologist as the needle penetrates the ligamentum flavum. Correct identification of the subarachnoid space is confirmed by free flow of CSF out of the hub of the needle.

The paramedian approach is used in patients where the midline may be difficult to identify (e.g., scoliosis) or the interspace may be challenging to pass a needle through (e.g., thoracic level for epidural placement, elderly patients with calcified ligaments or loss of disc space). Needle insertion is typically 1 cm from the midline. After the transverse process is contacted, and the needle is redirected cephalad and medial to pass through the interlaminar space. One of the main differences between the paramedian and midline approach is that the ligamentum flavum is the first resistance encountered with the paramedian approach. Again, correct identification of the subarachnoid space is confirmed by free flow of CSF out of the hub of the needle.

Assuming there is no blood exiting the needle and the patient has not experienced a paresthesia, administration of the local anesthetic can proceed. Common local anesthetics include lidocaine, chloroprocaine, and bupivacaine. Each local anesthetic has slightly different properties, which affect onset, duration, and potential for toxicity (see Chap. 6, Pharmacology of Local Anesthetics). When the syringe containing the local anesthetic is attached to the spinal needle, care must be taken to avoid moving the needle. The anesthesiologist’s hands are usually braced against the patient’s back while holding the spinal needle steady. Before injection of the local anesthetic, one should aspirate first and allow a small volume of CSF to enter the syringe. This can be confirmed by visualizing a CSF “swirl” when mixing with the local anesthetic in the syringe. The local anesthetic is injected slowly over 3–5 s. CSF can be aspirated at the end of the injection as well to confirm the needle has not moved from the spinal space while injecting. The onset of anesthesia will be rapid (within 60 s) with a spinal anesthetic.

Two of the most important factors determining the distribution of local anesthetic in the subarachnoid space are the baricity of the solution (density compared to CSF) and the position of the patient immediately after injection of the solution. Addition of a vasoconstrictor (e.g., epinephrine) and the type of local anesthetic selected influence the duration of the spinal block. Local anesthetic solutions are classified as hypobaric, isobaric, or hyperbaric based on their density relative to the density of CSF. Knowledge of the local anesthetic baricity can help the anesthesiologist control both the direction and extent of local anesthetic spread within the subarachnoid space.

Hyperbaric solutions usually contain glucose/dextrose. They allow for a greater cephalad spread of the local anesthetic. If a higher dermatomal level is needed, the patient may be placed in a head-down (Trendelenburg) position, allowing the hyperbaric solution to migrate cephalad. Likewise, if the surgery requires dense anesthesia for a perirectal procedure, the patient may be left in a sitting position for several minutes after completion of the spinal.

Hypobaric solutions are used less commonly in clinical practice. A patient undergoing hip arthroplasty may benefit from having the hypobaric solution “float up” to the operative side. Hypobaric solutions can be made by mixing the local anesthetic with sterile water, or normal saline.

Isobaric solutions tend to have limited spread within the subarachnoid space and are thought to produce a more profound motor block and longer duration of action. Isobaric solutions can be prepared by mixing the local anesthetic with normal saline or the patient’s CSF.

Addition of epinephrine (0.1–0.2 mg) or phenylephrine (2–5 mg) to the local anesthetic solution increases the duration of the spinal block. The resultant decrease in spinal cord blood flow and uptake of the local anesthetic prolongs the exposure to the nerve roots of the local anesthetic.

This type of regional anesthetic is most commonly performed in pediatric patients. After induction of general anesthesia the child is placed in the lateral position. The sacral cornu are identified as well as the sacral hiatus. The skin is prepared in sterile fashion. A needle is introduced perpendicular to the skin through the sacrococcygeal ligament (beneath the sacral hiatus), advanced slightly, then the angle is dropped and the needle is advanced slightly further into the epidural caudal canal. Confirmation of proper needle position can be obtained by rapidly injecting 3–5 mL of air or saline while the anesthesiologist’s fingers are palpating the skin directly over the needle. Skin swelling or crepitus indicates the needle has not penetrated the epidural space. Once proper position is confirmed, a syringe is connected to the end of the needle and aspirated to ensure no blood or CSF is obtained. Local anesthetic is then injected in slow 3–5 mL aliquots.