The spinal cord is a continuation of the medulla oblongata, occupying the upper two thirds of the vertebral canal. It is approximately 18 inches long and ends at the lower border of L1 in most adults. The spinal cord ends in the conus medullaris and cauda equina. Nerve roots are not fixed, as the spinal cord is, making it difficult to cause trauma to nerve roots; this makes it safe to place a spinal needle below L2 in the adult patient. The lower portion of the spinal cord becomes the filum terminale, which connects to the bone of the coccyx and holds the spinal cord in place.⁸^–¹⁰ The spinal cord is encased by three membranes: dura, arachnoid, and pia mater. The outermost membrane, dura mater, consists of two layers. Between the dura and the ligamentum flavum is the epidural space, a potential space filled with loose fatty tissue and blood vessels. Local anesthetics are injected in this space for epidural anesthesia. The arachnoid layer consists of a thin membranous sheath. The innermost layer is called the pia mater and is separated from the arachnoid mater by a subarachnoid space filled with cerebrospinal fluid (CSF). The subarachnoid space is where local anesthetics are deposited for spinal anesthesia (Fig. 25-2). The total CSF volume in the adult is approximately 100 to 150 mL, with 25 to 35 mL of the volume residing within the subarachnoid space. CSF, continually produced by the choroid plexuses, is reabsorbed into the blood through the arachnoid villi, granulation, and to a small extent through epidural veins. The specific gravity of CSF, which ranges between 1.004 and 1.009, affects the spread of local anesthetic solution depending on its baracity. A hyperbaric medication will move toward dependent areas because it is heavier than CSF, whereas a hypobaric solution will do the opposite. Isobaric medications stay within the general area of injection.⁵^,⁸^,⁹

FIG. 25-2 Extension of the spinal cord to the second lumbar vertebra.

(From Nagelhout J, Plaus K, editors: Nurse anesthesia, ed 4, St. Louis, 2010, Saunders.)

Thirty-one pairs of spinal nerves travel from the spinal cord and exit at their respective intervertebral foramina (C1-S5). There are 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal pair of spinal nerves. These nerves are blocked by the local anesthetic medication to produce anesthesia or analgesia. Spinal nerve roots vary in size and can be divided into dorsal roots (primarily sensory) and anterior roots (primarily motor).⁵^,⁸^,⁹

Spinal versus epidural blocks

Spinal and epidural blocks are neuraxial blocks. Both block spinal nerve roots, the site of administration differs as does the mechanism of action (Table 25-1). A spinal anesthetic blocks spinal nerve roots as they pass through the CSF, resulting in blockade of sensory, motor, and autonomic impulse transmission whereas epidural anesthesia and analgesia blocks spinal nerve roots outside the subarachnoid space. Epidurals require a large dose of local anesthetic to allow for diffusion through anatomic barriers, such as the dura, and spreads horizontally and vertically within the epidural space. The epidural space is vascular, and there is a risk of local anesthetic toxicity (see Chapter 24). Spinal anesthesia is accomplished by administering a small dose of local anesthetic within the subarachnoid space. Spinal anesthesia is a single injection through a needle and reduces the time for administration but does not allow for repeated dosing. Epidural anesthesia is administered through a catheter that is placed through a needle, may require more time to place, may be dosed again, and used for postoperative analgesia. Spinal anesthesia is limited to placement in the lower lumbar region below the termination of the spinal cord. Epidural catheter placement can occur in the lumbar or thoracic region. Spinal anesthesia is rapidly achieved and produces an intense sensory and motor block. Rapid onset can lead to profound hypotension. Epidural anesthesia onset is gradual, blockade is less profound, and decreases in blood pressure are less intense.⁷^,⁹

Table 25-1 Differences Between Spinal and Epidural Anesthesia

	SPINAL	EPIDURAL
Site or mechanism of action	Nerve roots blocked as they pass through the CSF	Nerve roots blocked outside the cerebrospinal fluid
Site of administration	Lower lumbar below the termination of spinal cord	Lumbar or thoracic area
Dose of local anesthetic	Small	Large
Instrument for administration	Needle	Catheter
Ability to repeat dose	No	Yes
Onset	Rapid, intense blockade, may lead to hypotension	Gradual; may have less intense blockade, decrease in blood pressure is more gradual

Contraindications

Absolute contraindications include patient refusal; cardiovascular diseases such as severe aortic–mitral stenosis and asymmetrical septal hypertrophy; severe uncorrected hypovolemia; documented allergy to local anesthetic; increased intracranial pressure; infection at the site of injection; sepsis or bacteremia; and coagulopathy.¹^,⁵^,⁷^,⁹ Coagulation-altering medications should be reviewed and current recommendations should be followed to avoid the potentially catastrophic complication of a neuraxial hematoma.¹⁰ Current guidelines related to anticoagulation therapy and regional anesthesia can be accessed and reviewed at http://asra.com/publications-anticoagulation-3rd-edition-2010.php.¹¹ Relative contraindications include preexisting neurologic disease, musculoskeletal abnormalities, prior back surgery, and untreated hypertension.¹^,⁵^,⁷^,⁹ Placing a neuraxial block through a tattoo is controversial.¹² The perianesthesia nurse should alert the anesthesia provider to potential contraindications.

Spinal anesthesia

Universal protocol, as outlined by The Joint Commission, should be followed and medications should be labeled before the procedure.¹³ An initial set of vital signs should be documented. Patients may be positioned in a lateral decubitus or sitting position with the back flexed to maximize the intervertebral opening (Figs. 25-3 and 25-4). Tuffier’s line is generally used to identify the 3rd and 4th lumbar spaces, but may not always be reliable.¹⁴ After the anesthesia provider has identified the desired interspace, he or she will follow sterile technique (mask, sterile gloves, hat) and proceed with preparing and placing sterile drapes. The perianesthesia nurse should maintain verbal contact with the patient to reassure and inform them of what to expect. Local anesthetic is used to anesthetize the skin and underlying tissue. A 25-gauge or smaller spinal needle is used in patients at risk for a postdural puncture headache (PDPH): patients younger than 50 years, females, pregnancy, and a prior history of PDPH. Needle design is an important consideration in at risk groups, with pencil point needles being less likely to cause a PDPH.¹^,¹⁵ A 22-gauge cutting tip needle can be used in patients older than 50 years. The spinal needle will transverse the supraspinous and interspinous ligaments, ligamentum flavum, and dura; this is noted by a change in the resistance. The stylet is removed and CSF is noted at the hub of the needle. Procaine, bupivacaine, or tetracaine are commonly used for spinal anesthesia. Epinephrine (0.1 to 0.2 mg) or phenylephrine (2 to 5 mg) can be added to increase duration of action. Local anesthetic is administered and patient positioned. Vital signs should be monitored at 1-minute intervals for the first 10 minutes and then, if stable, every 5 minutes. Blood pressure decreases of 20% or more should be treated with fluids and vasopressors (ephedrine or phenylephrine).¹^,⁵^,⁷^,⁹

FIG. 25-3 Lateral decubitus positioning for a neuraxial block. The assistant can help the patient assume the ideal position of “forehead to knees.”

(From Miller RD, et al: Miller’s anesthesia, ed 7, Philadelphia, 2009, Churchill Livingstone.)

FIG. 25-4 Sitting position for a neuraxial block. The assistant provides the patient with a foot rest (stool) and a pillow and prevents the patient from slumping to either side.

(From Miller RD, et al: Miller’s anesthesia, ed 7, Philadelphia, 2009, Churchill Livingstone.)

Progression of onset is noted by loss of autonomic or sympathetic function, sense of temperature, pain, touch, movement and proprioception. During recovery, function will return in the opposite order (Box 25-1). Spinal anesthetics are dosed for specific surgical procedures (Table 25-2). Level of anesthesia is monitored by dermatome level (Fig. 25-5). Monitoring regression of spinal anesthetic is aided with the knowledge of the dermatome level and can be assessed by the use of an alcohol wipe, a blunt needle that does not break the skin, or a light pinch.⁵^,⁶

BOX 25-1 Onset and Return of Neuraxial Blockade

Order of Loss of Function	Order of Return of Function
1. Autonomic, sympathetic function

1. Proprioception (sense of body location)

2. Sense of temperature

5. Sense of temperature

6. Proprioception (sense of body location)

6. Autonomic, sympathetic function

From Schick L, Windle PE, editors: Perianesthesia nursing core curriculum: preprocedure, Phase I and Phase II PACU nursing, ed 2, St. Louis, 2010, Saunders.

Table 25-2 Dermatome Level and Surgical Procedure

SENSORY LEVEL	SURGICAL PROCEDURE	CUTANEOUS LEVEL
Above T4		Pinky, inner arm, apex of axilla
T4	Cesarean section, upper abdomen, uterine	Nipple
T6-7	Lower abdomen	Xiphoid
T7-8	Tourniquet pain
T10	Hip and genitourinary	Umbilicus
L1-3	Lower extremities
S2-5	Perineal region (genitalia/buttocks)

Modified from Schick L, Windle PE, editors: Perianesthesia nursing core curriculum: preprocedure, phase I and phase II PACU nursing, ed 2, St. Louis, 2010, Saunders.

FIG. 25-5 Dermatomes.

(From Nagelhout J, Plaus K, editors: Nurse anesthesia, ed 4, St. Louis, 2010, Saunders.)

Epidural anesthesia

Epidural anesthesia or analgesia is similar to spinal anesthesia in regard to positioning, sterile technique, monitoring, progression, and assessment. Epidural anesthesia can be a single shot technique (i.e., caudal anesthesia) or more commonly as an intermittent or continuous technique for anesthesia or analgesia. Anesthesia may be administered for the same type of surgical procedures as spinal anesthetics (see Table 25-2), but may be less effective for procedures of the perineum because of sacral sparing. Epidurals are versatile for analgesic purposes and may be used for labor and delivery or postoperative analgesia for surgical procedures ranging from the thorax to lower extremities. Epidural needles are larger than spinal needles to allow for the insertion of a catheter. The average needle is 9 cm and is marked to indicate the depth of insertion. An epidural needle transverses the supraspinous and intraspinous ligament and seated in the ligamentum flavum. A special loss-of-resistance syringe with preservative-free saline is attached to the hub, and the needle is advanced until a loss of resistance is noted upon entering the epidural space. An epidural catheter is then inserted to a desired depth. Epidural catheters are marked to indicate the depth of insertion.¹^,⁵^,⁷^,⁹^,¹⁶ A 3-mL test dose of 1.5% lidocaine with 1:200,000 epinephrine is administered to help detect intravascular, subarachnoid, or subdural placement.¹⁷ If positive, the heart rate and systolic blood pressure will increase by at least 20 beats/min and 15 mm Hg, but may not be reliable in the presence of beta blockers, advanced age, and general anesthesia.¹⁸ A subarachnoid injection will result in a spinal anesthetic, whereas a subdural injection can result in an extensive block relative to a small dose of local anesthetic.¹⁹ Common local anesthetics used for epidural anesthesia include 2-chloroprocaine, lidocaine, bupivacaine, and ropivacaine. Epidurals are dosed based on the number of dermatomes that are to be blocked (1 to 2 mL per dermatome). Dosing occurs slowly with no more than 5 mL of local anesthetic administered at a time because there is a risk of local anesthetic toxicity. Epinephrine (1:200,000) can be used to decrease absorption and prolong duration. Epidural catheters should be monitored for signs of drainage or dislodgement. The site is covered with a sterile dressing and connections should be tight. The American Society of PeriAnesthesia Nurses position statement on safe medication administration²⁰ and local institutional policy should be followed for the administration of medications. When initiating a continuous infusion check dose and concentration of local anesthetic solution and confirm the infusion rate.