Spinal Anesthesia

Francis V. Salinas

De Q.H. Tran

KEY POINTS

1. Spinal blocks provide a reliable and rapid onset of dense surgical anesthesia. They should be performed below the L2-L3 intervertebral space to minimize the risk of mechanical trauma to the spinal cord.

2. Knowledge of dermatomal projections is paramount to determine whether the sensory block is adequate for the planned surgical procedure.

3. Subarachnoid distribution of local anesthetic agents determines the extent of sensorimotor and sympathetic block. Distribution can be manipulated through the interplay of local anesthetic baricity and patient position (gravity). Elimination (vascular absorption) of local anesthetics from the subarachnoid space determines the duration of action.

4. Common adjuncts used to prolong (and intensify) spinal anesthesia include α-adrenergic agonists and opioids.

6. Although preservative-free 2-chloroprocaine offers significant benefits for outpatient surgery, the Food and Drug Administration has not approved its administration for spinal blocks.

7. The midline approach is simple to learn and adequate for most patients. However, in subjects with difficult spinal anatomy, the operator could employ a paramedian approach, lumbosacral method, or ultrasound assistance.

8. Absolute contraindications to spinal anesthesia include patient refusal, infection at the puncture site, untreated hypovolemia, coagulopathy, and increased intracranial pressure.

9. Spinal anesthesia can be associated with major cardiovascular trespass (e.g., bradycardia, hypotension) and serious side effects (e.g., nerve damage, infection, neuraxial hematoma).

ADMINISTRATION OF LOCAL ANESTHETIC into the subarachnoid space results in a reliable and rapid onset of dense surgical anesthesia. Despite the technical simplicity of spinal blocks, the anesthesiologist should possess a thorough understanding of the anatomy of the lumbosacral spine, the determinants of subarachnoid local anesthetic distribution, and the factors that influence block duration. Furthermore, knowledge of the physiologic effects and potential complications related to spinal anesthesia is paramount to ensure patient safety.

I. Anatomy

A. Vertebral column. The vertebral column is composed of 33 vertebrae and 5 ligaments. Together, they form a protective exoskeleton around the spinal cord.

1. There are 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal vertebral segments (Fig. 6.1). Spinal anesthesia is typically performed in the lower lumbar region. The lumbar vertebra is composed of an anterior vertebral body and posterior bony elements. The latter include two pedicles, which project posteriorly from the vertebral body, and two flattened laminae, which connect the pedicles to form the vertebral arch. The anterior and posterior bony elements combine to delineate the vertebral foramen (Fig. 6.2). Vertebral foramina of adjoining vertebrae form the longitudinal spinal canal, which houses the spinal cord. The adjoining paired pedicles of each vertebra display superior and inferior notches. The latter form the intervertebral foramina through which the paired segmental spinal nerves exit the spinal canal (Fig. 6.2). A single spinous process projects posteriorly (and slightly caudally) from the posterior aspect of the vertebral arch at the midline junction of the paired laminae. The bony elements provide sites for muscular and ligamentous attachments

2. Five ligaments anchor the vertebral column (Fig. 6.3). The supraspinous ligament connects the tips of the spinous processes from the seventh cervical vertebra to the sacrum. The interspinous ligament connects adjoining spinous processes. The laminae of adjacent vertebral arches are connected by the tough, wedge-shaped ligamentum flavum, which is composed primarily of elastin. The ligamentum flavum binds the paired laminae of adjoining vertebrae together, thereby forming the posterior wall of the vertebral spinal canal. It is this posterior ligamentous “opening” (i.e., the intervertebral or interlaminar space) that a spinal needle traverses to reach the subarachnoid space. Anatomic studies demonstrate a 9% to11% incidence of gaps in the midline of the ligamenta flava.

3. The vertebral column displays characteristic curvatures in the lumbar and thoracic regions (Figs. 6.1 and 6.4). Local anesthetic solution injected at the peak height of the lumbar anterior convexity (lumbar lordosis) will distribute both caudad and cephalad to varying degrees depending on the baricity of the solution. The cephalad spread of hyperbaric local anesthetic solutions is typically limited to the mid-to-upper thoracic dermatomes because of pooling within the thoracic concavity (thoracic kyphosis).

FIGURE 6.1 Vertebral column, lateral (A) and posterior (B) views, illustrating the cervical, thoracic, lumbar, sacral, and coccygeal segments. Note the curvatures, intervertebral foramina, and interlaminar spaces. (Adapted from Cousins MJ, Bridenbaugh LD, eds. Neural Blockade in Clinical Anesthesia and Management of Pain. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 1998:205.)

B. Meninges. The spinal meninges consist of three membranes (the dura mater, arachnoid mater, and pia mater). In conjunction with the cerebrospinal fluid (CSF), the meninges cushion the spinal cord and nerve roots in the subarachnoid space (Fig. 6.5).

1. The dura mater (“tough mother”), the outermost and thickest meningeal membrane, is composed primarily of collagen fibrils, interspersed with elastic fibers and ground substance in an anatomic arrangement that allows ready passage of drugs (1). Historically, the dura was (incorrectly) assumed to be the primary barrier for drug diffusion from the epidural to the subarachnoid space. The dura mater forms the dural sac, which consists of a long tubular sheath contained within the surrounding spinal canal, which extends from the foramen magnum to the lower border of the second sacral vertebra. At this level, the dural sac fuses with the filum terminale. The dura mater also extends laterally along the spinal nerve roots, becoming continuous with the epineurium of spinal nerves at the level of intervertebral foramina.

2. The arachnoid mater, which is closely adherent to the inner surface of the dura mater, is composed of overlapping layers of flattened epithelial-like cells that are connected by frequent tight and occluding junctions (2). The arachnoid is not directly attached to the dura

but is held against its inner surface (dura-arachnoid interface) by the pressure of the CSF. During spinal anesthesia, the needle penetrates the dura and arachnoid simultaneously. Functionally, the arachnoid mater accounts for the resistance to drug diffusion through spinal meninges.

FIGURE 6.2 Typical lumbar vertebra illustrating superior and lateral views of the anterior vertebral body, the elements that form the vertebral arch (the paired pedicles and paired laminae), and single midline spinous process. Note the superior and inferior vertebral notches of the adjoining pedicles, which form the intervertebral foramen. (Adapted from Moore KL, Dalley AF. Clinically Oriented Anatomy. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2006:480.)

FIGURE 6.3 Sagittal section of vertebral column illustrating the supporting ligaments and their bony attachments. The interspinous ligaments connect adjacent spinous processes, and the ligamentum flavum connects adjacent laminae. (Adapted from Cousins MJ, Bridenbaugh LD, eds. Neural Blockade in Clinical Anesthesia and Management of Pain. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 1998:205.)

FIGURE 6.4 Normal curvature of the spinal column in the supine horizontal position. Hyperbaric solutions injected at the peak of lumbar lordosis will distribute (through gravity) to the lower sacral and thoracic concavities. (Adapted from Raj PP. Handbook of Regional Anesthesia. New York: Churchill Livingstone; 1985:225.)

3. The pia mater, which closely invests the surface of the spinal cord and nerve roots, is composed of three to six layers of cells (3). The subarachnoid space, which lies between the arachnoid mater and the pia mater and contains the CSF, constitutes the target compartment for spinal anesthesia. The pia mater extends to the conus medullaris, where it becomes the filum terminale. The latter anchors the spinal cord to the sacrum (Fig. 6.5).

FIGURE 6.5 Lumbosacral spinal cord and the spinal meninges (dura, arachnoid, and pia). Note also the terminal portion of the spinal cord (conus medullaris) and the nerve roots of the lower lumbar and sacral spinal cord segments, giving rise to the cauda equina. (Adapted from Cousins MJ, Bridenbaugh LD, eds. Neural Blockade in Clinical Anesthesia and Management of Pain. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 1998:209.)

C. Spinal cord

1. The spinal cord is a cylindrical structure that gives rise to 31 pairs of spinal nerves. In turn, each spinal cord segment gives rise to paired ventral motor and dorsal sensory roots. Motor and sensory roots cross the subarachnoid space separately before joining together (close to the intervertebral foramina) to form the mixed spinal nerves. Although larger than their ventral counterparts, dorsal nerve roots commonly divide into two or three separate bundles upon exiting the spinal cord. In contrast, most ventral roots exit as a single bundle (4). Moreover, as dorsal nerve root bundles course further laterally, they further subdivide into as many as 10 fascicles before the dorsal root becomes the dorsal root ganglion (5). Therefore, the larger multifilamentous dorsal nerve roots offer a substantially larger surface area for local anesthetic uptake compared to the smaller, unitary ventral roots. This partially explains the faster onset of sensory versus motor blockade.

2. The skin area supplied by a spinal nerve (and its corresponding spinal cord segment) is called a dermatome (Fig. 6.6). Dermatomal assessment for the loss of sensory functions (such as temperature, pinprick, and touch) provides a surrogate marker for the segmental distribution of surgical anesthesia.

FIGURE 6.6 Sensory dermatomes. (Adapted from Agur AMR, Lee MJ, eds. Grant’s Atlas of Anatomy. 10th ed. Philadelphia: Lippincott Williams & Wilkins; 1999:296.)

3. In adults, the spinal cord is shorter than the vertebral column. The caudal extent of the spinal cord (known as the conus medullaris [Fig. 6.5]) usually extends to the lower third of the first lumbar vertebral body, but may occasionally reach the upper third of the third lumbar vertebral body (6) and rarely the L4 level. Therefore, attempting spinal anesthesia at or above the L2-L3 intervertebral space (IVS) could result in mechanical trauma to the spinal cord in a small number of patients.

4. The discrepancy in length between spinal cord and vertebral column results in a progressive obliquity of the lower thoracic, lumbar, and sacral spinal nerve roots. The collection of spinal nerve roots caudal to the conus medullaris is known as the cauda equina (Fig. 6.5). The enlargement of the subarachnoid space containing the cauda equina is termed the lumbar cistern. Local anesthetics are deposited within the latter during the performance of spinal anesthesia.

D. Surface anatomy

1. Accurate identification of IVS levels is paramount for successful spinal anesthesia. The desired level is determined by the palpation of surface landmarks. A line connecting the iliac crests (i.e., the intercristal or Tuffier’s line) most commonly intersects the vertebral column at the L4-L5 IVS (Fig. 6.7) (6). However, the intercristal line may intersect the vertebral column as cephalad as the L3-L4 IVS and as caudad as the L5-S1 IVS (Fig. 6.8). Even experienced anesthesiologists incorrectly identify the IVS 70% of the time (7). Thus, inexperienced operators should preferentially attempt spinal anesthesia at a lower lumbar (L3-L4 or L4-L5) IVS to minimize the potential risk for spinal cord injury (8,9). Fortunately, lumbar flexion, which is commonly used to improves access to the intervertebral-interlaminar space, does not alter the position of the intercristal line (10).

II. Indications and contraindications

A. The injection of local anesthetics into the subarachnoid space results in (afferent) sensory block as well as varying degrees of (efferent) motor block. The cephalad extent of spinal anesthesia depends on the baricity of the local anesthetic solution as well as the patient’s position during the spinal block and the surgical procedure. The CSF concentration of local anesthetic decreases as the distance from the injection site starts to increase. This results in a gradient of afferent and efferent conduction block, which stems from the differential sensitivity of spinal nerve fibers to local anesthetic agents (Table 6.1). For instance, preganglionic efferent fibers are the most sensitive to local anesthetic conduction block and are commonly anesthetized two to six dermatome segments more cephalad than afferent sensory block. In turn, sensory block can be two to three dermatomes higher than efferent motor block (11). Although sensory block can be used as a surrogate marker for surgical anesthesia, tolerance to transcutaneous electrical stimulation of 10-mA, 50-Hz continuous square wave for 5 seconds constitutes a more reliable predictor of tolerance to surgical stimulation (12).

B. Knowledge of dermatomal projections is paramount to determine whether the sensory block is adequate for the planned surgical procedure, and to evaluate regression of the spinal block. For example, the fourth thoracic dermatome corresponds to the nipples, the sixth thoracic dermatome to the xyphoid process, and the tenth thoracic dermatome to the umbilicus (Table 6.2 and Fig. 6.6). It is also essential to remember that viscera are innervated differently and that anesthesia of an overlying cutaneous region does not inherently confer surgical anesthesia on an underlying visceral organ (Table 6.3).

CLINICAL PEARL

1. Although foot-ankle (S1-L5) and knee surgery (L3-L4) involve lumbosacral dermatomes, the use of thigh tourniquet (and tolerance to tourniquet pain) will often require a peak sensory block height to T10-T8 (13).

2. T6-T4 peak sensory levels are required for lower abdominal surgery such as inguinal herniorrhaphy, appendectomy, abdominal hysterectomy, or cesarean delivery. Nonetheless, patients may still report discomfort associated with (transient) traction on the peritoneum or abdominal viscera.

C. There exist absolute and relative contraindications to spinal anesthesia. Absolute contraindications include patient refusal, infection at the puncture site, severe untreated hypovolemia, coagulopathy, and increased intracranial pressure. Performance of spinal anesthesia in patients with preexisting neurologic deficits such as radicular or peripheral neuropathies (14,15) and demyelinating diseases (e.g., multiple sclerosis) (16) remains controversial. This issue is discussed in the chapter pertaining to complications of regional anesthesia (Chapter 14). Aortic stenosis, once considered an absolute contraindication to spinal anesthesia, does not necessarily preclude a carefully conducted spinal anesthetic (17). Although spinal blocks should not be performed in patients with untreated systemic infection, subjects with evidence of systemic infection may safely undergo spinal anesthesia if appropriate antibiotic therapy has been initiated before dural puncture and response to therapy (e.g., decrease in fever and granulocytosis) demonstrated (18).

III. Determinants of local anesthetic distribution and duration of action

A. The clinical pharmacology of local anesthetics and adjuvants is addressed in Chapters 1 and 2. This section will discuss only the determinants of local anesthetic distribution and duration

for spinal anesthesia. From a pharmacodynamic standpoint, subarachnoid distribution of local anesthetic agents determines the extent of sensorimotor and sympathetic block. Uptake of local anesthetics dictates the neuronal functions most affected by the spinal block. Elimination of local anesthetics from the subarachnoid space determines the duration of action.

FIGURE 6.7 Patient position for spinal or epidural blockade in the lateral decubitus position. The patient’s knees are drawn up toward the chest and the head flexed downward to provide the maximum anterior flexion of the vertebral column. The pillow should be placed under the head but not under the shoulders to avoid rotation of the spine. The hips and shoulders should be perpendicular to the surface of the bed, resisting the usual inclination of the patient to roll the superior shoulder forward. A line drawn between the posterior iliac crests usually crosses the spinal column at the L4-L5 interspace or L4 spinous process. Similarly, for thoracic epidural injection, a line between the inferior tips of the scapulae crosses the T7 spinous process.

FIGURE 6.8 Distribution of the vertebral level in segments at which the conus medullaris, the intercristal (Tuffier’s) line, and the dural sac cross. The segmental levels where the spinal cord ends, Tuffier’s line crosses, and the dural sac ends follow a normal distribution. Each vertebra was divided into four segmental thirds of the vertebral body—upper (U), middle (M), and lower (L)—and the intervertebral space. The most caudal distribution of the conus should not cross with the most cephalad level of the actual intercristal line in this patient population. (Adapted from Kim JT, Bahk J, Hung JH. Influence of age and sex on the position of the conus medullaris and Tuffier’s line in adults. Anesthesiology 2003;99(6):1359-1363.)

TABLE 6.1 Classification of afferent and efferent nerve fibers

Fiber class	Axon diameter (µm)	Myelin	Conduction velocity (m/s)	Innervation	Function
Aα	12-20	+++	75-120	Afferent from muscle spindle proprioceptors Efferent to skeletal muscle	Motor and reflex activity
Aβ	5-12	+++	30-75	Afferent from cutaneous mechanoreceptors	Touch and pressure
Aγ	3-6	++	12-35	Efferent to muscle spindles	Muscle tone
Aδ	1-5	++	5-30	Afferent pain and temperature nociceptors	“Fast” pain, touch, and temperature
B	<3	+	3-15	Preganglionic sympathetic efferent	Autonomic function
C	0.2-1.5	–	0.5-2.0	Afferent pain and temperature	“Slow” pain, temperature

TABLE 6.2 Surface anatomy and dermatomal levels

Surface anatomy	Sensory dermatome
Perineum	S2-S4
Lateral foot	S1
Knee and distal thigh	L3-L4
Inguinal ligament	T12
Umbilicus	T10
Tip of xyphoid process	T6
Nipple	T4
Inner aspect of forearm and arm	T1-T2
Thumb and index finger	C6-C7
Shoulder and clavicle	C5-C4

TABLE 6.3 Common surgical procedures appropriate for spinal anesthesia and recommended peak sensory block height

Surgical procedure	Recommended minimum peak block height
Perirectal and perineal Incision and drainage of rectal abscess Hemorrhoidectomy Transvaginal slings	L1-L2
Lower extremity surgery with tourniquet use Knee replacement Knee arthroscopy Below-knee amputation	T10-T8
Transurethral resection of prostate Cystoscopy and hysteroscopy Vaginal delivery Total hip replacement Femoral-popliteal bypass Varicose vein stripping	T10
Lower abdominal Hysterectomy (low transverse incision) Cesarean delivery Inguinal herniorrhaphy Appendectomy	T4

B. Determinants of subarachnoid local anesthetic distribution. Many factors have been proposed to explain the distribution of local anesthetic solutions in the subarachnoid space (19,20). After their injection, local anesthetics will initially spread by simple bulk flow. Subsequently, the most important determinant of distribution is the baricity of the local anesthetic solution. Other relevant factors include the total dose, the IVS selected for injection, and several patient characteristics.

1. Baricity. Baricity is defined as the ratio of the density of the local anesthetic solution to the CSF density at 37°C. Local anesthetic solutions that possess the same density as CSF are termed isobaric. Local anesthetic solutions that have a greater density than CSF are classified as hyperbaric, whereas solutions with a lower density than CSF are termed hypobaric. Hyperbaric solutions will sink to the most dependent areas, whereas hypobaric solutions will rise to the nondependent areas of the subarachnoid space. The effects of gravity are determined by the choice of patient position as well as by the natural curvatures of the spine. The mean density of CSF varies significantly among different patient subpopulations (Table 6.4). Thus, local anesthetics (e.g., plain bupivacaine 0.5% and plain lidocaine 2.0%) commonly classified as “isobaric” may in fact behave in a hypobaric manner. Table 6.4 lists the density range and classification of commonly used local anesthetics.

a. Hyperbaric spinal anesthesia. Hyperbaric solutions are commonly prepared by mixing the local anesthetic with dextrose. When the patient is placed in the supine position after the injection of a hyperbaric solution, the latter will distribute toward the lowest points of the thoracic (T6-T7) and sacral (S2) curvatures (Fig. 6.4) (21). In fact, pooling of hyperbaric local anesthetic solutions within the thoracic kyphosis has been postulated to explain its peak sensory block height in the midthoracic region (22).

(1) Injection of hyperbaric local anesthetic solutions with the patient in the sitting position (for 5 to 10 minutes) has been (mistakenly) advocated to restrict distribution to the lumbosacral dermatomes, producing a so-called “saddle block” (Fig. 6.9). Unfortunately, clinical studies have demonstrated that a hyperbaric spinal anesthetic
block initially restricted to the lumbosacral dermatomes will eventually distribute to a peak thoracic height equivalent to that which would have been achieved had the patient been immediately placed in the supine position (23).

TABLE 6.4 Density and baricity of cerebrospinal fluid in different patient subgroups and commonly used local anesthetics

	Mean (SD) density at 37°C	Range within 3 SD of the mean
Patient population
Men	1.00064 (0.00012)	1.00028-1.00100
Older women	1.00070 (0.00018)	1.00016-1.00124
Younger women	1.00049 (0.00004)	1.00037-1.00061
Pregnant/postpartum	1.00030 (0.00004)	1.00018-1.00042
Hyperbaric solutions^a
Lidocaine 5% in dextrose 7.5%	1.02650	1.01300-1.0142
Tetracaine 0.5% in dextrose 5%	1.0136 (0.0002)	1.01300-1.0142
Bupivacaine 0.5%-0.75% in dextrose 8.25%	1.02426 (0.00163)	1.01935-1.029131
Chloroprocaine 3%	1.00257 (0.00003)	1.00248-1.00266
Hypobaric solutions^b
Lidocaine 0.5% in water	0.99850	Hypobaric
Bupivacaine 0.35% in water	0.99730	Hypobaric
Tetracaine 0.2% in water	0.99250	Hypobaric
Bupivacaine 0.5%	0.99944 (0.00012)	0.99908-0.99980
Isobaric solutions (plain)
Lidocaine 2%^c	1.00004 (0.0006)	0.99986-1.00022
Tetracaine 0.5%^d	1.0000 (0.0004)	0.99880-1.00120
SD, standard deviation.
^aLocal anesthetic solutions with a baricity of >1.0015 can be expected to predictably behave in a hyperbaric manner. ^bLocal anesthetic solutions with a baricity of <0.9990 can be expected to predictably behave in a hypobaric manner. ^cMay act in an isobaric or hypobaric manner depending on patient population. ^dTetracaine 1% diluted 1:1 with 0.9% saline.