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Spinal anesthesia: Technique

Spinal anesthesia: Technique
Literature review current through: Jan 2024.
This topic last updated: May 17, 2023.

INTRODUCTION — Spinal anesthesia is a type of neuraxial anesthesia; local anesthetic (LA) is injected into cerebrospinal fluid (CSF) in the lumbar spine to anesthetize nerves that exit the spinal cord. Spinal anesthesia is most commonly used for anesthesia and/or analgesia for a variety of lower extremity, lower abdominal, pelvic, and perineal procedures. Spinal anesthesia is also occasionally used for spine surgery.

This topic will discuss the relevant anatomy, techniques, and management of spinal anesthesia. Indications, contraindications, preoperative evaluation, physiologic effects, and complications of spinal anesthesia are discussed separately. (See "Overview of neuraxial anesthesia", section on 'Physiologic effects of neuraxial anesthesia'.)

Techniques for epidural and combined spinal-epidural (CSE) are discussed separately. (See "Epidural and combined spinal-epidural anesthesia: Techniques".)

Spinal anesthesia techniques for labor and delivery are also discussed separately. (See "Neuraxial analgesia for labor and delivery (including instrumental delivery)" and "Anesthesia for cesarean delivery", section on 'Spinal anesthesia'.)

ANATOMY — Spinal anesthesia is performed by placing a needle between the lumbar vertebrae and through the dura to inject anesthetic medication. Anatomy of the bony spine and vertebrae are discussed in more detail separately (figure 1 and figure 2). (See "Spinal column injuries in adults: Types, classification, and mechanisms", section on 'Anatomy'.)

Anatomy related specifically to the performance of spinal anesthesia is discussed here.

Vertebral level Spinal anesthesia is performed no higher than the mid to low lumbar vertebral level to avoid puncturing the spinal cord with the spinal needle. In most patients, the spinal cord terminates as the conus medullaris at the lower border of the first lumbar vertebral body (L1), though it may end lower [1]. Therefore, the spinal needle is inserted at the L3 to L4 or L4 to L5 interspace. Other interspaces, eg L2 to 3 or L5 to S1, may be less ideal; L2 to 3 is close to the spinal cord and injection at L5 to S1 requires greater spread of local anesthetic (LA) to anesthetize thoracic dermatomes, which is necessary for some surgical procedures.

The intercristal line (ie, the line between the posterior superior iliac crests) is used as a rough guide for spinal needle placement. In many patients, this line crosses the body of L4, though it may cross the spine from L1/L2 to L4/L5, and tends to be higher in females and in patients with obesity [2,3]. Because landmarks do not accurately predict the lumbar interspace, the spinal needle should be inserted at or below the intercristal line.

Ligaments – The epidural space is the space between the dural sac and the inside of the bony spinal canal. The tough ligamentum flavum forms the posterior border of the epidural space at each interlaminar space. The interspinous ligament stretches between the spinous processes of successive vertebrae, and the supraspinous ligament anchors the tips of the spinous processes in a continuous column (figure 3).

Meninges – Within the bony vertebral canal, the spinal cord is surrounded by three membranes: the pia mater, the arachnoid mater, and the dura mater (innermost to outmost). The dura and arachnoid maters loosely adhere to each other in the spinal canal and comprise the "dural sac" in which the spinal cord is suspended. The subarachnoid space within the dural sac is located between the pia and arachnoid maters and contains cerebrospinal fluid (CSF), spinal nerves, and blood vessels. A loose trabecular network exists between the pia and dura-arachnoid.

CSF – The central nervous system (CNS) is surrounded by CSF, an ultrafiltrate of blood. CSF is formed continuously by the choroid plexuses and serves to protect the brain and spinal cord by providing a cushion. It also serves as a conduit for the delivery of spinal anesthetic agents to the spinal cord. CSF circulates in the spinal canal, with both bulk flow and oscillatory movements. This flow may explain some of the movement of anesthetic agents toward the brain after injection into the lumbar subarachnoid space. The density of CSF at body temperature averages 1.0003±0.0003 g/mL; density relative to the injected LA solution affects the distribution of spinal block [4]. (See 'Baricity' below.)

Nerves – The dorsal and ventral spinal nerve roots emerge from the spinal cord at each vertebral level and combine to form the spinal nerves. The lumbar and sacral nerve roots extend past the conus medullaris as the cauda equina and exit the vertebral canal between successive lumbar and sacral vertebrae (figure 4).

Autonomic nerves, both sympathetic and parasympathetic, are blocked by spinal anesthesia, in addition to sensory and motor nerves. The extent of sympathetic block depends on the height of the spinal block. Preganglionic sympathetic nerve fibers arise from the intermediolateral tract of the spinal cord from T1 to L2. The axons exit the spinal cord with the ventral nerve root of the spinal nerves and synapse with cell bodies in the ganglion of the sympathetic trunk. Sympathetic cardioaccelerator fibers arise from T1 to T4.

The splanchnic component of the parasympathetic nervous system travels in the ventral roots of S2 to S4 and supplies the organs of the pelvis, including the bladder, reproductive organs, descending colon, and rectum. Thus, all of the sacral parasympathetic nerves are blocked with spinal anesthesia, and some of the sympathetic nerves are blocked, depending on the cephalad extent of neural block.

Dermatomes – A dermatome is defined as the cutaneous area supplied by a single spinal nerve root (figure 5). The term "spinal level" refers to the most cephalad dermatome anesthetized by the spinal anesthetic. The level required for a specific surgery is determined by the dermatome level of the skin incision and by the level required for surgical manipulation; these two requirements may be very different. As an example, a low abdominal incision for cesarean delivery is made at the T11 to T12 dermatome, but a T4 spinal level is required to prevent pain with peritoneal manipulation. The sensory and visceral spinal levels required for common surgical procedures are shown in tables (table 1A-B).

Surface landmarks that correspond to dermatome levels include the following:

Inguinal ligament – T12

Umbilicus – T10

Tip of xiphoid process – T7

Nipple – T4

Fifth finger – C8

PREOPERATIVE EVALUATION — A medical history and anesthesia-directed physical examination should be performed for all patients who undergo any type of anesthesia; when spinal anesthesia is planned, the lower back should be examined.

When considering spinal anesthesia, we focus the preoperative evaluation on those medical conditions that may alter the physiologic response to spinal anesthesia or increase the risk of complications. These issues are discussed separately. (See "Overview of neuraxial anesthesia", section on 'Preoperative evaluation'.)

TECHNIQUE

Preparation for spinal anesthesia — For spinal anesthesia, the standard and emergency anesthesia equipment and medications should be prepared as they would be for general anesthesia. (See "Induction of general anesthesia: Overview", section on 'Preparation for anesthetic induction'.)

Most commonly, a disposable spinal kit is used, which contains needles, drugs, labels, and other required equipment. The kit is usually placed on a cart or table on the side of the clinician's dominant hand.

We prepare a phenylephrine infusion and a syringe of ephedrine (5 mg/mL) preoperatively; a high percentage of patients require administration of a vasoconstrictor during spinal anesthesia. Anticholinergic medication (ie, atropine and glycopyrrolate) and epinephrine should be immediately available.

Standard American Society of Anesthesiologists (ASA) monitors should be applied (ie, blood pressure, electrocardiography, oxygen saturation) prior to initiation of spinal anesthesia. Other monitoring (eg, intraarterial pressure monitoring) and the extent of venous access are dictated by the patient's medical status and the planned procedure.

A preprocedure time-out should be performed, which includes confirmation of the following:

Patient identifiers (ie, name, medical record number, date of birth)

Allergies

Planned surgical procedure

Surgical and anesthesia consent with site marked, if applicable

Coagulation status (ie, anticoagulant administration, coagulation laboratory values if applicable)

With the exception of discussion of the patient's coagulation status, these checklist components are a part of the World Health Organization (WHO) Surgical Safety Checklist components performed before the induction of anesthesia.

Premedication — Light sedation may be administered if necessary prior to spinal needle placement. Deep sedation should be avoided in order to allow patient cooperation with positioning and feedback (eg, the occurrence of pain or paresthesia) during the procedure. Any sedation should be administered in reduced doses, titrated to effect, anticipating the sedation that accompanies spinal block. (See "Overview of neuraxial anesthesia", section on 'Central nervous system'.)

Positioning for spinal procedure — Optimal patient positioning is critical to the success of neuraxial procedures. In this setting, the goals of positioning are to avoid rotation of the spine and to create a straight path for needle insertion between the bones of the spine (figure 6). Flexion of the spine opens the space between the spinous processes and is most important when a midline approach is used. (See 'Approaches' below.)

The sitting and lateral decubitus positions are used most commonly and are chosen according to clinician preference, planned patient position during the surgical procedure, patient body habitus, and patient comfort (figure 7). As an example, the lateral position may be used for a patient with a hip fracture, with the fracture side up. The jackknife position (ie, prone with hips flexed) is occasionally used for spinal anesthesia placement when this will be the position for surgery (eg, for rectal procedures).

The clinician can stand or sit to perform the procedure; the table height and patient position should be adjusted to prevent the need for the clinician to bend or reach.

Sitting position – The sitting position is most common and may be particularly useful for larger patients in whom the bony landmarks are difficult to feel; if the patient sits straight and level, the midline may be easier to estimate compared with the lateral position. The sitting position is usually preferred for very low sacral spinal anesthesia with hyperbaric local anesthetic (LA) drugs (ie, saddle block). (See 'Baricity' below.)

In the sitting position, the patient's legs should hang off the side of the bed, with the feet planted on a nonmobile stool. The backs of the knees should be against the edge of the bed so that the patient's back is as close as possible to the clinician standing on the other side of the bed. The patient is asked to "slouch" symmetrically with shoulders over the hips to allow flexion of the lumbar spine. We usually have the patient sit with the arms bent at the elbows with forearms and hands resting lightly on the thighs. Alternatively, the patient can rest the arms on a padded Mayo stand; the patient should not lean forward, which would increase the lumbar lordosis. In addition, positioning aids are available that are designed to support the patient in the sitting position during neuraxial block (figure 8).

Lateral decubitus position – Either left or right lateral decubitus position can be used and may be chosen based on the baricity of the spinal drug preparation. As an example, if hyperbaric medication is used for a left leg procedure, the patient should be positioned with the left side down. If baricity relative to surgical position is not a concern, the left lateral position is usually more comfortable for a right-handed clinician.

The patient's back should be close to the edge of the table nearest the clinician, parallel to the edge of the bed and vertical, with the hips aligned one above the other. The thighs are drawn up with the hips maximally flexed, and the patient is asked to "push out" or "round" the lower back. Patients instructed to "curl up" often actually curl their upper back, and since the bottom shoulder is fixed to the mattress, this results in rotation of the spine. The dependent shoulder may need to be pulled forward by an assistant. Patients with broad hips may require a pillow under and between the legs in order to prevent forward tilting.

Prone position – This position is occasionally used for spinal anesthesia for rectal or pilonidal surgery. The patient is positioned on chest supports, with the hips flexed (ie, the jackknife position). In this setting, hypobaric or isobaric spinal medication is used to primarily anesthetize low lumbar and sacral nerve roots. (See 'Baricity' below.)

Preprocedure ultrasonography — We routinely use preprocedural ultrasonography in patients whose anatomic landmarks are difficult to palpate. Preprocedural ultrasonography is increasingly used to help define anatomy prior to spinal anesthesia. Ultrasound can be used to identify the interspace for needle placement, and to estimate the depth of the subarachnoid space, particularly in patients with obesity or with difficult anatomy [5-12]. A curved array low-frequency (2 to 5 MHz) is commonly used and the back is scanned in both the parasagittal and transverse planes [13]. The midline and lumbar interspinous spaces are identified and marked on the back, the depth to the subarachnoid space is estimated, and the spinal needle is inserted in the marked space.

A 2020 systematic review and meta-analysis of preprocedural ultrasound use before attempted neuraxial anesthesia compared with the traditional palpation technique in obstetric patients (18 studies, 1844 patients) found that ultrasound use was associated with increased first needle pass rate success in patients with perceived difficult anatomy, but not in patients whose surface landmarks were easy to palpate [14]. Use of ultrasonography was associated with longer time to identify the interspace, but there was no difference in procedural time or failure rate between landmark and ultrasound techniques.

Studies have shown that use of ultrasound is associated with more accurate identification of the lumbar interspace than palpation [15]. Large studies are needed to determine whether this improves the safety of spinal anesthesia. (See "Ultrasound guidance for neuraxial anesthesia techniques".)

Aseptic technique — Strict aseptic technique is required for all aspects of the spinal anesthesia procedure. In 2017, the ASA published a Practice Advisory for the Prevention, Diagnosis, and Management of Infectious Complications Associated with Neuraxial Techniques [16]. We agree with the critical components of the Practice Advisory, including the following:

The clinician should:

Wear a cap and mask, covering mouth and nose.

Remove jewelry, including rings and watches.

Wash hands prior to the procedure.

Wear sterile gloves.

The skin of the patient's back should be:

Widely cleaned using individual antiseptic packets of chlorhexidine, preferably with alcohol, allowing adequate time for the solution to dry, according to the antiseptic package insert. The skin prep solution should be discarded before opening the spinal tray and preparing the drug solutions. Contamination of equipment with the prep solution must be avoided to prevent introduction of neurotoxic solution into the subarachnoid space. (See "Lumbar puncture: Technique, contraindications, and complications in adults", section on 'Aseptic technique'.)

Draped with a sterile drape.

At the author's institution, additional precautions include:

Change masks between cases

In addition to removing jewelry, remove false fingernails

Wash hands with alcohol solution or perform surgical scrub

All providers in the immediate environment wear a cap and mask

The patient wears a cap

Clean the tops of nonsterile drug ampules with alcohol or chlorhexidine

Choice of spinal needle — We use a small-diameter (ie, 24 to 27 gauge) pencil-point spinal needle with an introducer needle for spinal anesthesia. Larger-diameter needles and cutting needle tips [17,18] are both associated with an increased risk of postdural puncture headache (PDPH) compared with smaller-diameter and pencil-point needles [19]. (See "Overview of neuraxial anesthesia", section on 'Postdural puncture headache'.)

Disposable needles are used for spinal anesthesia. All spinal needles have a close-fitting, removable stylet that prevents skin and subcutaneous tissue from plugging the needle during insertion. Spinal needles are available in various gauges (ie, needle diameter) and with several needle tip shapes, as well as various lengths for use in larger patients. The commonly used needles and available sizes are shown in the table (table 2).

Spinal needles can be classified according to the needle tip, as follows (figure 9):

Cutting-tip needles – Quincke spinal needles have sharp cutting tips, with the hole at the end of the needle.

Pencil-point needles – Whitacre, Sprotte, and other similar needles have a closed tip shaped like that of a pencil, with the hole on the side of the needle near the tip. These needles are designed to minimize leak of cerebrospinal fluid after puncture and reduce the chance of PDPH.

An introducer needle is usually used to place the higher-gauge (ie, smaller-diameter, 24 to 27 gauge) spinal needles. These fine needles are very flexible and more difficult to direct through tissue than lower-gauge (greater-diameter) needles. The introducer needle is used to puncture the skin and is inserted into the interspinous ligament, when a midline approach is used, along the desired needle path. The spinal needle is then inserted through it.

A stiffer, larger-diameter spinal needle (ie, 22 gauge) may be required for older patients, in whom spinal needle placement may be more challenging. These patients are often unable to flex the lumbar spine and may have calcified ligaments that make the use of fine needles difficult or impossible. In addition, older patients are at lower risk for PDPH than younger patients regardless of spinal needle size [20]. In this setting, we try a 24 or 25 gauge pencil-point needle and use a 22 gauge needle if necessary.

Approaches — The spinal needle can be inserted using a midline or a paramedian approach. The midline approach is simpler; the paramedian approach requires mental triangulation and an estimate of the depth from the skin to the dura. However, the midline approach may be unsuccessful in patients who are unable to flex the spine to open the space between adjacent spinous processes (figure 6). Paramedian needle placement requires less spinal flexion.

For either approach, the needle is usually inserted at the L4 to L5 or L3 to L4 interspace. In most patients, a line between the iliac crests crosses the body of L4 or the L4 to L5 interspace. (See 'Anatomy' above.)

Midline approach technique — The aim is to place the spinal needle precisely in the midline, which is usually defined by the spinous processes with optimal positioning. The procedure is performed as follows:

Palpate the interspace between two spinous processes at the chosen spinal level. With a 25 gauge needle, raise a skin wheal with 1% lidocaine, in the midline of the spine, in the lower third of the interspace. Infiltrate the subcutaneous tissue with lidocaine into the interspinous ligament.

Insert an introducer needle at a slight cephalad angle, through the supraspinous ligament, until slightly firm tissue is felt, suggesting that the tip is in the interspinous ligament (figure 3).

Insert the 24 to 27 gauge spinal needle into and through the introducer needle. The needle passes through the ligamentum flavum, followed by the epidural space, and then the dura-arachnoid. Changes in resistance are felt as the needle passes through each of these layers. A pop is often felt when the dura is pierced. The depth of the dura from the skin in patients of normal body habitus is 5.1±1.0 cm [21].

After a pop or loss of resistance is appreciated, remove the stylet and watch for cerebrospinal fluid (CSF) flow into and through the hub of the needle. CSF flow may be very slow through fine-gauge needles, particularly if the patient is in the lateral position.

If CSF does not appear, rotate the needle (only if pencil-point needle) and, if necessary, adjust the needle tip (eg, advance, redirect, or withdraw the needle and repeat the procedure).

If the needle hits bone during spinal needle placement, note the depth, withdraw the needle into the introducer, redirect and reinsert in a slightly cephalad direction. If bone is contacted again, redirect as follows (figure 10):

If bone is contacted at a shallower depth, the needle is likely contacting the more cephalad spinous process in the interspace, and should then be redirected in a caudad direction.

If bone is contacted deeper, the needle is likely contacting the more caudad spinous process in the interspace, and the needle should then be redirected in an even more cephalad direction.

If bone is contacted at the same depth, the needle is likely contacting lamina, and the tip is off the midline. The midline should be reassessed, and the needle redirected accordingly. Patients can often report that they perceive the needle to one side or the other.

If using a 20 or 22 gauge spinal needle, perform the procedure in the same way without an introducer needle.

Paramedian approach technique — The paramedian approach is often used for patients who are unable to flex the spine, or when the midline approach fails (figure 11).

Palpate the interspace between two spinous processes at the chosen spinal level. With a 25 gauge needle, raise a skin wheal with 1% lidocaine one centimeter lateral to the midline at the cephalad border of the inferior spinous process. Infiltrate the subcutaneous tissue with lidocaine.

Insert the introducer needle at a 10 to 15 degree angle from the midline in a slightly cephalad direction (ie, 75 to 80 degrees from skin). The aim is to puncture the dura in the midline. With this approach, the introducer does not pass through the supraspinous or interspinous ligaments.

Insert a 24 to 27 gauge spinal needle through the introducer needle and advance through the tough ligamentum flavum until a pop is felt as the needle pierces the dura.

Remove the stylet and watch for CSF flow into and through the hub of the needle. CSF flow may be very slow through the very fine-gauge needles.

If CSF does not appear, rotate the needle and, if necessary, adjust the needle tip (eg, advance, redirect, or withdraw the needle and repeat the procedure).

If the needle hits bone, it is most likely vertebral lamina. Withdraw the needle into the introducer, redirect slightly, and advance the spinal needle again. Several attempts at passing the needle may be required.

If using a 20 or 22 gauge spinal needle, perform the procedure in the same way without an introducer needle.

Spinal injection — Once CSF flows to the end of the spinal needle, the LA solution is injected.

Stabilize the spinal needle to prevent movement while connecting the LA syringe to the hub of the spinal needle. We place the lateral edge of the nondominant hand against the patient's back, using the index finger and thumb to hold the needle hub while connecting the syringe (figure 12).

After securely connecting the LA syringe, aspirate approximately 0.2 mL of CSF into the syringe to confirm subarachnoid placement of the needle tip.

Inject the LA solution slowly, at a rate of ≤0.5 mL/second.

Although the practice is not evidence-based, many clinicians aspirate approximately 0.2 mL after completing the injection to confirm that the needle tip remained correctly positioned through injection. Other clinicians aspirate a small amount of CSF halfway through the injection, reasoning that it is too late to reposition the needle if the need to do so is discovered at the end of the injection. If CSF is aspirated, it should be reinjected.

We aspirate during injection only if the patient or needle obviously moved during the procedure.

Remove the introducer, spinal needle, and syringe as one unit.

Post-injection positioning — The timing and specifics of patient positioning after anesthetic injection depend on the required level of block, the baricity of the spinal solution, and the position used for the spinal injection. (See 'Baricity' below.)

Hyperbaric spinal drugs – If a hyperbaric solution is injected for a perineal surgical procedure, the patient should be left sitting for several minutes to allow the anesthetic solution to sink to the sacral nerve roots to achieve what is called a "saddle block." In contrast, if a high thoracic spinal level is required, the patient should be placed in the supine position immediately after injection. The tilt of the operating table should be adjusted based on the level of the block. (See 'Assessment of level of block' below.)

Isobaric or hypobaric drug – If a plain (slightly hypobaric) solution is injected in the sitting position, or in the lateral position for bilateral surgery, the patient should be placed supine immediately after injection. If a plain solution is injected in the lateral position for surgery on the nondependent side (eg, hip replacement), the patient should remain in the lateral position and positioned for surgery. (See 'Baricity' below.)

Continuous spinal — Spinal anesthesia is usually initiated as a single-shot technique. However, occasionally, clinicians site a catheter in the subarachnoid space to enable prolongation of anesthesia by the intermittent injection of local anesthetic solution. Most clinicians use an epidural needle and catheter for continuous spinal anesthesia. The epidural needle is advanced as described for spinal anesthesia. After the needle tip has punctured the dura, the epidural needle stylet is removed and the epidural catheter is inserted through the needle and advanced 2 to 4 cm into the subarachnoid space. The epidural needle is carefully withdrawn without moving the catheter, the epidural catheter connector is secured to the catheter, and CSF is allowed to backflow through the catheter to the hub of the epidural catheter connector (this is important so that the air in the dead space of the catheter is not injected into the subarachnoid space). The dead space of an epidural catheter is approximately 0.3 mL, and this volume must be taken into account when injecting the spinal anesthetic solution. Drugs used for continuous spinal anesthesia mimic those used for single-shot spinal anesthesia. (See 'Choice of spinal drugs' below.)

Advantages of the continuous spinal technique include the option to titrate the initial dose, and the maintenance of anesthesia for a prolonged period. We usually decrease the initial dose to one-half to two-thirds of the usual single-shot dose and then titrate to effect. Additional doses of anesthetic solution can be injected as needed to prolong the block.

The use of continuous spinal anesthesia has fallen out of favor because of reports of neurologic complications, including cauda equina syndrome when LA was administered through small-gauge catheters (28 gauge, so-called microcatheters) for continuous spinal anesthesia [22], and because of the high risk of PDPH using larger-gauge catheters. A large retrospective study of continuous spinal anesthesia reported a 33.1 percent incidence of PDPH in obstetric patients who received continuous spinal with a microcatheter (28 gauge) [23].

A continuous spinal catheter is now commercially available in the United States, though experience with its use is limited [24,25].

Choice of spinal drugs — For single-shot spinal anesthesia, LAs and adjuvant medications must be chosen to achieve the required spinal level and duration of anesthesia and recovery. Particularly for ambulatory procedures, time to micturition and ambulation are critical to a satisfactory perioperative experience. The most important determinants of the extent of sensory block are the dose and baricity (relative to the patient's position) of the anesthetic solution. Less important variables include patient age, body mass index, orientation of the pencil-point spinal needle orifice, and the angle of the needle relative to the neuraxis [26].

Baricity — Baricity refers to the ratio of the density of a solution to the density of cerebrospinal fluid (CSF). Solutions with the same density as CSF have a baricity of 1 and are called isobaric. Solutions more dense than CSF are called hyperbaric; those less dense than CSF are called hypobaric. The baricity of LA solutions used clinically are shown in a table (table 3).

Baricity influences the distribution of anesthetic solution within the subarachnoid space; hyperbaric solutions tend to "sink" within the CSF relative to the site of injection, and hypobaric solutions rise relative to the site of injection, while gravity has no effect on the distribution of isobaric solutions in CSF. Thus, the patient's position during and after injection of spinal drugs influences the extent of spinal block with hyperbaric or hypobaric drugs.

The baricity of the anesthetic solution, along with the patient's position, can be used to increase the density and duration of spinal block at the site of surgery. As an example, a saddle block (ie, low lumbar and sacral nerve block) can be achieved by injecting a hyperbaric solution in the sitting position and keeping the patient sitting for several minutes. Conversely, spinal anesthesia for rectal surgery can be performed with a hypobaric solution injected with the patient in a jackknife position.

LA solutions are made hyperbaric by adding dextrose to LA (eg, bupivacaine 0.75% in 8.25% dextrose is commercially available). A hypobaric solution can be mixed by adding sterile water to a plain solution of LA (eg, bupivacaine 0.5% in saline). LA solutions in saline are slightly hypobaric but behave and are used clinically as if they are isobaric.

In general, hyperbaric solutions result in faster onset, greater extent of sensory block, and shorter duration of block compared with isobaric solutions [27]. A double-blinded study of duration and extent of spinal anesthesia that randomized patients to receive hyperbaric drug (0.5% bupivacaine with 5 or 8% glucose) or minimally hypobaric drug (plain 0.5% bupivacaine) found that the hyperbaric solutions produced greater cephalad spread of block compared with the hypobaric drug (ie, T6 versus T10) [28].

In our practice, we use hyperbaric solutions when the desired sensory level is higher than T10 (eg, cesarean delivery or postpartum tubal ligation), and we use isobaric (plain) solutions when a T10 sensory level is sufficient (eg, total knee or hip arthroplasties).

Local anesthetics — Bupivacaine is the most commonly used LA for spinal anesthesia in the United States. Others include lidocaine, tetracaine, ropivacaine, and 2-chloroprocaine. Not all of the clinically used formulations of LAs have been approved for intrathecal injection by the US Food and Drug Administration (FDA), though they have a record of safety when used for spinal anesthesia. Only preservative-free preparations of LAs and other adjuvants should be used for spinal anesthesia (table 3).

All LAs are neurotoxic in high concentrations. However, when mixed with preservative-free water, saline, or dextrose solutions to achieve the concentrations used clinically, neurotoxicity is rare.

Studies of the clinical characteristics of spinal LAs usually measure two-dermatome regression of sensory blockade as an indicator of duration of surgical anesthesia. Time to two dermatome regression is approximately the same as the duration of surgical anesthesia; the density of neuroblockade wanes from the corresponding surgical dermatomes simultaneously with regression of cephalad sensory blockade. By the time the cephalad extent of sensory blockade has regressed two dermatomes, the density of neuroblockade at the site of surgery is inadequate for surgical anesthesia.

Bupivacaine – Bupivacaine is a long-acting amide LA. Bupivacaine is our LA of choice for spinal anesthesia for cesarean delivery, lower extremity joint arthroplasty, and urologic and gynecologic pelvic procedures. Although low-dose bupivacaine (eg, 4 to 6 mg) has been described for outpatient anesthesia, the prolonged recovery time relative to shorter-acting LAs or general anesthesia makes bupivacaine less desirable for ambulatory surgery.

Dose – Doses of bupivacaine range from 6 to 15 mg (eg, cesarean delivery 12 mg; lower extremity joint arthroplasty 12.5 to 15 mg)

Duration of surgical anesthesia – 1.5 to 2.5 hours

Available formulations – Hyperbaric 0.75% in 8.25% dextrose, slightly hypobaric 0.5% in saline

Tetracaine – Tetracaine is a long-acting ester LA that was commonly used several decades ago. Although tetracaine is still used in some centers, it has largely been replaced with bupivacaine.

Dose – 5 to 20 mg

Duration of surgical anesthesia/two-dermatome regression – 1.5 to 2.5 hours

Available formulations – 1% solution, slightly hypobaric

Lidocaine – Lidocaine is a short-acting amide LA. Intrathecal lidocaine is associated with a high incidence of transient neurologic symptoms; thus, its use for spinal anesthesia has fallen out of favor. However, given its short duration of action and rapid recovery, it is still a useful drug in the ambulatory setting [29]. We occasionally use plain lidocaine (ie, diluted in saline) for spinal anesthesia for short-duration procedures (ie, less than one hour), understanding the increased risk for transient neurologic symptoms.

Dose – 40 to 100 mg

Duration of surgical anesthesia – 45 to 75 minutes

Available formulations – Hyperbaric 5% in 7.5% dextrose, slightly hypobaric 2%

Mepivacaine – Mepivacaine is a slightly longer-acting amide LA compared with lidocaine. Similar to lidocaine, mepivacaine is associated with a relatively high incidence of transient neurologic symptoms [29]. (See "Overview of neuraxial anesthesia", section on 'Transient neurologic symptoms'.)

Dose – 50 to 70 mg

Duration of surgical anesthesia – 45 to 75 minutes

Available formulations – Slightly hypobaric 1%, 1.5%, and 2%

Ropivacaine – Ropivacaine is a pure L-enantiomer amide LA. Outside the United States, ropivacaine is used commonly for spinal anesthesia. It is approximately 40 percent less potent than bupivacaine [30] and may achieve a lower motor to sensory block ratio compared with bupivacaine (ie, less dense motor block relative to the density of sensory block).

In our opinion, there are no advantages to using ropivacaine rather than bupivacaine for spinal anesthesia.

Dose – 15 to 20 mg

Duration of surgical anesthesia – 75 to 120 minutes

Available formulations – Slightly hypobaric 0.5 to 1% in saline

2-Chloroprocaine 2-Chloroprocaine is a short-acting ester LA, usually used for short-duration gynecologic, urologic, orthopedic and general surgery procedures, particularly ambulatory procedures [31-35]. Early concerns of neurotoxicity have been attributed to the preservative sodium bisulfite. In 2018, the US FDA approved 1% 2-chloroprocaine for intrathecal use. Preservative-free 2% and 3% 2-chloroprocaine solutions have been used off label in the United States and in Europe [32,35].

Dose – 20 to 60 mg [32,35,36]

Duration of surgical anesthesia – 30 to 50 minutes

Available formulations – 1% (isobaric), 2%, and 3% (slightly hyperbaric) in saline [37]

Prilocaine - Prilocaine is an amide LA with rapid onset and intermediate duration of action which is not approved for spinal use in the United States, but may be used for spinal anesthesia elsewhere. It is an alternative to lidocaine and mepivacaine, with the advantage of a lower incidence of transient neurologic symptoms (1 to 2 percent) [38-41].

Dose – 30 to 60 mg

Duration of surgical anesthesia – 60 minutes

Available formulations – Hyperbaric 2% in saline (available in Europe)

Adjuvants — Several classes of medications can be added to the LA solution to improve or change the distribution of the block.

DextroseLidocaine and bupivacaine are commercially prepared with dextrose in order to make the solutions hyperbaric. In the absence of added dextrose, plain LA solutions are slightly hypobaric, but are used clinically as if they were isobaric.

Opioids Opioids may be added to the LA solution used for spinal anesthesia, to improve intraoperative analgesia, and for postoperative analgesia. The addition of an opioid is particularly helpful for blocking the discomfort of visceral manipulation (eg, manipulation of the uterus during cesarean delivery). A meta-analysis including over 800 patients who had spinal anesthesia for cesarean delivery with and without various intrathecal opioids found the need for intraoperative analgesia supplementation decreased from 24 to 4 percent with the addition of opioid to the LA [42].

Morphine – Morphine is a hydrophilic opioid that can be added to LA primarily to provide postoperative analgesia and is commonly used for cesarean delivery. With doses from 75 to 200 mcg, onset of analgesia is within 30 to 60 minutes, with a duration of 12 to 36 hours [42]. Common side effects include nausea, vomiting, and pruritus. The duration of analgesia and incidence of pruritus appear dose-related with a ceiling effect. A less common but serious side effect is delayed respiratory depression (6 to 18 hours after the intrathecal injection). Intrathecal morphine is widely used as a component of multimodal analgesia following cesarean delivery as well as other abdominal surgeries and lower extremity joint surgery.

The use of neuraxial morphine and patient monitoring are discussed in more detail separately. (See "Continuous epidural analgesia for postoperative pain: Technique and management", section on 'Monitoring during epidural analgesia' and "Post-cesarean delivery analgesia", section on 'Hydrophilic opioids (morphine and hydromorphone)'.)

Fentanyl and sufentanil Lipid-soluble opioids such as fentanyl (10 to 25 mcg) and sufentanil (2.5 to 10 mcg) are commonly added to intrathecal LA to improve intraoperative analgesia [43], and for labor analgesia. For example, we routinely add fentanyl 15 mcg to bupivacaine for cesarean delivery anesthesia. Sufentanil is less commonly used in the United States than in Europe because the concentrated solution (50 mcg/mL) makes accurate measurement of low doses difficult. Lipid-soluble opioids provide minimal postoperative analgesia because of short duration of action and are not associated with delayed respiratory depression. Pruritus is a common, dose-related side effect of intrathecal fentanyl and sufentanil [44].

Alpha adrenergic agonists – The addition of an alpha-adrenergic agonist (eg, epinephrine, clonidine) to local anesthesia increases the density of sensory and motor block, prolongs the duration of action, and may contribute to postoperative analgesia by acting at alpha-adrenergic receptors in the spinal cord.

Epinephrine – We add 100 to 200 mcg of epinephrine to bupivacaine when an additional 30 minutes of spinal anesthesia is desirable. Epinephrine may augment spinal anesthesia by its direct alpha-adrenergic agonist activity on central nervous system receptors and by causing vasoconstriction, leading to decreased vascular absorption and subsequent increased neural tissue uptake of the LA.

A meta-analysis including 1271 patients randomized to receive spinal anesthesia with and without epinephrine found a weighted mean increase of 35 minutes in the duration of sensory block with epinephrine [45]. Epinephrine doses of 100 mcg or less prolonged sensory and motor block but were associated with a greater incidence of hypotension and postoperative nausea and vomiting (PONV) compared with no epinephrine. Doses between 100 and 200 mcg prolonged sensory and motor block without an increase in hypotension or PONV compared with no epinephrine. A potential mechanism for this differential effect between low and higher dose epinephrine is a differential effect on beta- and alpha-adrenergic receptors at varying doses.

Clonidine Clonidine is not routinely used in the United States for spinal anesthesia, but it may be used for selected patients who require prolonged anesthesia/analgesia or when other adjuncts cannot be used.

A meta-analysis including 1445 patients randomized to receive spinal anesthesia with and without the addition of clonidine (15 to 150 mcg) found a linear, dose-dependent prolongation of time until two-segment regression of sensory block, with an increase in mean duration of block from 14 to 75 minutes compared with no clonidine [46]. Clonidine was associated with more episodes of intraoperative hypotension (relative risk [RR] 1.81).

Phenylephrine – Phenylephrine has been used as an adjunct to spinal LAs; its mechanism of action and effects are similar to those of epinephrine. However, we do not add phenylephrine to spinal drugs because of its association with transient neurologic symptoms [47,48].

Magnesium Magnesium sulfate is an antagonist of the N-methyl-D-aspartate (NMDA) receptor and thus may modify nociceptive modulation. Small studies have shown that the addition of magnesium to intrathecal LA/opioid solutions prolongs the duration of spinal anesthesia [49]. However, safety data are limited (ie, regarding neurotoxicity). We believe that further study is required before the routine intrathecal use of magnesium can be recommended.

MANAGEMENT AFTER SPINAL INJECTION — After spinal injection, the patient must be monitored as for general anesthesia, and the adequacy of the spinal block assessed.

Hemodynamic management — Hemodynamic changes are common after spinal anesthesia; blood pressure (BP) and heart rate must be monitored closely and changes treated promptly. (See "Overview of neuraxial anesthesia", section on 'Cardiovascular'.)

BP

Monitoring – BP should be measured frequently after spinal injection to allow rapid treatment of hypotension with fluids and vasopressors as needed. We measure BP every 2.5 minutes for the first 20 minutes, and every 2.5 to 5 minutes thereafter, depending on hemodynamic stability and vasopressor requirement.

Goal BP – The optimal goal for BP depends on the clinical setting and patient medical status. As examples, for cesarean delivery, we aim to keep systolic BP close to baseline, while for a young healthy patient having lower extremity surgery, a systolic BP 80 percent of baseline would likely be safe and well-tolerated.

Prophylaxis for hypotension – A practical strategy for avoiding hypotension includes a rapid bolus of crystalloid at the time of induction/neuraxial placement (co-load), in conjunction with administration of vasopressors as needed. Co-loading fluids (either crystalloid or colloid) has been shown to be as effective at minimizing hypotension for cesarean delivery as preloading colloid, and better than preloading crystalloid [50-52].

Depending on the hemodynamic goals, vasopressors may be administered prophylactically (if the goal is to maintain BP at baseline) or for treatment of hypotension (if the goal is the maintain BP greater than 80 percent baseline).

Treatment of hypotension – Hypotension can be treated with intravenous (IV) fluid administration and with vasopressor administration (eg, ephedrine 5 to 10 mg IV boluses, phenylephrine bolus of 40 to 160 mcg IV or infusion of 20 to 200 mcg/minute IV). Limited data suggest that norepinephrine may be beneficial for the prevention and treatment of spinal hypotension during cesarean delivery. This is discussed separately. (See "Anesthesia for cesarean delivery", section on 'Vasopressors'.)

Bradycardia Bradycardia associated with hypotension should be treated promptly with atropine (0.4 to 0.6 mg IV) or glycopyrrolate (0.2 to 0.4 mg IV), ephedrine (5 to 10 mg IV, repeated as necessary up to 25 to 50 mg IV), and if necessary epinephrine (5 to 10 mcg IV).

For severe bradycardia or cardiac arrest, full resuscitation doses of epinephrine should be administered promptly (100 mcg to 1 mg IV) and advanced cardiac life support (ACLS) protocols followed. (See "Overview of neuraxial anesthesia", section on 'Cardiovascular'.)

Hemodynamic management during spinal anesthesia for cesarean delivery is discussed in more detail separately. (See "Anesthesia for cesarean delivery", section on 'Hemodynamic management'.)

Assessment of level of block — A sensory level may be detectable as soon as two to three minutes after injection of the spinal anesthetic. We usually begin testing approximately three to five minutes after the intrathecal injection. We begin with testing to cold stimulus followed by pinprick or touch after an adequate sensory level to cold is documented.

Surface landmarks useful for estimating dermatome levels include the following:

Inguinal ligament – T12

Umbilicus – T10

Tip of xiphoid process – T7

Nipple – T4

Fifth finger – C8

Various methods are used to assess the level of sensory block prior to skin incision, including loss of sensation to a cold stimulus (eg, ice or alcohol swab), pinprick, or touch. The stimulus is applied at a level below the expected level of the block, where the patient is unlikely to feel the stimulus, and then reapplied sequentially higher along the flank until normal sensation returns.

The level at which sensation is lost depends on the stimulus, with loss of cold sensation at a higher level than pinprick, and loss of touch sensation lower still [53]. One study of patients who underwent cesarean delivery with spinal anesthesia compared cold, pinprick, and touch for assessment of the sensory level. The median sensory level to cold was approximately two dermatomes cephalad to the sensory level to pinprick, which in turn was approximately two dermatomes cephalad to the sensory level to touch [54]. In a separate study, the level at which the patient first felt any touch was two dermatomes more caudad than the level at which touch felt normal [55]. Thus, ascertaining "first touch" (eg, using a dull plastic needle cap or the rounded tip of a tongue depressor) is the most reliable way to determine that the sensory level is adequate for surgical anesthesia.

If the extent of sensory block is inadequate after approximately five minutes and hypobaric or hyperbaric solution was injected, the patient's position may be manipulated to augment intrathecal spread of the LA solution. As an example, if hyperbaric solution was used and the patient has a T6 sensory level, but a T4 level is desired, the patient may be placed in Trendelenburg position until the desired level is obtained.

Sedation during spinal anesthesia — We administer sedation as needed, in small doses titrated to effect, anticipating the sedation that may occur as a direct effect of spinal anesthesia. (See "Overview of neuraxial anesthesia", section on 'Central nervous system'.)

We often administer a low-dose propofol infusion (eg, 25 to 75 mcg/kg/minute) for intraoperative sedation in this setting. Midazolam (eg, 1 to 2 mg) may also be administered to provide mild sedation. An opioid (eg, fentanyl 50 to 100 mcg IV) may be required for pain related to positioning (eg, shoulder pain in the lateral decubitus position).

Inadequate or failed spinal anesthesia — Inadequate or failed spinal anesthesia, defined as the need to repeat the neuraxial procedure, convert to general anesthesia, or abort the planned surgery, occurs in approximately 3 to 4 percent of planned spinal anesthetics for hip and knee arthroplasties [56,57]. Consistent risk factors elucidated from retrospective studies include younger patient age and longer surgical duration. Other reported factors include lower body mass index, low lumbar (L4-5 and L5-S1) compared with high lumbar (L2-3) needle insertion, and 22 g compared with 25 g spinal needle. Possible mechanisms include technique errors, drug errors (including inadequate dose, inadequate duration of action), anatomic variations, and failure of the local anesthetic to distribute in the subarachnoid space [58]. Failure to adequately test the block before the surgical incision is also a reason for intraoperative failure. (See "Epidural and combined spinal-epidural anesthesia: Techniques", section on 'Assessment of level of block'.)

Management options if failed spinal is detected prior to surgical incision include repeating a neuraxial procedure (spinal, epidural, or combined spinal-epidural [CSE]) or conversion to general anesthesia. Once surgical incision has been made, the only option is conversion to general anesthesia. Occasionally, particularly if the surgical procedure is almost complete, the surgeon can inject local anesthetic into the field and the anesthesiologist can supplement with intravenous sedation.

If a second neuraxial procedure is chosen, we recommend choosing a different interspace, as anatomic variability may contribute to inadequate spinal anesthesia and this is less likely to occur in a different interspace. If there is no sensory block after the initial spinal attempt, spinal anesthesia may be repeated with the original drug dose. If partial block was established with the initial attempt, a reduced dose of spinal drug should be used, though the required dose is unpredictable. An alternative is to use a catheter-based technique (epidural or CSE), which allows the injection of an initial low dose, followed by titration of additional drug if the block is still inadequate.

Management of failed epidural anesthesia is discussed separately (see "Epidural and combined spinal-epidural anesthesia: Techniques", section on 'Troubleshooting inadequate anesthesia'). Importantly, spinal anesthesia should be induced with care after failed epidural anesthesia because of an increased risk of high or total spinal anesthesia [59].

RECOVERY — Patients should recover in the post anesthesia care unit (PACU) or recovery room after spinal anesthesia. In most cases, the anesthetic is still in effect when the patient is transported to the PACU; blood pressure (BP) must be measured frequently until the sympathectomy resolves, and precautions should be in place to prevent injury to insensate lower extremities.

PACU discharge criteria for ambulatory surgery should include verification of lower extremity motor strength and ability to void. (See "Overview of neuraxial anesthesia", section on 'Urinary retention'.)

PATIENTS WITH SUSPECTED OR CONFIRMED COVID-19 — Spinal anesthesia is not contraindicated in patients with COVID-19. Regional anesthesia may avoid the need for general anesthesia and airway management, with associated aerosolization of airway secretions and viral spread. The American Society of Regional Anesthesia and Pain Medicine and the European Society of Regional Anesthesia and Pain Therapy have published practice recommendations for neuraxial anesthesia and peripheral nerve blocks for patients with COVID-19. (See "Overview of neuraxial anesthesia", section on 'Patients with suspected or confirmed COVID-19'.)

UpToDate has added information on many aspects of COVID-19, including infection control, airway and other aspects of anesthetic management, and general and intensive care, in topic reviews linked here, and others.

(See "COVID-19: Perioperative risk assessment, preoperative screening and testing, and timing of surgery after infection".)

(See "COVID-19: Management in hospitalized adults".)

(See "Overview of infection control during anesthetic care", section on 'Considerations during aerosol-generating procedures'.)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Post dural puncture headache" and "Society guideline links: Local and regional anesthesia".)

SUMMARY AND RECOMMENDATIONS

Use and definition – Spinal anesthesia is a type of neuraxial anesthesia that is used for a variety of lower extremity, lower abdominal, pelvic, and perineal procedures (table 1A and table 1B).

A spinal needle is inserted into the subarachnoid space at the mid- to lower-lumbar level to inject an anesthetic solution into cerebrospinal fluid (CSF) (figure 3). (See 'Anatomy' above.)

Patient positioning – Spinal anesthesia can be performed in the sitting; lateral decubitus; or, less commonly, prone position, with the spine flexed to open the space between the spinous processes and to create a straight path between the spinal vertebra (figure 7). (See 'Positioning for spinal procedure' above.)

Choice of spinal needle – We use a small-diameter (ie, 24 to 27 gauge) pencil-point spinal needle with an introducer (figure 9). Larger-diameter needles and cutting needle tips are both associated with an increased risk of postdural puncture headache (PDPH) compared with smaller-diameter and pencil-point needles. (See 'Choice of spinal needle' above.)

Midline versus paramedian approach – Spinal anesthesia can be performed using a midline or paramedian approach. The midline approach may be easier, while the paramedian approach may be more successful for patients who are unable to flex the lumbar spine (figure 11). (See 'Approaches' above.)

Spinal anesthesia drugs – Spinal anesthetic drugs are chosen to achieve the required spinal level and duration of anesthesia (table 3). (See 'Choice of spinal drugs' above.)

Duration of block is primarily determined by the choice of local anesthetic (LA). (See 'Local anesthetics' above.)

The most important determinants of the extent of sensory block are the dose and baricity of the anesthetic solution. (See 'Baricity' above.)

Adjuvants may be added to LA solutions to alter the baricity, improve the quality, or extend the duration of spinal block. (See 'Adjuvants' above.)

Monitoring during spinal anesthesia – After spinal anesthesia is performed, the patient must be monitored as for general anesthesia, with close attention to blood pressure (BP) and heart rate. The level of spinal block should be assessed prior to the start of the surgical procedure. (See 'Management after spinal injection' above.)

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References

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