Neuraxial blocks may be used alone or in conjunction with general anesthesia for many procedures below the neck. As a primary anesthetic, neuraxial blocks have proved most useful in lower abdominal, inguinal, urogenital, rectal, and lower extremity surgery. Lumbar spinal surgery may also be performed under spinal anesthesia. Upper abdominal procedures (eg, gastrectomy) have been performed with spinal or epidural anesthesia, but because it can be difficult to safely achieve a sensory level adequate for patient comfort, these techniques are less commonly used.
If a neuraxial anesthetic is being considered, the risks and benefits must be discussed with the patient, and informed consent should be obtained. The patient must be mentally prepared for neuraxial anesthesia, and neuraxial anesthesia must be appropriate for the type of surgery. Patients should understand that they will have little or no lower extremity motor function until the block resolves. Procedures that require maneuvers that might compromise respiratory function (eg, pneumoperitoneum or pneumothorax) or those operations that are unusually of long duration are typically performed with general anesthesia, with or without neuraxial blockade.
Major contraindications to neuraxial anesthesia include lack of consent, coagulation abnormalities, severe hypovolemia, elevated intracranial pressure (particularly with an intracranial mass), and infection at the site of injection. Other relative contraindications include severe aortic or mitral stenosis and severe left ventricular outflow obstruction (hypertrophic obstructive cardiomyopathy); however, with close monitoring and control of the anesthetic level, neuraxial anesthesia can be performed safely in patients with stenotic valvular heart disease, particularly if extensive dermatomal spread of anesthesia is not required (eg, “saddle” block spinal anesthetics).
Relative and controversial contraindications are also shown in Table 45–1. Inspection and palpation of the back can reveal surgical scars, scoliosis, skin lesions, and whether or not the spinous processes can be identified. Although preoperative screening tests are not required in healthy patients undergoing neuraxial blockade, appropriate testing should be performed if the clinical history suggests a coagulation abnormality. Neuraxial anesthesia in the presence of sepsis or bacteremia could theoretically predispose patients to hematogenous spread of the infectious agents into the epidural or subarachnoid space.
TABLE 45–1Contraindications to neuraxial blockade. ||Download (.pdf) TABLE 45–1 Contraindications to neuraxial blockade.
| Infection at the site of injection |
| Lack of consent |
| Coagulopathy or other bleeding diathesis |
| Severe hypovolemia |
| Increased intracranial pressure |
| Sepsis |
| Uncooperative patient |
| Preexisting neurological deficits |
| Demyelinating lesions |
| Stenotic valvular heart lesions |
| Left ventricular outflow obstruction (hypertrophic obstructive cardiomyopathy) |
| Severe spinal deformity |
| Prior back surgery at the site of injection |
| Complicated surgery |
| Prolonged operation |
| Major blood loss |
| Maneuvers that compromise respiration |
Patients with preexisting neurological deficits or demyelinating diseases may report worsening symptoms following a neuraxial block. It may be impossible to discern effects or complications of the block from preexisting deficits or unrelated exacerbation of preexisting disease. For these reasons, some risk-averse practitioners argue against neuraxial anesthesia in such patients. A preoperative neurological examination should thoroughly document any deficits. In a retrospective study examining the records of 567 patients with preexisting neuropathies, 2 of the patients developed new or worsening neuropathy following neuraxial anesthesia. Although this finding indicates a relatively low risk of further injury, study investigators suggest that an injured nerve is vulnerable to additional injury, increasing the likelihood of poor neurological outcomes. However, a history of preexisting neurological deficits or demyelinating disease is at best a relative contraindication, and the balance of perioperative risks in this patient population may favor neuraxial anesthesia in certain select patients.
Regional anesthesia requires at least some degree of patient cooperation. This may be difficult or impossible for patients with dementia, psychosis, or emotional instability. The decision must be individualized. Unsedated young children may not be suitable for pure regional techniques; however, regional anesthesia is frequently used with general anesthesia in children.
Neuraxial Blockade in the Setting of Anticoagulants & Antiplatelet Agents
Whether a block should be performed in the setting of anticoagulants and antiplatelet agents can be problematic. The American Society of Regional Anesthesia and Pain Medicine (ASRA) has issued guidelines on this subject. Because guidelines are frequently revised and updated, practitioners are advised to seek the most recent edition. Fortunately, the incidence of epidural hematoma is reported to be infrequent (1 in 150,000 epidurals). The use of anticoagulant and antiplatelet medications continues to increase, placing an ever-larger number of patients at potential risk of epidural hematomas. However, because of the rarity of epidural hematomas, most guidelines are based on expert opinion and case series reviews, as clinical trials are not feasible.
If neuraxial anesthesia is to be used in patients receiving warfarin therapy, a normal prothrombin time and international normalized ratio usually will be documented prior to the block, unless the drug has been discontinued for weeks. Anesthesia staff should always consult with the patient’s primary physicians whenever considering the discontinuation of antiplatelet or antithrombotic therapy. New agents such as the direct thrombin inhibitor dabigatran and the factor Xa inhibitors rivaroxaban and apixaban are increasingly encountered by anesthesia staff (Figure 45–9).
Sites of action of anticoagulant drugs. Clotting factors are indicated by Roman numerals. Warfarin reduces production of factors VII, IX, X, and prothrombin. Heparin and LMWH inhibit factor Xa and thrombin. Fondaparinux, rivaroxaban, and apixaban are direct factor Xa inhibitors. Dabigatran is a direct thrombin inhibitor. HMWK, high-molecular-weight kininogen; LMWH, low-molecular-weight heparin. (Reproduced with permission from Benzon H, Avram M, Green D, Bonow R. New oral anticoagulants and regional anesthesia. Br J Anaesth. 2013 Dec;111(Suppl 1):i96-i113.)
Two drug half-life intervals have been suggested as the time from drug discontinuation until neuraxial procedures are performed. However, depending upon patient factors two half-life intervals may not be sufficient to mitigate the increased risk of bleeding following neuraxial procedures. A period from drug discontinuation of up to five to six drug half-life intervals may be necessary to avoid increased bleeding risk. Thrombin clotting time assays can be used to detect the effects of dabigatran. Likewise, factor Xa inhibitors can be assessed through assays of factor Xa inhibition. Anesthesia providers planning neuraxial procedures should consult closely with the patient’s primary providers to discern if suspension of anticoagulation is advised when considering a neuraxial technique. Bridging therapy with low-molecular-weight heparin can be considered during the time that oral anticoagulation is suspended if there is increased thrombotic risk. It has been suggested that new oral anticoagulants can be resumed 24 to 48 h following a neuraxial procedure or removal of an epidural catheter.
By themselves, aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) drugs do not increase the risk of spinal hematoma from neuraxial anesthesia procedures or epidural catheter removal. This assumes a normal patient with a normal coagulation profile who is not receiving other medications that might affect clotting mechanisms. In contrast, more potent agents should be stopped, and neuraxial blockade should generally be administered only after their effects have worn off. The waiting period depends on the specific agent: for ticlopidine it is 14 days; clopidogrel, 7 days; prasugrel, 7 to 10 days; ticagrelor, 5 days; abciximab, 48 h; and eptifibatide, 8 h. Neuraxial techniques should be avoided in patients receiving antiplatelet medications until platelet function has been recovered. Metabolites of clopidogrel and prasugrel block the P2Y12 receptor, impeding platelet aggregation. Ticagrelor directly inhibits the P2Y12 receptor. Both prasugrel and ticagrelor have greater platelet inhibition compared with clopidogrel. In patients with a recently placed cardiac stent, discontinuation of antiplatelet therapy can result in stent thrombosis and acute ST-segment elevation myocardial infarction. Risks versus benefits of a neuraxial technique should be discussed with the patient and the patient’s primary physicians.
C. Standard (Unfractionated) Heparin
“Minidose” subcutaneous heparin prophylaxis is not a contraindication to neuraxial anesthesia or epidural catheter removal. In patients who are to receive systemic heparin intraoperatively, blocks may be performed 1 h or more before heparin administration. A bloody epidural or spinal catheter placement does not necessarily require cancellation of surgery, but discussion of the risks with the surgeon and careful postoperative monitoring is needed. Removal of an epidural catheter should occur 1 h prior to, or 4 h following, subsequent heparin dosing.
Neuraxial anesthesia should be avoided in patients on therapeutic doses of intravenous heparin and with increased partial thromboplastin time. If the patient is started on heparin after the placement of an epidural catheter, the catheter should be removed only after discontinuation or interruption of heparin infusion and evaluation of the coagulation status. The risk of spinal hematoma is unclear in the setting of the anticoagulation required for cardiac surgery, and we have used epidural techniques for pain control after such procedures as off-pump coronary bypass only in the rare instances where the anticipated benefits exceed the perceived risks. Prompt diagnosis and evacuation of symptomatic epidural hematomas increase the likelihood that neuronal function will be preserved.
D. Low-Molecular-Weight Heparin (LMWH)
A number of cases of spinal hematoma were reported associated with neuraxial anesthesia after patients began receiving low-molecular-weight heparin (LMWH) enoxaparin (Lovenox) in the United States in 1993. Many of these cases involved intraoperative or early postoperative LMWH use, and several patients were receiving concomitant antiplatelet medication. The ASRA guideline provides useful recommendations for risk reduction in such circumstances. Some of the draft recommendations include that if an unusually bloody needle or catheter placement occurs, LMWH should be delayed until 24 h postoperatively, because this trauma may increase the risk of spinal hematoma. If postoperative LMWH thromboprophylaxis will be utilized, epidural catheters should be removed 4 h prior to the first LMWH dose.
E. Fibrinolytic or Thrombolytic Therapy
Neuraxial anesthesia should not be performed if a patient has received fibrinolytic or thrombolytic therapy.
ASRA has developed a smartphone application (ASRA Coags Regional app) to assist in the perioperative management of patients taking drugs that affect coagulation.
Should lumbar neuraxial anesthesia, when used in conjunction with general anesthesia, be performed after induction of general anesthesia? This is controversial. The major arguments for having the patient asleep are that (1) most patients, if given a choice, would prefer to be asleep, and (2) the possibility of sudden patient movement causing injury is markedly diminished. The major argument in favor of neuraxial blockade administration only while the patient is still awake is that the patient can alert the clinician to paresthesias and pain on injection in this circumstance, both of which have been associated with postoperative neurological deficits. Although many clinicians are comfortable performing lumbar epidural or spinal puncture in anesthetized or deeply sedated adults, there is greater consensus, although not unanimous opinion, that thoracic and cervical punctures should, except under unusual circumstances, only be performed in awake, responsive patients. Pediatric neuraxial blocks, particularly caudal and epidural blocks, are usually performed under general anesthesia.
Neuraxial blocks must be performed only in a facility in which all the equipment and drugs needed for intubation, resuscitation, and general anesthesia are immediately available. Regional anesthesia is greatly facilitated by adequate patient premedication. Nonpharmacological patient preparation is also very helpful. The patient should be told what to expect so as to minimize anxiety. This is particularly important in situations in which premedication is not used, as is typically the case in obstetric anesthesia. Supplemental oxygen via a face mask or nasal cannula may be required to avoid hypoxemia when sedation is used. Minimum monitoring requirements include blood pressure and pulse oximetry for labor analgesia. Monitoring for blocks rendered in surgical anesthesia is the same as that in general anesthesia.
Spinous processes are usually palpable and help to define the midline. Ultrasound can be used when landmarks are not palpable (Figure 45–10). The spinous processes of the cervical and lumbar spine are nearly horizontal, whereas those in the thoracic spine slant in a caudal direction and can overlap significantly (Figure 45–2). Therefore, when performing a lumbar or cervical epidural block (with maximum spinal flexion), the needle is directed with only a slight cephalad angle, if at all, whereas for a thoracic block, the needle must be angled significantly more cephalad to enter the thoracic epidural space. In the cervical area, the first palpable spinous process is that of C2, but the most prominent one is that of C7 (vertebra prominens). With the arms at the side, the spinous process of T7 is usually at the same level as the inferior angle of the scapulae (Figure 45–11). A line drawn between the highest points of both iliac crests (Tuffier’s line) usually crosses either the body of L4 or the L4–L5 interspace. Counting spinous processes up or down from these reference points identifies other spinal levels. A line connecting the posterior superior iliac spine crosses the S2 posterior foramina. In slender persons, the sacrum is easily palpable, and the sacral hiatus is felt as a depression just above or between the gluteal clefts and above the coccyx, defining the point of entry for caudal blocks.
A: Transducer position to image paramedian epidural space at the lumbar spine, longitudinal view. B: Corresponding ultrasound image. Post. Long. Lig., posterior longitudinal ligament; Lig. Flavum, ligamentum flavum; Ant. Dura Mater, anterior dura mater; Post. Dura Mater, posterior dura mater. (Reproduced with permission from Hadzic, A. Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia. 2nd ed. New York, NY: McGraw-Hill; 2012.)
Surface landmarks for identifying spinal levels.
The anatomic midline is often easier to identify when the patient is sitting than when the patient is in the lateral decubitus position (Figure 45–12). This is particularly true with obese patients. Patients sit with their elbows resting on their thighs or a bedside table, or they can hug a pillow. Flexion of the spine (arching the back “like an angry cat”) maximizes the “target” area between adjacent spinous processes and brings the spine closer to the skin surface (Figure 45–13).
Sitting position for neuraxial blockade. Note an assistant helps in obtaining maximal spinal flexion.
The effect of flexion on adjacent vertebrae. A: Posterior view. B: Lateral view. Note the target area (interlaminar foramen) for neuraxial blocks increases in size with flexion.
Many clinicians prefer the lateral position for neuraxial blocks (Figure 45–14). Patients lie on their side with their knees flexed and pulled high against the abdomen or chest, assuming a “fetal position.” An assistant can help the patient assume and hold this position.
Lateral decubitus position for neuraxial blockade. Note again the assistant helping to provide maximal spine flexion.
C. Buie’s (Jackknife) Position
This position may be used for anorectal procedures utilizing an isobaric or hypobaric anesthetic solution (see later discussion). The advantage is that the block is done in the same position as the operative procedure, so that the patient does not have to be moved following the block. The disadvantage is that CSF will not freely flow through the needle, so that correct subarachnoid needle tip placement will need to be confirmed by CSF aspiration. A prone position is typically used when fluoroscopic guidance is required.
The spine is palpated, and the patient’s body positioned so that a needle passed parallel to the floor will stay midline as it courses deeper (Figure 45–4). The depression between the spinous processes of the vertebrae above and below the level to be used is palpated; this will be the needle entry site. A sterile field is established with an appropriate antibacterial solution. A fenestrated sterile drape is applied. After the preparation solution has dried, a skin wheal is raised at the level of the chosen interspace with local anesthetic using a small (25-gauge) needle. A longer needle can be used for deeper local anesthetic infiltration.
Next, the procedure needle is introduced in the midline. Remembering that the spinous processes course caudad from their origin at the spine, the needle will be directed slightly cephalad. The subcutaneous tissues offer little resistance to the needle. As the needle courses deeper, it will enter the supraspinous and interspinous ligaments, felt as an increase in tissue resistance. The needle also feels more firmly implanted in the back (like “an arrow in a target”). If bone is contacted superficially, a midline needle is likely hitting the lower spinous process. Contact with bone at a deeper level usually indicates that the needle is in the midline and hitting the upper spinous process, or that it is lateral to the midline and hitting a lamina. In either case, the needle must be redirected. As the needle penetrates the ligamentum flavum, an obvious increase in resistance is encountered. At this point, the procedures for spinal and epidural anesthesia differ.
For epidural anesthesia, a sudden loss of resistance (to injection of air or saline) is encountered as the needle passes through the ligamentum flavum and enters the epidural space. For spinal anesthesia, the needle is advanced through the epidural space and penetrates the dura–subarachnoid membranes, as signaled by freely flowing CSF.
The paramedian technique may be selected, particularly if epidural or subarachnoid block is difficult, particularly in patients who cannot be positioned easily (eg, severe arthritis, kyphoscoliosis, or prior spine surgery) (Figure 45–15). Many clinicians routinely use the paramedian approach for thoracic epidural puncture. After skin preparation and sterile draping (as previously described), the skin wheal for a paramedian approach is raised 2 cm lateral to the inferior aspect of the superior spinous process of the desired level. Because this approach is lateral to most of the interspinous ligaments and penetrates the paraspinous muscles, the needle may encounter little resistance initially and may not seem to be in firm tissue. The needle is directed and advanced at a 10° to 25° angle toward the midline. If bone is encountered at a shallow depth with the paramedian approach, the needle is likely in contact with the medial part of the lower lamina and should be redirected mostly upward and perhaps slightly more laterally. On the other hand, if bone is encountered deeply, the needle is usually in contact with the lateral part of the lower lamina and should be redirected only slightly craniad, more toward the midline (Figure 45–16).
Paramedian approach. A needle that encounters bone at a shallow depth (a) is usually hitting the medial lamina, whereas one that encounters bone deeply (b) is farther lateral from the midline. A: Posterior view. B: Parasagittal view.
C. Assessing Level of Blockade
With knowledge of the sensory dermatomes (see appendix), the extent of sensory block can be assessed by a blunted needle or a piece of ice.
D. Ultrasound-Guided Neuraxial Blockade
Although it has not, as of yet, transformed the practice of neuraxial blockade in the same manner as it has for other procedures, ultrasound guidance can facilitate neuraxial blockade in patients with poorly palpable landmarks. As with other uses of ultrasound, specific training is required for practitioners to identify correctly the landmarks and interspaces necessary for neuraxial blockade.
Spinal needles are commercially available in an array of sizes lengths, and bevel and tip designs (Figure 45–17). All should have a tightly fitting, removable stylet that completely occludes the lumen to avoid tracking epithelial cells into the subarachnoid space. Broadly, they can be divided into either sharp (cutting)-tipped or blunt-tipped needles. The Quincke needle is a cutting needle with end injection. The introduction of blunt tip (pencil-point) needles has markedly decreased the incidence of postdural puncture headache. The Whitacre and other pencil-point needles have rounded points and side injection. The Sprotte is a side-injection needle with a long opening. It has the advantage of more vigorous CSF flow compared with similar gauge needles. However, this can lead to a failed block if the distal part of the opening is subarachnoid (with free flow CSF), the proximal part is not past the dura, and the full dose of medication is not delivered. In general, the smaller the gauge needle (along with use of a blunt-tipped needle), the lower will be the incidence of headache.
Larger catheters designed for epidural use are frequently employed for continuous spinal anesthesia following accidental dural puncture during performance of epidural anesthesia. Catheters must be carefully labeled as being subarachnoid, as opposed to epidural, to avoid the potential for inadvertent dosage.
Specific Technique for Spinal Anesthesia
The midline or paramedian approaches, with the patient positioned in the lateral decubitus, sitting, or prone positions, can be used for spinal anesthesia. As previously discussed, the needle is advanced from skin through the deeper structures until two “pops” are felt. The first is penetration of the ligamentum flavum, and the second is penetration of the dura–arachnoid membrane. Successful dural puncture is confirmed by withdrawing the stylet to verify free flow of CSF. With small-gauge needles (<25 gauge), aspiration may be necessary to detect CSF. If free flow occurs initially, but CSF cannot be aspirated after attaching the syringe, the needle likely will have moved. Persistent paresthesias or pain with injection of drugs should prompt the clinician to withdraw and redirect the needle.
Factors Influencing Level of Spinal Block
Table 45–2 lists factors that have been shown to affect the level of neural blockade following spinal anesthesia. The most important determinants are baricity of the local anesthetic solution, position of the patient during and immediately after injection, and drug dosage. In general, the larger the dosage or more cephalad the site of injection, the more cephalad the level of anesthesia that will be obtained. Moreover, migration of the local anesthetic cephalad in CSF depends on its density relative to CSF (baricity). CSF has a specific gravity of 1.003 to 1.008 at 37°C. Table 45–3 lists the specific gravity of anesthetic solutions. A hyperbaric solution of local anesthetic is denser (heavier) than CSF, whereas a hypobaric solution is less dense (lighter) than CSF. The local anesthetic solutions can be made hyperbaric by the addition of glucose or hypobaric by the addition of sterile water or fentanyl. Thus, with the patient in a head-down position, a hyperbaric solution spreads cephalad, and a hypobaric anesthetic solution moves caudad. A head-up position causes a hyperbaric solution to settle caudad and a hypobaric solution to ascend cephalad. Similarly, when a patient remains in a lateral position, a hyperbaric spinal solution will have a greater effect on the dependent (down) side, whereas a hypobaric solution will achieve a higher level on the nondependent (up) side. An isobaric solution tends to remain at the level of injection. Anesthetic agents lacking glucose may be mixed with CSF (at least 1:1) to make their solutions isobaric. Other factors affecting the level of neural blockade include the level of injection and the patient’s height and vertebral column anatomy. The direction of the needle bevel or injection port may also play a role; higher levels of anesthesia are achieved if the injection is directed cephalad than if the point of injection is oriented laterally or caudad.
TABLE 45–2Factors affecting the dermatomal spread of spinal anesthesia. ||Download (.pdf) TABLE 45–2 Factors affecting the dermatomal spread of spinal anesthesia.
|Most important factors |
| Baricity of anesthetic solution |
| Position of the patient |
| During injection |
| Immediately after injection |
| Drug dosage |
| Site of injection |
|Other factors |
| Age |
| Cerebrospinal fluid |
| Curvature of the spine |
| Drug volume |
| Intraabdominal pressure |
| Needle direction |
| Patient height |
| Pregnancy |
TABLE 45–3Specific gravities of some spinal anesthetic agents. ||Download (.pdf) TABLE 45–3 Specific gravities of some spinal anesthetic agents.
Hyperbaric solutions tend to move to the most dependent area of the spine (normally T4–T8 in the supine position).
With normal spinal anatomy, the apex of the thoracolumbar curvature is T4 (Figure 45–18). In the supine position, this should limit a hyperbaric solution to produce a level of anesthesia at or below T4. Abnormal curvatures of the spine, such as scoliosis and kyphoscoliosis, have multiple effects on spinal anesthesia. Placing the block becomes more difficult because of the rotation and angulation of the vertebral bodies and spinous processes. Finding the midline and the interlaminar space may be difficult. The paramedian approach to lumbar puncture may be preferable in patients with severe scoliosis and kyphoscoliosis. Reviewing radiographs of the spine before attempting the block may be useful. Spinal curvature affects the ultimate level by changing the contour of the subarachnoid space. Previous spinal surgery can similarly result in technical difficulties in placing a block. Correctly identifying the interspinous and interlaminar spaces may be difficult at the levels of previous laminectomy or spinal fusion. The paramedian approach may be easier, or a level above the surgical site can be chosen. The block may be incomplete, or the level may be different than anticipated, due to postsurgical anatomic changes.
The position of the spinal canal in the supine position (A) and lateral decubitus position (B). Note the lowest point is usually between T5 and T7, where a hyperbaric solution tends to settle once the patient is placed supine.
Lumbar CSF volume inversely correlates with the dermatomal spread of spinal anesthesia. Increased intraabdominal pressure or other conditions that cause engorgement of the epidural veins, thus decreasing CSF volume, are associated with greater dermatomal spread for a given volume of injectate. This would include conditions such as pregnancy, ascites, and large abdominal tumors. In these clinical situations, higher levels of anesthesia are achieved with a given dose of local anesthetic than would otherwise be expected. For spinal anesthesia on a term parturient, some clinicians reduce the dosage of anesthetic by one-third compared with a nonpregnant patient, particularly when the block will be initiated with the patient in the lateral position. Age-related decreases in CSF volume are likely responsible for the higher anesthetic levels achieved in the elderly for a given dosage of spinal anesthetic. Severe kyphosis or kyphoscoliosis can also be associated with a decreased volume of CSF and often results in a higher than expected level, particularly with a hypobaric technique or rapid injection.
Many local anesthetics have been used for spinal anesthesia in the past, but only a few are currently in use (Table 45–4). Only preservative-free local anesthetic solutions are used. Addition of vasoconstrictors (α-adrenergic agonists, epinephrine [0.1–0.2 mg]) and opioids enhance the quality or prolong the duration of spinal anesthesia, or both. Vasoconstrictors seem to delay the uptake of local anesthetics from CSF and may have weak spinal analgesic properties. Opioids and clonidine can likewise be added to spinal anesthetics to improve both the quality and duration of the subarachnoid block.
TABLE 45–4Dosages, uses, and duration of commonly used spinal anesthetics.1,2 ||Download (.pdf) TABLE 45–4 Dosages, uses, and duration of commonly used spinal anesthetics.1,2
|Drug ||Preparation ||Dose (mg) ||Procedures || |
|2-Chloroprocaine ||1%, 2%, 3% ||30–60 ||Ambulatory, T8 ||1–2 ||Not recommended (flu-like symptoms) |
|Lidocaine ||2% ||40–50 ||Ambulatory, T8 ||1–2 ||Only modest effect, not recommended |
|Mepivacaine3 ||1.5% ||30 (T9) ||Ambulatory surgery, knee scope, TURP ||1–2 ||Not recommended |
|45 (T6)4 ||1.5–3 |
|60 (T5) ||2–3.5 |
|Bupivacaine ||0.5% ||7.5 ||Ambulatory lower limb ||1–2 || |
|10 ||THA, TKA, femur ORIF ||2 || |
|15 ||3 ||4–5 |
|Bupivacaine ||0.75% in 8.25% dextrose ||4–10 ||Perineum, lower limbs5 ||1.5–2 ||1.5–2.5 |
|12–14 ||Lower abdomen |
|12–18 ||Upper abdomen |
|Ropivacaine ||0.5%, 0.75% ||15–17.5 ||T10 level ||2–3 ||Does not prolong block |
|18–22.5 ||T8 level ||3–4 |
|1% + 10% dextrose (equal volumes D10 and ropivacaine) ||18–22.5 ||T4 level ||1.5–2 |
|Tetracaine ||1% + 10% dextrose (0.5% hyperbaric) ||4–8 ||Perineum/lower extremities ||1.5–2 ||3.5–4 |
|10–12 ||Lower abdomen |
|10–16 ||Upper abdomen |
|Adjuvant ||Dose (mcg) ||Duration (h) ||Comments/Side Effects |
|Fentanyl ||10–25 ||1–2 ||Itching; nausea; urinary retention; sedation; ileus; respiratory depression (delayed with morphine—↓ dose with elderly or sleep apnea) |
|Sufentanil ||1.25–5 ||1 |
|Morphine ||125–250 ||4–24 |
|Epinephrine ||100–200 || ||Prolongs nerve exposure to local anesthetic + α-adrenergic modulation |
|Phenylephrine ||1000–2000 || ||Hypotension. Prolongs tetracaine but not bupivacaine. Extends tetracaine better than epinephrine does. May cause TNS |
|Clonidine ||15–150 || ||Hypotension. Sedation. Prolongs motor and sensory block |
Until very recently in North America, hyperbaric spinal anesthesia was more commonly used than hypobaric or isobaric techniques. The level of anesthesia is then dependent on the patient’s position during and immediately following the injection. In the sitting position, “saddle block” can be achieved by keeping the patient sitting for 3 to 5 min following injection, so that only the lower lumbar nerves and sacral nerves are blocked. If the patient is moved from a sitting position to a supine position immediately after injection, the agent will move more cephalad to the dependent region defined by the thoracolumbar curve. Hyperbaric anesthetics injected intrathecally with the patient in a lateral decubitus position are useful for unilateral lower extremity procedures. The patient is placed laterally, with the extremity to be operated on in a dependent position. If the patient is kept in this position for about 5 min following injection, the block will tend to be denser and achieve a higher level on the operative dependent side.
If regional anesthesia is chosen for surgical procedures involving hip or lower extremity fracture, hypobaric or isobaric spinal anesthesia can be useful because the patient need not lie on the fractured extremity.