Clinical aspects

Description

Lumbar puncture procedure

Requirements

• At least 1 trained assistant to hold the patient • Sterile gloves • Dressing pack • Sterile drapes and procedure tray • Skin preparation: povidone iodine solution (Betadine) or chlorhexidine • Local anesthetic 1% or 2% lidocaine, 2ml syringe, 25G needle • CSF tubes (4) • Spinal needle – 22 gauge is preferable • Spinal manometer with 3-way stopcock • One glucose (oxalate – grey-topped) blood tube

Needles. Based on the tip designs of the needles, 3 groups of spinal needles are available; these are named Quincke, Whitacre, and Sprotte needles.

Although the larger diameter needle might be associated with the higher incidence of postdural puncture headache, the narrower the needle, the more flexible it is. This greater flexibility of a thinner needle often results in greater difficulty when passing it through tough tissue. Ideally, a thinner needle should be chosen--a pencil-point (27 G) when possible. But with a rigid back, 22 G with a Quincke tip is ideal.

Holst and colleagues using Quinke needles demonstrated a positive correlation in needle diameter and CSF loss (14). In a study limited to women, Vallejo and colleagues compared 5 spinal needles for lumbar puncture and demonstrated a 9% incidence of postdural puncture headache with Quinke needle as compared to 5% incidence with the Atraucan needle; both are cutting-tip needles (37). Incidence of postdural puncture headache was 2.8%, 4%, and 3.1% with Sprotte, Gertie-Marx, and Whitacre pencil-point needles, respectively. Based on their study, O’Connor and colleagues conclude that the use of small diameter and pencil-tipped needles will cause less CSF leakage through human dura (25; 02).

Identifying the correct space. The intercristal line (Tuffier’s line) is a theoretical line that joins the tops of the 2 iliac crests and passes through the fourth lumbar vertebra (L4). Once this space is identified, the other spaces can be easily counted and identified by palpation and extension. Because the spinal cord ends at the upper border of the L1 vertebra in the majority of individuals, L3-4 interspace is clear of the spinal cord, thereby avoiding injury to the cord. As a variation, the cord can end below the L1 vertebra--hence, the recommendation that lumbar puncture should not normally be attempted above the L3-4 interspace.

Positioning. Both the sitting and the lateral positions have been used. The spine is less likely to be rotated or laterally flexed if the patient is sitting and leaning forward, though the lateral position is more comfortable for patient and operator. One should learn to perform the procedure equally well in both positions, including the paramedian approach when the midline approach with the needle proves difficult.

Lumbar puncture may be performed on a child with the child lying on their side or sitting up.

Aim for maximum flexion of the spine (curl into fetal position), but avoid overflexing the neck, especially in infants as this may cause respiratory compromise. Ask an adolescent to slouch rather than bend from their hips.

Ensure that the plane of the back is exactly at 90 degrees to the bed, making sure that the hips and shoulders are in line. Flexing the back maximally ensures opening up the spinous processes. In addition, you can feel the gaps between the spinous processes.

Draw an imaginary line between the top of the iliac crests. This intersects the spine at approximately the L3-4 interspace (mark this if necessary).

The conus medullaris finishes near L3 at birth, but at L1-2 by adulthood. However, it ends below this level in a substantial proportion of patients; therefore, it is ideal to recommend that lumbar puncture or spinal anesthesia should not normally be attempted above the L3-4 interspace. Lumbar punctures are usually done at the L3-4 or L4-5 interspace.

Pre-spinal tap preparation. The following are important when preparing for spinal tap:

• Wash hands and aseptically put on sterile gloves. • Prepare the skin with povidone-iodine or chlorhexidine and set up sterile drapes. • Allow adequate time for the skin preparation to dry. • Take the tops off the tubes, ensuring that they remain sterile. • Infiltrate the skin with 1% lignocaine using a 25G needle. • Assemble the 3-way stopcock and manometer. Be certain you know which tap opens to the manometer and closes all the others, and then close the entry to the manometer and open it to the atmosphere.

Procedure. Wearing nonsterile gloves, locate the L3-L4 interspace by palpating the right and left posterior superior iliac crests and moving the fingers medially toward the spine. The interspace above will be L2-L3, and the interspace below will be L4-L5. Mark the entry site at the widest space of choice with a thumbnail or a marker.

Open the spinal tray, change to sterile gloves, and prepare the equipment. Open the numbered plastic tubes, and place them upright. Assemble the stopcock on the manometer, and draw the lidocaine into the 10-mL syringe.

After informing the patient about the use of a cold antiseptic solution, use the skin swabs and antiseptic solution to clean the skin in a circular fashion; start with at the L3-L4 interspace and then include 1 space above and below. Place a sterile drape below the patient and a fenestrated drape on the patient. Use the 10-mL syringe to administer a local anesthetic. After raising a skin wheal using the 25-gauge needle, switch to a 20-gauge needle to anesthetize the deeper tissue. Insert the needle all the way to the hub, aspirate to confirm that the needle is not in a blood vessel, and then inject a small amount as the needle is withdrawn a few centimeters. Continue this process above, below, and to the sides very slightly using the same puncture site.

Next, stabilize the 20- or 22-gauge needle with the index fingers, and advance it through the skin wheal using the thumbs. Orient the bevel parallel to the longitudinal dural fibers to increase the chances that the needle will separate the fibers rather than cut them; in the lateral recumbent position, the bevel should face up, and in the sitting position, it should face to 1 side or the other. An alternative variation of the technique involves removing the stylet once the needle is in the ligament and advancing very slowly without stylet, watching for CSF to flow back. Using this technique, unintentional entry past the subarachnoid space is avoided.

For measurement of the opening pressure, the patient must be in the lateral recumbent position. After fluid is returned from the needle, attach the manometer through the stopcock, and note the height of the fluid column. The patient’s legs should be straightened, and they should be fully relaxed during the measurement of the opening pressure, or a falsely elevated pressure will be obtained. Give time for the pressure to stabilize.

The needle should be passed slowly towards the dura. There is a particular feel to the passage of the spinal needle through the ligamentum flavum. The appearance of CSF at the hub of the needle always confirms the correct placement of the spinal needle. Although placing a needle into the CSF space of a slim individual is easy, placing a spinal needle in the correct place when faced with obese, pregnant, elderly or uncooperative patients consistently takes skill and considerable experience. More than anything the success of a lumbar puncture depends on the correct position of the patient.

Collect at least a cc of cerebrospinal fluid in each of the 4 plastic tubes, starting with tube 1. If the CSF flow is too slow, changing the needle position might enhance the flow. At the end of collecting the CSF, replace the stylet, and remove the needle. After applying brief pressure at the puncture site, clean the area, apply a sterile dressing, and place the patient in the supine position.

Post procedure varies between hospitals and outpatient clinics. Lying flat in bed with rest for 1 to 4 hours is generally the rule. However, there is no clear evidence suggesting that routine bed rest after dural puncture is beneficial for the prevention of postprocedural headache (01). Peralta and colleagues looked at the utility of a single prophylactic intrathecal morphine to decrease the incidence or severity of postdural puncture headache and did not find any meaningful benefit (27).

At times, bedside lumbar puncture could prove difficult for even the most experienced, confident operator. Under these circumstances one should not shy away from asking another qualified person to do the procedure or consider doing the procedure under fluoroscopy.

Paramedic approach to lumbar puncture. Prehospital providers (EMTs and paramedics) may become involved with this phase of patient management, particularly in a 9-1-1 setting when patients present with acute meningitis or encephalitis and have rapidly progressing symptoms. High index of suspicion for meningitis is warranted, especially when the patient has had a sore neck and altered mental status. Paramedics on scene should collect a detailed history from all concerned and relay this to the ER staff for an early diagnosis. When indicated, contact precautions and respiratory isolation should be established. Care providers in this scenario should wear masks and gloves, and where indicated patient can wear a surgical mask. The main aim will be to stabilize the patient regarding airways, breathing, and cardiac function. In the acute phase of care, intravenous access should ideally be established, with fluid therapy to establish euvolemia. Seizure precautions should be established, and seizures, should they occur, should be managed according to established protocols.

Following the transport of any suspected meningitis patient, regardless of etiology, thorough decontamination of the ambulance and transport stretcher should be carried out with commercial chemical cleaning solutions in line with the manufacturer’s cleaning recommendations; however, a 1-to-100 bleach-to-water solution is effective as well. At the time of this writing lumbar puncture is a procedure exclusive to physicians, and a paramedic should not perform the procedure, though a paramedic might assist in holding the patient in the proper position.

Epidural injection procedure

The famously known 4 p’s apply to epidural injections: position, preparation, projection, and puncture.

Preparation. An informed consent should be obtained after explaining and discussing with the patient all options as well as risks and benefits of the procedure. At this time, a decision should be made on the type of technique to be used, either a single-shot continuous catheter or intermittent bolus technique. Although Crawford needle is appropriate for a 1-shot technique, a Tuohy needle is usually the choice for epidural catheter insertion. To identify the epidural space, one must choose to use the loss of resistance versus the hanging drop technique.

Positioning. As with lumbar puncture, the success of an epidural injection also depends on the correct position of the patient. Three positions are routinely used for the administration of epidural anesthesia: lateral decubitus, sitting, and prone.

Lateral decubitus. The patient is positioned with their back parallel with the side of the operating room table in a fetal position with the thighs flexed up, and the neck is flexed forward. This position is often used for caudal approach, especially in children, as this allows for the maintenance of a patent airway because the caudal technique is often performed under general anesthesia in pediatric patients. In this position, the provider often has less dependence on an assistant for positioning. The provider also has the ability to administer more sedation.

Sitting. In this position, the patient’s feet are placed on a stool, with the patient sitting up with an arched back, head flexed, arms hugging a pillow, or on a table in front. The patient should place their feet on a stool and sit up straight, head flexed, arms hugging a pillow, or on a table in front of them. Flexing the back maximally ensures opening up the spinous processes.

Prone position. This is often used for caudal approach in adults.

Procedure. In adults, in the lumbar area, skin to ligamentum flavum depth ranges from 3.5 to 6 cm; the average range is approximately 4 cm. The average thickness of the ligamentum flavum is 5 to 6 mm. One must be extra cautious in the dorsal (thoracic) area to avoid dural puncture and spinal cord injury, as the spinal canal is narrowest here.

After a sterile preparation, place a skin wheal at the determined site of insertion using a local anesthetic, followed by anesthetizing the deeper tissues. Anatomical structures transverse before reaching the epidural space include skin, subcutaneous tissue, supraspinous ligament, and interspinous ligament. Identifying the midline helps immensely to locate the epidural space. The general concept of epidural anesthesia or analgesia is to provide local administration of the anesthetic or analgesic agent into the epidural space. The level of segmental block depends on the distance that the drug diffuses in the rostral or caudal directions as well as volume, concentration, and potency of the drug.

Typically, an 18-gauge needle is used to penetrate through the skin and ligamentum flavum into the epidural space. As the needle is advanced through the ligamentum flavum, resistance to injection of air or saline is continuously or frequently checked. When the tip of the needle is within the ligamentum flavum, air or saline cannot be readily injected. Immediately past the ligamentum flavum, there is a loss of resistance, and air or saline can be injected; this indicates that the needle tip has entered the epidural space.

Loss of resistance technique. Once the needle is placed into the ligamentum flavum, remove the stylet. Attach a glass syringe with 2 to 3 ml of preservative-free normal saline and a small (0.25 ml) air bubble. The needle is held steady by the non-dominant hand, and the dominant hand holds the syringe. Steady pressure is applied to the plunger to compress the air bubble. Slowly and steadily advance the needle until loss of resistance is noted and the air bubble and saline get sucked in.

Hanging drop technique. Place the needle into the ligamentum flavum. Next, apply a drop of preservative-free normal saline to the hub of the needle. Apply slow, steady pressure to the needle until the hanging drop gets sucked in as the epidural space contains subatmospheric pressure.

In addition to localization of the needle tip within the epidural space, injection of air or saline pushes the dura away from the needle tip, thus, reducing the risk of puncturing or entering the subarachnoid space. A flexible catheter is then inserted through the needle bore and passed approximately 2 to 3 cm into the epidural space. To prevent migration of the catheter out of the epidural space during labor and delivery in obstetric patients, the catheter can be inserted 4 to 5 cm. The needle is withdrawn, and the catheter immobilized so that multiple injections of medications into the epidural space can be performed (24).

Aspiration of the catheter for CSF is attempted to determine if the catheter tip is within the subarachnoid space. Test doses (small volumes) of an anesthetic and epinephrine are routinely injected to determine if the catheter tip is in the subarachnoid space (leading to unexpected spinal block) or intravenous vessel (causing tachycardia from the epinephrine). A test dose consists of 3 ml of 1.5% preservative-free lidocaine with 1:200,000 epinephrine. Aspiration of the catheter for CSF and the injection of test doses should be performed before each injection of medication to ensure that the catheter tip has not migrated through the dura into the subarachnoid space. Mahajan and coworkers recommend that the catheter should be inserted 1 to 2 hours preoperatively in an awake patient (21). This provides ample time to place the catheter and accurately assess the level of sensory analgesia with local anesthetic drug before surgery begins. Accurate positioning of the catheter is only confirmed by bilateral sensory block. Anything other than an effective bilateral block suggests that the catheter may not be correctly positioned, with pleural puncture as one of the possibilities (12).

Among the various methods epidural anesthesia providers use to identify the epidural space, some use air, some use fluid, and others use a combination of air and fluid during the loss of resistance technique. It has long been speculated that loss of resistance to air results in a lesser quality of analgesia compared with loss of resistance to only fluid. In a study that also included a systematic review with meta-analysis of 4 older studies, Sanford and colleagues found inconclusive evidence in determining whether a difference in analgesia quality results from the use of air or fluid during the loss of resistance technique (31).

Instrumentation for epidural injection has included a novel spring-loaded syringe, which is a potentially useful loss of resistance syringe that provides a reliable, objective endpoint for identification of the epidural space.

Epidrum® is a device that identifies the epidural space with an epidural Tuohy needle (32). It is an optimal pressure, loss of resistance device for identifying the epidural space. In addition to offering good Tuohy needle control, Epidrum also helps in performing epidural anesthesia quickly compared to the loss of resistance or hanging drop technique.

Monitoring

Vital signs, pulse oximetry, level of consciousness, block progression, and signs and symptoms of toxicity should be monitored continuously. Numbness of the arms and hands, problems with breathing, and altered level of consciousness might suggest block progression. Hypotension, if greater than 20% of baseline, and bradycardia should be treated aggressively.

Postoperative care should include assessment of block regression, followed by full return of baseline motor and sensory functions. In the likely event of hypotension, patient should be treated with a Trendelenburg position, additional intravenous fluids, oxygen, and vasopressors as needed. If urinary retention occurs, it should be dealt with appropriately.

Indications

Lumbar puncture. Lumbar puncture is indicated in the diagnosis of bacterial, fungal, mycobacterial, and viral CNS infections as follows and, in certain settings, for help in the diagnosis of subarachnoid hemorrhage, with a normal CT scan of the brain.

• Idiopathic intracranial hypertension (pseudotumor cerebri) • Guillain-Barré syndrome • Carcinomatous meningitis • Normal pressure hydrocephalus • Certain CNS malignancies • CNS syphilis • CNS vasculitis • Multiple sclerosis or other demyelinating diseases • Paraneoplastic syndromes and dementia

Lumbar puncture is also needed as a therapeutic or diagnostic maneuver in the following situations (35; 11; 22):

• Spinal anesthesia • Intrathecal administration of chemotherapy • Intrathecal administration of antibiotics • Injection of contrast media for myelography or for cisternography

Lumbar epidural anesthesia or analgesia. Lumbar epidural anesthesia or analgesia is indicated for regional anesthesia of the lumbosacral segments during obstetric, gynecologic, urologic, orthopedic, and general surgical procedures and for postoperative pain control. It is often performed in conjunction with general anesthesia to permit lighter, general anesthesia followed by postoperative analgesia. Lumbar epidural analgesia has also been used in patients with severe pain in the lumbosacral segments, such as from cancer or reflex sympathetic dystrophy. Based on their study, Choi and colleagues concluded that it is possible to offer regional block to women with inherited bleeding disorders provided their coagulation defects have normalized, either spontaneously during pregnancy or following adequate hemostatic cover (05).

Contraindications

Lumbar puncture. Lumbar puncture should not be performed in the following situations:

• Increased intracranial pressure versus impending herniation (exception being idiopathic intracranial hypertension or pseudotumor cerebri) • Large intracranial space occupying lesions • Bleeding diathesis • Coagulopathy • Decreased platelet count lower than 50,000 • Infection at lumbar puncture site • Sepsis • Severe vertebral deformities (scoliosis or kyphosis). In the hands of an inexperienced physician, this might require lumbar puncture under fluoroscopy (Ross 2003; 34).

Lumbar puncture and intracranial hypertension. The risk of holding or postponing a spinal tap because of concern of the risk of brain herniation is small. In these patients antibiotics should be started immediately along with a mannitol infusion, with other interventions to control increased intracranial pressure, including attention to airway, breathing, and circulation. This should be immediately followed by a brain CT and not a spinal tap. Hence, a detailed neurologic examination is essential before deciding on a lumbar puncture. Although tell-tale signs of herniation might include worsening level of consciousness (Glasgow coma scale of 11 or lower) and brainstem signs (including restricted gaze, loss of Doll’s eye, dysconjugate gaze, posturing, papillary changes, or irregular respirations), impending herniation may prove difficult to diagnose because brain CT may be normal or show no evidence of herniation. Hence, a normal CT scan in acute bacterial meningitis does not equate to a safe lumbar puncture. With the evidence available, lumbar puncture is temporally strongly associated with brain herniation and is also considered causative in precipitating the same.

Cerebral herniation occurs in about 5% of patients with acute bacterial meningitis, accounting for about 30% of the mortality (17).

Current international guidelines recommend cerebral CT before lumbar puncture in many adults with suspected acute bacterial meningitis, due to concern about lumbar puncture-induced cerebral herniation. Still the guideline emphasis is on early treatment based on symptoms. Glimaker and colleagues argue that performing CT prior to lumbar puncture implies a risk of delayed acute bacterial meningitis treatment, which may be associated with a fatal outcome (10). They further feel that firm evidence for lumbar puncture-induced herniation in adult acute bacterial meningitis is absent and brain CT cannot discard herniation. In their opinion, “the recommendation to perform CT before lumbar puncture may contribute to an avoidable delay of lumbar puncture and acute bacterial meningitis treatment, and the inappropriate use of the diagnostic treatment sequence of brain CT scan, followed by lumbar puncture, followed by antibiotics and corticosteroids should be avoided in adults with suspected acute bacterial meningitis by omitting needless contraindications for lumbar puncture, thus, eliminating an unnecessary fear of immediate lumbar puncture.”

Lumbar puncture with coagulopathy. Foerster and colleagues in their studies found that even in thrombocytopenic patients, an epidural hematoma would be a relatively rare complication following lumbar puncture (08). They further opine that despite the large number of punctures performed on patients with platelet counts below 100,000 mm-3 (n = 1108), further studies are necessary to determine a lower safe platelet count threshold for the performance of lumbar punctures in healthy patients undergoing neuraxial anesthesia.

Only meager published data are available regarding the provision and safety of neuraxial techniques in patients with common bleeding diatheses. The minimum "safe" factor levels and platelet count for neuraxial techniques remain undefined in both the obstetric and general populations. Based on the available information, evidence-based recommendations in the setting of hemophilia, von Willebrand disease, or idiopathic thrombocytopenic purpura cannot be offered (05).

In patients with leukemia where abnormal coagulation test results after standard treatment with fresh frozen plasma persisted, prophylactic use of a single dose of 90 microg/kg recombinant activated factor VII had been used with the rapid correction of abnormal coagulation test results previously not corrected by fresh frozen plasma, thus, avoiding a delay of diagnostic lumbar punctures and intrathecal chemotherapy (07).

Lumbar epidural anesthesia and analgesia. In addition to the contraindication mentioned earlier, some other relative contraindications for lumbar epidural anesthesia and analgesia include allergies to an anesthetic or analgesic agent, anticoagulation, systemic or local infection, and lumbar spinal stenosis. The American Society of Regional Anesthesia and Pain Medicine (ASRA) recommends that epidural catheters be removed with the international normalized ratio of 1.4 or less (15). This generally correlates with normal or near normal hemostasis with clotting factor activities greater than 40%, thereby potentially avoiding the risk of bleeding, though this is subject to timely revision with more studies. An international normalized ratio greater than 1.4 is typically associated with factor VII activity less than 40% and the potential for inadequate clotting.

Cohn and colleagues analyzed complications of 761 short-term intrathecal macrocatheters in obstetric patients (06). They revealed that intrathecal catheters are dependable and an option for labor analgesia and surgical anesthesia for cesarean delivery with miniscule serious and long-lasting complications.

Results

Goals. The goals of lumbar epidural anesthesia are to induce regional anesthesia of the lumbosacral segments and avoid or reduce the depth of general anesthesia. The goals of lumbar epidural analgesia are to provide regional analgesia in postoperative or chronic pain patients and avoid or reduce the need for systemic analgesic control.

Effects of epidural anesthesia on motor function and sympathetic innervation resulting in a reduction in vital capacity and forced expiratory volume in 1 s (FEV (1.0) are negligible under lumbar and low thoracic epidural anesthesia. However, going higher up the vertebral column, these effects can increase up to 20% or 30% of baseline. Still, these effects are so small that the beneficial effects far overweigh the side effects. Postoperatively, early extubation, improvement in pain therapy, and improved diaphragmatic function are noted. Overall, epidural anesthesia not only provides excellent anesthesia and analgesia but also improves postoperative outcome and reduces postoperative pulmonary complications compared with anesthesia and analgesia without epidural anesthesia (13).

A variety of anesthetic or analgesic agents can be injected. Typical anesthetics include lidocaine, bupivacaine, etidocaine, tetracaine, chloroprocaine, prilocaine, procaine, dibucaine, and mepivacaine. Epinephrine is frequently added to induce local vasoconstriction and reduce systemic uptake of the local anesthetic agent. Besides reducing systemic toxicity, epinephrine increases the local potency of the anesthetic drug. Analgesic agents injected include fentanyl, morphine, and droperidol.

Epidural catheterizations provide regional anesthesia and analgesia during childbirth or surgical procedures. Ultrasound imaging can reduce the risk of failed or traumatic lumbar punctures and epidural catheterizations, as well as the number of needle insertions and redirections (30). Ultrasound may be a useful adjunct for these procedures (04).

Like in lumbar puncture, the performance of epidural catheterizations and palpation of anatomical landmarks as well as visual imagery of the local anatomy, the angle of needle progression, and assessment of the distance from the skin to the target space, rely on the skill, confidence, and experience of the operator. Obesity, local edema, or anatomical variations might make the procedure more difficult to perform.

Fluoroscopy is often used as a rescue modality after failed lumbar punctures (39). Fluoroscopy is expensive, not readily available or portable, requires multiple operators, and involves radiation exposure (23). Hence, ultrasound is recommended over fluoroscopy. Jacobson and colleagues have utilized fluoroscopy-guided curved-needle transforaminal approach in patients with spinal muscular atrophy requiring nusinersen intrathecal injections (16). These patients have a challenging spine with complete interlaminar osseous fusion and the modified needle has made the technique fully successful.

Forty-five milligrams of lidocaine, if injected intrathecally, will result in a spinal anesthetic. Fifteen micrograms of epinephrine, if injected intravascularly, will result in a 20% or more increase in heart rate (36).

Adverse effects

Epidural anesthesia. Apart from technical complications such as needle contacting spinous process, needle contacting lamina, inability to thread the catheter, continuous return of fluid suggestive of needle traversing the dura into the subarachnoid space, blood return suggesting needle entry in to epidural vein, other complications include pain, paresthesia and pain with injection as well as a failed epidural. Factors leading to failed epidural include false loss of resistance, misplaced local anesthetic, unilateral block, segmental sparing, and visceral pain. Catheter migration might occur, leading to intravascular or intrathecal injections. It is ideal to consider aspiration before each dose to check the position of the needle and also to increase the dose in small increments.

A retrospective review revealed a relatively high rate of unintended intradiscal injections that occurs in the performance of the retrodiscal approach for transforaminal epidural steroid injections (20). Unfortunately, this carries a likely risk of disc injury induced by the needle puncture. Rana and colleagues have shown that inserting an intrathecal catheter after a recognized accidental dural puncture significantly reduced the need for an epidural blood patch (29).

Special considerations

Pregnancy. Lumbar puncture and epidural injections can be carried out in pregnancy when indicated without any added risks as in normal adults.

Scientific basis

Injection of a local anesthetic into the lumbar epidural space results in diffusion of the drug in the rostral and caudal directions. Direct contact of the drug with the spinal roots as they cross the epidural space and possibly the dorsal root ganglia causes anesthesia at those levels (24). Thus, the level of anesthesia is defined by the extent of rostral and caudal diffusion of the drug as well as the concentration, volume, and potency of the drug.

The therapeutic effect of local anesthetics is to produce anesthesia by temporary blockade of nerve conduction through reversible inhibition of sodium channels. However, animal studies have demonstrated direct neurotoxic effects of anesthetics at high concentrations, such as are sometimes produced during epidural anesthesia (09; 33). Breakdown of the blood-brain or blood-nerve barrier, such as with intraneural or subarachnoid injection, leads to neurotoxicity at lower concentrations of local anesthetics (09).

The length of the lumbar section of the vertebral column is relatively short, and the dimensions of the lumbar epidural space are fairly constant; this results in only small differences in cranial spread of blockade after injection of local anesthetic at 3 different lumbar interspaces. In contrast, the thoracic part of the spinal column is longer, and it adjoins many different anatomical structures and spaces. Also, thoracic vertebrae and epidural space vary greatly in shape and size, and the above facts result in varying distribution of neural blockade following epidural injection.

In general, less local anesthetic is required to produce a given level of epidural anesthesia in pregnant patients. Engorgement of epidural veins by increased intraabdominal pressure has often been implied as the mechanism for this phenomenon. During pregnancy, onset of blockade of nerve conduction by local anesthetic is faster, and blockade is more intense (03). The recommendation that epidural catheters should be sited at an intervertebral space that represents the middle of the area of surgical incision is no longer tenable when one considers the different patterns of distribution after single injection or continuous infusion of local anesthetic. Also, sympathicolysis, sympathetic epidural blockade in a particular area of the body, may be considered as important as satisfactory analgesia. Naturally, epidural insertion sites for various surgical indications vary to accomplish both goals (38). Epinephrine-augmented hypotensive epidural anesthesia is an effective method to avoid the use of a tourniquet during total knee arthroplasty without the negative effects on perioperative hemoglobin values (19). Parvizi and colleagues retrospectively ascertained by chart review the incidence of epidural hematoma in 11,235 patients who had 12,991 knee arthroplasties and who received oral anticoagulation and epidural anesthesia for their surgery (26). For 1030 patients (1038 knees) whose charts were reviewed in detail, the mean international normalized ratio at the time of removal of the epidural catheter was 1.54 (range 0.93 to 4.25). Although administration of epidural anesthesia in patients with coagulopathy can be detrimental, they recognized no cases of epidural hematoma causing neurologic symptoms in patients receiving controlled oral anticoagulation after total knee arthroplasty. Kawaguchi and associates reported 2 cases of epidural anesthesia using ultrasound imaging and concluded that using ultrasound imaging before epidural puncture in obese children is safer and more educational for residents (18).