Lumbar Plexus Block

Figure 6: Landmarks for the lumbar plexus block. The needle insertion site is labeled 3-4 cm lateral to the intersection of the (horizontal) line passing through spinous processes and iliac crest (perpendicular) line.

After disinfecting with an antiseptic solution, the skin and paravertebral muscles are anesthetized by infiltrating local anesthetic subcutaneously at site of needle insertion.

The fingers of the palpating hand are firmly pressed against the paravertebral muscles to stabilize the landmark and decrease the skin-nerve distance (Figure 8). The palpating hand should not be moved during the entire block procedure so that precise redirections of the angle of needle insertion can be made, if necessary. The needle is inserted at an angle perpendicular to the skin with the nerve stimulator set initially to deliver a current of 1.5 mA (1.5 mA, 2 Hz, 0.1-0.3 ms). As the needle is advanced, local twitches of the paravertebral muscles are obtained first at a depth of a few centimeters. The needle is advanced further, until twitches of the quadriceps muscle are obtained (usually at a depth of 6-8 cm). After these twitches are observed, the current should be lowered to produce stimulation between 0.5 and 1.0 mA. Motor response should not be present at a current less than 0.5 mA, which could indicate needle placement in the dural sleeve. At this point, 25 to 35 mL of local anesthetic is injected slowly while avoiding resistance to injection and with frequent aspirations to rule out inadvertent intravascular needle placement. A resultant, typical spread of the local anesthetic solution is demonstrated in Figure 9.

Troubleshooting

When insertion of the needle does not result in quadriceps muscle stimulation, the following maneuvers should be followed:

1. Withdraw the needle to the skin level, redirect it 5° to 10° cranially, and repeat the procedure.

2. Withdraw the needle to the skin level, redirect it 5° to 10° caudally, and repeat the procedure.

3. Withdraw the needle to the skin level, redirect it 5° to 10° medially, and repeat the procedure.

4. Withdraw the needle to the skin level, reinsert it 2 cm caudally or cranially, and repeat the procedure.

Block Dynamics and Perioperative Management

A lumbar plexus block can be associated with significant patient discomfort because the needle passes through multiple muscle planes. Adequate sedation and analgesia are necessary to ensure a still and cooperative patient. We often use midazolam 4 to 6 mg after the patient is positioned and alfentanil 500 to 750 µg just before needle insertion. A typical onset time for this block is 20 to 30 minutes, depending on the type, concentration, and volume of local anesthetic and the level at which the needle is placed. The first sign of the onset of blockade is usually a loss of sensation in the saphenous nerve territory (medial skin below the knee).

Continuous Lumbar Plexus Block

Figure 9: Radiograph demonstrating distribution of the radiopaque solution within the psoas muscle after a lumbar plexus injection.

A continuous lumbar plexus blockade is an advanced regional anesthesia technique, and adequate experience with the single-injection technique is a prerequisite to ensure its efficacy and safety. Otherwise, the technique is quite similar to the single-injection procedure, except that a Tuohy-style needle is preferred. The needle opening should be directed caudad and laterally to facilitate threading of the catheter in the direction of the plexus. The technique can be used for postoperative pain management in patients undergoing hip, femur, and knee surgery.

Equipment

A standard regional anesthesia tray is prepared with the fol- lowing equipment:

Sterile towels and gauze packs

Two 20-mL syringes containing local anesthetic

Sterile gloves, marking pen, and surface electrode

A syringe plus 25-gauge needle with local anesthetic for     skin infiltrationor skin infiltration

Peripheral nerve stimulator

Catheter kit (including a 8- to 10-cm large-gauge     stimulating needle and catheter)

Landmarks and Patient Positioning

As for the single-injection technique, the patient is positioned in the lateral decubitus position with the side to be blocked up and with a slightly forward pelvic tilt (Figure 4). An assistant helps maintain flexion of the spine, as in positioning a patient for an epidural or spinal block in the lateral position.

The landmarks for a continuous lumbar plexus block are the same as for the single-injection technique (Figure 6):

1. Iliac crests (intercristal line)

2. Midline (spinous processes)

3. Needle insertion site 4 cm lateral to the intersection of landmarks 1 and 2

Technique

Figure 10: Insertion of the catheter in the lumbar plexus. The catheter is inserted approximately 5 cm beyond the needle tip.

The skin and subcutaneous tissues are anesthetized with local anesthetic. The needle is attached to the nerve stimulator (1.5 mA, 2 Hz). The palpating hand should be firmly pressed and anchored against the paraspinal muscles to facilitate needle insertion and redirection of the needle when necessary. An 8-10-cm Tuohy-style continuous block needle is inserted at a perpendicular angle and advanced until the quadriceps muscle contractions are obtained at 0.5 to 1.0 mA. A 5-10 mL of local anesthetic or other injectate (e.g., D5W) is injected to "open up" a tissue space and facilitate catheter advancement. The catheter is threaded through the needle for approximately 5 cm beyond the tip of the needle (Figure 10). The needle is withdrawn, the catheter secured, and the remaining anesthetic is injected via the catheter. Before administration of the local anesthetic, the needle and/or catheter are checked for inadvertent intravascular and intrathecal placement. This is done by performing an aspiration test and administering a test dose of epinephrine-containing local anesthetic.

Continuous Infusion

Continuous infusion is always initiated after an initial bolus of dilute local anesthetic through the needle or catheter. For this purpose, 0.2% ropivacaine (15 to 20 mL) is used most commonly. The infusion is maintained at 5 mL/h with 5 mL/h patient-controlled regional analgesia bolus dose.

Complications and How to Avoid Them

The lumbar plexus block is an advanced nerve block technique that carries a potential for serious complications if proper precautions are not strictly followed. Some complications and methods to decrease the risk of them are listed in Table 2.