Figure 33–1. Psoas muscle (1) is shown exposed from within abdominal cavity with lumbar plexus branches (2, 3).

Distribution of Anesthesia

Injection of local anesthetic during lumbar plexus block most commonly results in spread of the injectate within the body of the psoas muscle around the lumbar branches (L2–4), with cephalad spread to the lumbar nerve roots.¹⁷

Figure 33–2. Lumbar plexus branches.

Figure 33–3. Distribution of anesthesia and analgesia after a successful lumbar plexus block.

The femoral nerve supplies motor fibers to the quadriceps muscle (knee extension), the skin of the anteromedial thigh, and the medial aspect of the leg below the knee and foot. The obturator nerve sends motor branches to the adductors of the hip and a highly variable cutaneous area on the medial thigh or knee joint. The lateral femoral cutaneous and genitofemoral nerves are purely cutaneous nerves. Figure 33-3 illustrates the cutaneous innervation of the lumbar plexus.

Choice of Local Anesthetic

Lumbar plexus blockade requires a relatively large volume of local anesthetic to achieve anesthesia of the entire plexus. The choice of the type and concentration of local anesthetic should be based on whether the block is planned for surgical anesthesia or pain management. Because of the vascular nature of the area and the potential for inadvertent intravascular injection, rapid absorption from the deep muscle beds, and epidural spread, then rapid, forceful injections should be avoided. Epinephrine is almost routinely used as a vascular marker. The most commonly used local anesthetic for this block in our institution is alkalinized 2-chloroprocaine 3% 1:300,000 epinephrine in outpatients having knee arthroscopy.^{18, 19} Some common choices of local anesthetics for this block are listed in Table 33-1.

Table 33–1.

Local Anesthetic Choices for Lumbar Plexus Block

Technique

The patient is in the lateral decubitus position with a slight forward tilt (Figure 33-4). The foot on the side to be blocked should be positioned over the dependent leg so that twitches of the quadriceps muscle and/or patella can be seen easily.

Figure 33–4. Patient position for lumbar plexus block.

Figure 33–5. Landmarks for lumbar plexus block. 1, Midline. 1, Iliac crest. 1, Needle insertion site is labeled 4 cm lateral to the midline.

A standard regional anesthesia tray is prepared with the following equipment:

• Sterile towels and 4-in. x 4-in. gauze packs

• 20-mL syringes with local anesthetic

• Sterile gloves, marking pen, and surface electrode

• One 1½-in., 25-gauge needle for skin infiltration

• A 10-cm long, short-bevel, insulated stimulating needle

• Peripheral nerve stimulator

Landmarks for the lumbar plexus block include (Figure 33-5):

1. Midline (spinous processes)

2. Iliac crest

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

After a cleaning with an antiseptic solution, the skin is anesthetized by infiltrating local anesthetic subcutaneously at the determined needle insertion site.

The fingers of the palpating hand are firmly pressed against the paravertebral muscles to stabilize the landmark and decrease the skin-nerve distance. The needle is inserted at a perpendicular angle to the skin (Figure 33-6). The nerve stimulator should be initially set to deliver current intensity of 1.5 mA. As the needle is advanced, local twitches of the paravertebral muscles are obtained first at a depth of a few centimeters. The needle is then advanced further until twitches of the quadriceps muscle are obtained (usually at the depth of 6–8 cm). After the twitches are obtained, the current should be lowered to obtain stimulation between 0.5 mA and 1.0 mA.

Figure 33–6. Needle insertion for the lumbar plexus block. The needle is inserted perpendicular to the body plane or with a slight medial orientation (shown).

At this point, 25–35 mL of local anesthetic is slowly injected with frequent aspiration to rule out inadvertent intravascular placement of the needle.

Clinical Pearls

Successful lumbar plexus blockade depends on the dispersion of the local anesthetic in the fascial plane (psoas muscle) where the roots of the plexus are situated. Thus, the goal of the nerve stimulation is to identify this plane by eliciting stimulation of one of the roots.

Stimulation at currents less than 0.5 mA should not be sought when using this technique. Durai sleeves thickly envelop the roots of the lumbar plexus. Motor stimulation with a low current may indicate placement of the needle inside a durai sleeve. An injection inside this sheath can result in tracking of the local anesthetic toward the epidural or subarachnoid space, with consequent epidural or spinal anesthesia.

When insertion of the needle does not result in quadriceps muscle stimulation, the maneuvers outlined in Table 33-2 should be followed.

Block Dynamics & Perioperative Management

A lumbar plexus block can be associated with significant patient discomfort during nerve localization owing to the needle passage through multiple muscle planes. Adequate sedation and analgesia are necessary to ensure a still and tranquil patient. Typically, we use midazolam 4–6 mg after the patient is positioned and alfentanil 500–750 meg just before needle insertion. A typical onset time for this block is 15–25 minutes, depending on the type, concentration, and volume of local anesthetic and the level at which the needle is placed.

Table 33–2.

Troubleshooting Procedures During Lumbar Plexus Blocks

For example, although an almost immediate onset of anesthesia in the anterior thigh and knee can be achieved with an injection at the L3 level, additional time is required for local anesthetic to block the lateral thigh (LI) or obturator nerve (L5). The first sign of the onset of blockade is usually the loss of sensation in the saphenous nerve territory (medial skin below the knee).

CONTINUOUS LUMBAR PLEXUS BLOCK

Continuous lumbar plexus blockade is an advanced regional anesthesia technique, and adequate experience with the single-shot technique is a prerequisite to ensure its efficacy and safety. The technique is similar to the single-shot injection except that the Tuohy needle is preferable. The needle opening should be directed cephalad to facilitate threading of the catheter. This technique can be used for postoperative pain management in patients undergoing hip, femur, and knee surgery.²⁰ Because a large volume of local is required anesthetic to accomplish analgesia, continuous infusion requires intermittent boluses for success. Consequently, some feel that its advantages over continuous femoral block for postoperative analgesia are questionable at best and that continuous lumbar plexus block should not be in routine use for postoperative analgesia.²¹