Lumbar Plexus Blocks

A. Lumbar Plexus/Psoas Compartment Block

Rita Merman

Patient Position: Lateral, with the surgical side up.

Indications: Anesthesia and immediate postoperative analgesia for hip, femur, or knee surgery.

Needle Size: 21-gauge, 100-mm insulated needle.

Volume: 20 to 30 mL 0.5% or 0.2% ropivacaine depending on the indication.

Anatomic Landmarks: The intercrestal line and the spinous process.

Approach and Technique: The spinous processes of L4, L5 is identified. A parallel line to the spinous process line is then drawn, originating from the postsuperior iliac spine (parasagittal line usually 4.5 to 5 cm lateral to midline). Next, a vertical line is drawn at the level of the highest point on the iliac crest (intercrestal line). The intersection of the intercrestal line with the parasagittal line determines the site of introduction of the needle (Fig. 13-1). The insulated needle, connected to a nerve stimulator (1.5 mA, 2 Hz, 0.1 ms) is introduced perpendicularly to the skin and advanced slowly in search of the transverse process of L4. The introduction of the needle is first associated with a direct stimulation of paravertebral muscles. When the transverse process is contacted, the needle is withdrawn and redirected either caudal or cephalad in search of the femoral nerve. The stimulation of the femoral nerve produces a quadriceps contraction and a patella snap. After appropriate positioning of the needle maintaining the motor response with a current of less than 0.5 mA and negative aspiration for blood, 2 mL of ropivacaine is slowly injected. The patient is asked to move both feet to verify that the injection is not intrathecal. Then 5 ml of local anesthetic is injected slowly alternating with repeated aspiration for blood (Fig. 13-2).

Figure 13-1. The intersection of the intercrestal line with the parasagittal line determines the site of introduction of the needle. Iliac Spine (IS). Postsuperior iliac spine (PSIS).

Tips

The lumbar spine may be flexed as in the positioning for a lumbar epidural or spinal technique to help identify surface anatomy landmarks.
More cephalad approaches to the lumbar plexus should be avoided. With a more cephalad approach there is an increased risk of renal puncture and renal hematoma.

Figure 13-2. X-ray indicating appropriate positioning.

Figure 13-3. Evaluation of lumbar plexus blocks. Sensory innervation with ice. A: Femoral nerve. B: Lateral femoral cutaneous nerve. C: Obturator nerve.
One may elicit quadriceps muscle contraction, without hitting the transverse process.
If the stimulation of the femoral nerve in the lumbar plexus does not occur within 2 cm of the transverse process, the needle is withdrawn from the skin and reintroduced after increasing the needle angulations by 10° laterally.
If the transverse process is not contacted at a depth of 9 cm or deeper, and quadriceps contraction is not elicited, the needle should be withdrawn and redirected medially.
Hamstring contraction indicates stimulation of the L4 component of the sacral trunk at the level of the nerve root. The needle needs to be withdrawn from the skin and redirected laterally.
A scrotum/labial paresthesia indicates stimulation of genitofemoral nerve (L1 branch of lumbar plexus), which lies anteriomedial in the psoas compartment. The needle needs to be withdrawn from the skin and redirected more laterally and less caudally.
Stimulation of iliohypogastric nerve causes abdominal wall contraction. In this case, redirect the needle more medially.
Because of the risk of epidural spread, it is important to monitor blood pressure during the injection of local anesthetic to limit the epidural spread. A decrease in arterial blood pressure is the first symptom suggesting such diffusion.
Exercise caution with the needle depth to avoid intraperitoneal injection. Non-obese patients require a needle of no more than 100 mm in length, inserted to a depth of 70 to 90 mm. However, with morbidly obese patients the femoral nerve may be reached

at a depth of 150 mm. Even in these patients it is recommended to first start with a 100-mm needle, because there is no correlation between weight and the depth of the lumbar plexus.

Figure 13-4. Evaluation of motor function against resistance. A: Femoral nerve. B: Obturator nerve.
The operator should ensure that a direct stimulation of the psoas muscle is not mistaken for a contraction of the quadriceps muscle by having an assistant place a hand on the quadriceps.
It is possible to identify the psoas compartment (a region containing the lumbar plexus) by using a loss-of-resistance approach. In fact, to reduce the number of punctures, it is possible to combine the use of the nerve stimulator and the loss-of-resistance approach by using an insulated needle connected to both a nerve stimulator (1.5 mA, 2Hz, 0.1 ms) and a loss-of-resistance syringe filled with 5 mL of air. As the needle goes through the posterior part of the psoas muscle a direct stimulation of the muscle is elicited. This muscle contraction disappears and a loss of resistance is felt when the needle enters the psoas compartment. After negative aspiration for blood, the local anesthetic solution can be injected slowly.
The aspiration of blood suggests the puncture of a paravertebral vein and indicates that the needle needs to be redirected laterally.
Local anesthesia is required to minimize patient discomfort using a 25-gauge, 38-mm needle and 5 mL of 1% lidocaine.
Moderate sedation should be provided in most cases with 1 to 2 mg of midazolam and 50 to 100 μg of fentanyl.
Evaluation of lumbar plexus blocks: sensory innervation with ice (femoral [A], lateral femoral cutaneous nerve [B], and obturator [C]; Fig. 13-3) and motor function against resistance (femoral [A] and obturator [B]; Fig. 13-4).