Psoas Compartment Block
The psoas compartment block uses a technique in which a needle
is placed into the space between the psoas major and quadratus lumborum muscles.
A large volume of injected solution anesthetizes the hip and anterolateral thigh.
[39]
Clinical Applications
The psoas compartment block offers a single injection rather than
three separate needle insertions for anesthesia of the lumbar plexus. The technique
must be combined with a sciatic block for anesthesia of the entire lower extremity.
Psoas compartment block is often used to provide postoperative analgesia for patients
undergoing major knee and hip surgery.
Technique: Posterior Approach
The patient is placed in the lateral position, with hips flexed
and operative extremity uppermost. A line is drawn to connect the iliac crests (i.e.,
intercristal line), identifying the fourth lumbar spine. After skin preparation,
a skin wheal is raised 3 cm caudad and 5 cm lateral to the midline on the side to
be blocked. A 21-gauge, 10-cm stimulating needle is then advanced perpendicular
to the skin entry site until it contacts the fifth lumbar transverse process. The
needle is redirected cephalad until it slides off the transverse process. The lumbar
plexus is identified by elicitation of a quadriceps motor response. When the needle
is in place, 30 mL of solution is injected.
Based on anatomic imaging studies, Capdevila and colleagues[40]
modified the classic psoas technique. Needle insertion site is the junction of the
lateral third and medial two thirds of a line between the spinous process of L4 and
a line parallel to the spinal column passing through the posterior superior iliac
spine. (The spinous process of L4 was estimated to be approximately 1 cm cephalad
to the upper edge of the iliac crests.) The needle is advanced perpendicularly to
the skin until contact with the transverse process of L4 is obtained and advanced
under the transverse process until quadriceps femoris muscle twitches are elicited.
Despite a difference between men and women in the depth of the lumbar plexus (median
values, 8.5 and 7.0 cm, respectively), the distance from the L4 transverse process
to the lumbar plexus was comparable (median value, 2 cm) in both sexes. The investigators
stressed the importance of achieving contact with the L4 transverse process to establish
appropriate needle depth and position.
Figure 44-12
A, Cutaneous distribution
of the lumbosacral nerves. B, Cutaneous distribution
of the peripheral nerves of the lower extremity.
Technique: Perivascular Approach
The perivascular approach (i.e., 3-in-1 block) to the psoas compartment
is based on the premise that injection of a large volume of local anesthetic within
the femoral canal while maintaining distal pressure will result in proximal spread
of the solution into the psoas compartment and consequent lumbar plexus block.[41]
The key anatomic assumption is that the fascial sheath surrounding the lumbar roots
extends into the femoral canal and acts as an enclosed conduit for the spread of
local anesthetic solutions. The patient lies in the supine position. The inguinal
ligament is marked as a line connecting the public tubercle and the anterior superior
iliac spine. The femoral artery is marked. A 22-gauge, 5-cm needle is advanced
lateral to the artery in a cephalad direction until a paresthesia or nerve stimulator
response is obtained. The needle is held immobile while distal pressure is applied
digitally to the femoral sheath. A total of 20 to 40 mL of solution is injected
incrementally after negative aspiration. Reliable anesthesia of the femoral and
lateral femoral cutaneous nerves can be predircted with 20 mL. However, obturator
nerve block may not occur even with volumes greater than 30 mL.
Side Effects and Complications
The deep needle placement with the posterior (psoas compartment)
approach increases the risk of possible epidural, subarachnoid, or intravascular
injection. Peripheral nerve damage is also a potential risk with this technique.
A side effect of the paravertebral approach to the lumbar plexus is the development
of a sympathetic block from extravasation of local anesthetic. This unilateral sympathectomy
is usually of little consequence. Because one of the reasons for choosing a lower
extremity block over spinal or epidural blockade is prevention of sympathectomy,
the advantage of a psoas compartment block is diminished if this effect occurs.
