9 Psoas Block
9.1 Anatomical Overview
The anterior rami of the first four lumbar nerves lie between the deep and the superficial origins of the psoas major muscle and form the lumbar plexus. The ramus of the fourth lumbar nerve divides into cranial, medial, and caudal branches:
The cranial part supplies the femoral nerve.
The medial part supplies the obturator nerve.
Iliohypogastric nerve and ilioinguinal nerve.
The first branch from the lumbar plexus, the iliohypogastric nerve, lies at the lateral border of the psoas major muscle. It is usually followed by the ilioinguinal nerve, passing through the psoas muscle and running almost parallel.
The next nerve passing through psoas major is the genitofemoral nerve, which divides at a variable level into the genital branch and the femoral branch.
Lateral cutaneous nerve of the thigh.
A further branch of the lumbar plexus lying at the lateral border of the psoas major muscle is the lateral cutaneous nerve of the thigh, which reaches the muscular lacuna far laterally close to the anterior superior iliac spine.
The biggest branch, the femoral nerve, runs in the groove between the iliacus and psoas major muscles and passes through the muscular lacuna to the thigh.
The last branch, the obturator nerve, is the only one that runs medial to the psoas major muscle and reaches the obturator canal after passing beneath the external iliac artery and vein and crossing the linea terminalis of the lesser pelvis. It passes through and perforates the parietal pelvic fascia to reach the obturator canal (Fig. 9.2 and Fig. 9.3).
There is no evidence of a compartment in the anatomical sense. The lumbar plexus permeates the psoas muscle. The term “psoas compartment block” should be replaced by “psoas block.”
9.2 Technique of Psoas Block
9.2.1 Classical Technique (according to Chayen)
The technique, originally called “psoas compartment block”, was described in 1976 by Chayen (Chayen et al 1976). In the following years, other versions of the technique were published, including continuous anesthesia (e.g., Geiger 1999).
A dorsal line between the iliac crests marks the spinous process of the 4th lumbar vertebra. A 3-cm interspinal line is drawn caudally from the spinous process of L4. From the caudal end of this line, a 5-cm line is drawn laterally at a right angle toward the side to be blocked. This second line ends a little before the medial border of the iliac crest, cranial to the posterior superior iliac spine, and corresponds to the insertion site (Fig. 9.5).
Position of the Patient
The patient is positioned on his or her side with the legs drawn up and the spine flexed with the side to be anesthetized uppermost (Fig. 9.6). Alternatively, the patient may assume a sitting position, similar to that used for neuraxial anesthesia (Fig. 9.7).
Using continuous stimulation with a current of 0.5 to 1.0 mA, a needle 10 to 12 cm long is advanced at a right angle to the skin in strictly sagittal direction (Fig. 9.6, Fig. 9.7, Fig. 9.8). Contact between the needle tip and the transverse process of the 5th lumbar vertebra is first sought at a depth of 5 cm to a maximum of 8 cm (Fig. 9.7 and Fig. 9.8). After making bone contact and withdrawing the needle about 4 cm, the needle is changed to a more cranial direction and advanced again.
The needle should never be advanced more than 2.5 cm beyond the first bone contact, and never more than 11 cm overall (Fig. 9.8).
When the needle has passed the transverse process of the 5th lumbar vertebra, the loss of resistance after passing through the quadratus lumborum muscle and transversalis and psoas fascia indicates that the psoas compartment has been reached (Fig. 9.9, Fig. 9.10, Fig. 9.11, Fig. 9.12, Fig. 9.13).
Muscle contractions of the quadriceps (anterior thigh) indicate that the tip of the needle is in the correct position close to the femoral nerve (Fig. 9.14). The desired response corresponds to the motor block obtained by femoral nerve blockade.
After careful aspiration, a test dose of 3 mL of local anesthetic is injected to exclude an incorrect intrathecal position. This is followed by injection of 40 mL of a medium-acting or long-acting local anesthetic. After each 10 mL, aspiration should be repeated to exclude an accidental intravascular position. Initially, 40 mL of prilocaine 1% or mepivacaine 1% (10 mg/mL) or 30 mL of prilocaine 1% (10 mg/mL) and 10 mL of ropivacaine 0.75% (7.5 mg/mL) can be injected (Büttner and Meier 1999, Geiger 1999, Meier and Büttner 2001).
Continuous Psoas Block
The anatomical orientation corresponds to the landmarks given by Chayen (see above). The puncture is made with a 12-cm stimulation needle, which allows a catheter to be introduced. When advancing the needle, the transverse process of the 5th lumbar vertebra does not absolutely have to be contacted. Contractions of the quadriceps indicate the immediate vicinity of the femoral nerve.
After correct stimulation, negative aspiration, and a test dose of a local anesthetic (3 mL of a medium-acting local anesthetic to exclude an intrathecal position), 30 mL of local anesthetic is injected. The catheter is advanced caudally (Fig. 9.15). Slight resistance at the end of the needle during advancement is normal and is caused by the transition of the needle tip to the tissue. This slight resistance is usually easily overcome (Geiger 1999).
A trial aspiration is performed to exclude an intravascular position and another test dose is given through the catheter to exclude an intrathecal position.
Initially, 20 mL of prilocaine 1% (10 mg/mL) and 10 mL of ropivacaine 0.5% (5 mg/mL) can be injected; for continuous administration, 5 to 15 mL/h of ropivacaine 0.2% (2 mg/mL) can be injected or bolus injections of 20 mL of ropivacaine 0.2 to 0.375% (2–3.75 mg/mL) can be given. The maximum recommended dose of ropivacaine is 37.5 mg/h (Büttner and Meier 1999, Meier 2001, Meier and Büttner 2001; Fig. 9.16 and Fig. 9.17).
9.2.2 Psoas Blockade with Ultrasound
Curved array: 2 to 5 MHz
Penetration depth: 10 to 15 cm
Needle: 10 to 12 cm
An overview of the position of the kidneys is possible in the frontal plane (Fig. 9.19). The neural structures in the psoas muscle can usually not be visualized in adults by ultrasound due to the large skin-to-nerve distance. The nerves run in the posterior third of the psoas major muscle.
In the next step, the distance from the skin to the costal processes (transverse processes) can be determined in the parasagittal plane (long axis) using ultrasound (Fig. 9.19). In the long axis, the transducer is moved laterally from the sagittal plane starting from the level of L4/5 toward the side to be anesthetized. First the articular process becomes visible, appearing as a more or less continuous, wavy hyperechoic line with interrupted acoustic shadows below it caused by the bony structures of the articular process (Fig. 9.20). When the transducer is moved further laterally, the costal processes (transverse processes) come into view, visualized as intermittent bony acoustic shadows interspersed with soft tissue (psoas major; Fig. 9.20). When the transducer is moved further laterally, the lateral border of the transverse processes can be visualized as the loss of the acoustic shadows. The kidney may appear in the image here.
This method gives greater certainty with respect to the estimated penetration depth of the needle (Ilfeld et al 2010).
The median depth of the costal process was determined to be 5.0 cm (interquartile 4.5–5.5 cm; range 3.5–7.5 cm). To produce a response using the nerve stimulator, the needle must be advanced another 2.5 cm (interquartile 2.0–3.0 cm; range 0.2–4.0 cm). The lumbar plexus thus lies at a depth of 7.5 cm (interquartile 7.0–8.0; range 5.0–9.5 cm) (Ilfeld et al 2010).
This study (Ilfeld et al 2010) showed that the injection site described by Capdevilla et al (2002) at the junction from the middle to the lateral third on a transverse line from L4 to a line running parallel to the spine through the posterior superior iliac spine was too lateral by 0.75 cm in 50% of the patients examined. This is largely consistent with the results of a study by Heller et al (2009).
In the plane between the transverse processes of L4/5, the transducer is then rotated in the transversal plane. Immediately lateral to the spinous process, the typical structure of the articular process can be visualized in cross-section with the bony acoustic shadow. The psoas major can often be visualized further laterally and somewhat deeper. At an even greater penetration depth behind the psoas major, a hyperechoic line can be seen that marks the junction from the psoas major to the peritoneal cavity. Intestinal movement can often be seen beneath it. The sagittal diameter of the psoas major at the level of L4/5 is reported to be 4.7 ± 0.4 cm (Takai et al 2011); at L3/4 the sagittal diameter is somewhat smaller (about 4 cm).