Hadzic's Peripheral Nerve Blocks and Anatomy for Ultrasound-Guided Regional Anesthesia, 2nd

PART 2: LATERAL APPROACH

General Considerations

The lateral approach to a popliteal blockade is similar to the intertendinous block in many aspects. The main difference is that the technique involves placement of the needle from the lateral aspect of the leg, therefore obviating the need to position the patient in the prone position. Nerve stimulation principles, volume requirements, and block onset time are the same. The block is well suited for surgery on the calf, Achilles tendon, ankle, and foot. It also provides adequate analgesia for a calf tourniquet.

Functional Anatomy

The sciatic nerve consists of two separate nerve trunks, the tibial and common peroneal nerves (Figure 20.2-1). A common epineural sheath envelops these two nerves at their outset in the pelvis. As the sciatic nerve descends toward the knee, the two components eventually diverge in the popliteal fossa, giving rise to the tibial and common peroneal nerves. This division of the sciatic nerve usually occurs 5–7 cm proximal to the popliteal fossa crease.

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FIGURE 20.2-1. Anatomy of the popliteal fossa crease. Image tibial nerve. Image common peroneal nerve before its division. Image epineural sheath of the common sciatic nerve. Image tendon of semitendinosus and semimembranous. Imagebicep femoris tendon.

Distribution of Blockade

The lateral approach to popliteal block also results in anesthesia of the entire distal two thirds of the lower extremity with the exception of the skin on the medial aspect of the leg (Figure 20.2-2). Cutaneous innervation of the medial leg below the knee is provided by the saphenous nerve, the terminal extension of the femoral nerve. Depending on the level of surgery, the addition of a saphenous nerve block may be required for complete surgical anesthesia.

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FIGURE 20.2-2. Sensory distribution of anesthesia accomplished with popliteal sciatic block. All shaded areas except medial aspect of the leg (blue, saphenous nerve) are anesthetized with the popliteal block.

Single-Injection Popliteal Block (Lateral Approach)

Equipment

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

• Sterile towels and gauze packs

• Two 20-mL syringes containing local anesthetic

• A 3- to 5-mL syringe plus 25-gauge needle with local anesthetic for skin infiltration

• A 10-cm, 21-gauge short-bevel insulated stimulating needle

• Peripheral nerve stimulator

• Sterile gloves; marking pen

Landmarks and Patient Positioning

The patient is in the supine position. The foot on the side to be blocked should be positioned so that the motor response of the foot or toes can be easily observed (Figure 20.2-3). This is best achieved by placing the leg on a footrest with the heel and the foot protruding beyond the footrest. This positioning allows for easy visualization of foot twitches during nerve localization. The foot should form a 90° angle to the horizontal plane of the table.

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FIGURE 20.2-3. Maneuver to accentuate landmarks for popliteal sciatic block. The patient is asked to flex the leg at the knee, which accentuates the popliteal fossa and hamstring muscles.

Landmarks for the lateral approach to a popliteal block include the following (Figure 20.2-4 and 20.2-5):

1. Popliteal fossa crease

2. Vastus lateralis muscle

3. Biceps femoris muscle

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FIGURE 20.2-4. Popliteal fossa is marked with the knee flexed.

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FIGURE 20.2-5. The main landmark for popliteal sciatic block is in the groove between the vastus lateralis Image and the biceps femoris muscles Image.

The needle insertion site is marked in the groove between the vastus lateralis and biceps femoris muscles 7-8 cm above the popliteal fossa crease. Note that the lateral femoral epicondyle is another landmark that can be used with this technique. It is easily palpated on the lateral aspect of the knee 1 cm cephalad to the popliteal fossa crease (Figures 20.2-6 and 20.2-7).

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FIGURE 20.2-6. Needle insertion site is labeled at 7 cm proximal to the popliteal fossa crease in the groove between the vastus lateralis and bicep femoris muscles.

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FIGURE 20.2-7. A needle insertion point for lateral approach sciatic popliteal block.

TIP

• In patients with an atrophic biceps femoris muscle (e.g., prolonged immobility), the iliotibial aponeurosis can prove to be a more consistent landmark). In this case, the needle insertion site is in the groove between the vastus lateralis and the iliotibial tract.

Maneuvers to Facilitate Landmark Identification

Landmarks can be better appreciated using the following steps:

• Lifting the foot off the table accentuates the biceps femoris and vastus lateralis muscles, and helps the recognition of the groove between the two muscles.

• The groove between the vastus lateralis and biceps femoris can be located by firmly pressing the fingers of the palpating hand against the adipose tissue in the groove approximately 8 cm above the popliteal fossa crease.

Technique

The operator should be seated facing the side to be blocked. The height of the patient’s bed is adjusted to allow for a more ergonomic position and greater precision during block placement. This position also allows the operator simultaneously to monitor both the patient and the responses to nerve stimulation.

The site of estimated needle insertion is prepared with an antiseptic solution and infiltrated with local anesthetic using a 25-gauge needle. It is useful to infiltrate the skin along a line rather than raise a single skin wheal. This allows needle reinsertion at a different site when necessary without the need to anesthetize the skin again.

A 10-cm, 21-gauge needle is inserted in a horizontal plane perpendicular to the long axis of the leg between the vastus lateralis and biceps femoris muscles (Figure 20.2-8), and it is advanced to contact the femur. Contact with the femur is important because it provides information about the depth of the nerve (typically 1–2 cm beyond the skin to femur distance) and about the angle at which the needle must be redirected posteriorly to stimulate the nerve (Figure 20.2-9). The current intensity is initially set at 1.5 mA. With the fingers of the palpating hand firmly pressed and immobile in the groove, the needle is withdrawn to the skin level, redirected 30° below the horizontal plane, and advanced toward the nerve.

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FIGURE 20.2-8 Needle insertion for lateral approach to popliteal block.

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FIGURE 20.2-9. Needle insertion strategy for lateral approach to popliteal sciatic block. (A) needle is first inserted to contact femur. (B) After contact with the femur, the needle is withdrawn back to the skin and redirected 30° posteriorly to local the sciatic nerve. Note the needle passage through the biceps femoris muscle before entering the popliteal fossa crease. This explains why local bicep femoris muscle twitch is often obtained during needle advancement. 1 - Semimembranosus-semitendinosus muscles, 2 - Biceps Femoris, 3 - Femur, 4 - Popliteal artery and vein, 5 - Common peroneal nerve, 6 - TIbial nerve

GOAL

The ultimate goal of nerve stimulation is to obtain visible or palpable twitches of the foot or toes at a current of 0.2–0.5 mA.

TIPS

• The needle passes through the biceps femoris muscle, often resulting in local twitches of this muscle during needle advancement. Cessation of the local twitches of the biceps muscle should prompt slower needle advancement because this signifies that the needle is in the popliteal fossa and in close proximity to the sciatic nerve.

• When stimulation of the sciatic nerve is not obtained within 2 cm after cessation of the biceps femoris twitches; the needle is probably not in plane with the nerves and should not be advanced further because of the risk of puncturing the popliteal vessels.

After initial stimulation of the sciatic nerve is obtained, the stimulating current is gradually decreased until twitches are still seen or felt at 0.2–0.5 mA. This typically occurs at a depth of 5–7 cm. At this point, the needle should be stabilized and, after aspiration test for blood, 30–40 mL of local anesthetic is injected slowly.

Troubleshooting

When the sciatic nerve is not localized on the first needle pass, the needle is withdrawn to the skin level and the following algorithm is used:

1. Ensure that the nerve stimulator is functional, properly connected to the patient and to the needle, and set to deliver the current of desired intensity.

2. Ensure that the leg is not externally rotated at the hip joint and that the foot forms a 90° angle to the horizontal plane of the table. A deviation from this angle changes the relationship of the sciatic nerve to the femur and the biceps femoris muscle.

3. Mentally visualize the plane of the initial needle insertion and redirect the needle in a slightly posterior direction (5–10° posterior angulation).

4. If step 3 fails, withdraw the needle and reinsert it with an additional 5–10° posterior redirection.

5. Failure to obtain a foot response to nerve stimulation should prompt reassessment of the landmarks and leg position. In addition, the stimulating current should be increased to 2 mA.

TIPS

• When motor response can be elicited only with current of ≥0.5 mA, tibial nerve response (Figure 20.2-10) may be associated with a higher success rate of anesthesia of both divisions of the nerve.

• Isolated twitches of the calf muscles should not be accepted as reliable signs because they can be the result of stimulation of the sciatic nerve branches to the calf muscles that may be outside the sciatic nerve sheath.

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FIGURE 20.2-10. Motor responses of the foot obtained with stimulation of the sciatic nerve in the popliteal fossa. Stimulation of the tibial nerve results in plantar flexion and inversion of the foot. Stimulation of the common peroneal nerve results in dorsi flexion and inversion of the foot.

Table 20.2-1 lists the common responses that can occur during block placement using a nerve stimulator and the proper course of action needed to obtain twitches of the foot.

TABLE 20.2-1 Some Common Responses to Nerve Stimulation and Course of Action

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Block Dynamics and Perioperative Management

This technique may be associated with patient discomfort because the needle transverses the biceps femoris muscle, and adequate sedation and analgesia are necessary. Administration of midazolam (2–4 mg intravenously) and a short-acting narcotic (alfentanil 250–to 750 g) ensures patient comfort and prevents patient movement during needle advancement. Inadequate premedication can make it difficult to interpret the response to nerve stimulation because of patient movement during needle advancement. A typical onset time for this block is 15–30 minutes, depending on the type, concentration, and volume of local anesthetic used. The first signs of onset of the blockade are usually a report by the patient that the foot “feels different” or there is an inability to wiggle their toes. With this block, sensory anesthesia of the skin is often the last to develop. Inadequate skin anesthesia despite an apparently timely onset of the blockade is common, and it can take up to 30 minutes to develop. Local infiltration by the surgeon at the site of the incision is often all that is needed to allow the surgery to proceed.

Continuous Popliteal Block (Lateral Approach)

The technique is similar to the single-injection except that slight angulation of the needle cephalad is necessary to facilitate threading of the catheter. Securing and maintaining the catheter are easy and convenient with this technique. A lateral popliteal block is suitable for surgery and postoperative pain management in patients undergoing a wide variety of lower leg, foot, and ankle surgeries.

Equipment

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

• Sterile towels and gauze packs

• Two 20-mL syringes containing local anesthetic

• Sterile gloves, marking pen, and surface electrode

• A 3- to 5-mL syringe plus 25-gauge needle with local anesthetic for skin infiltration

• Peripheral nerve stimulator

• Catheter kit (including an 8- to 10-cm 18–19 gauge stimulating needle and catheter)

Either nonstimulating (conventional) or stimulating catheters can be used. During the placement of a conventional non-stimulating catheter, the stimulating needle is advanced until appropriate twitches are obtained. Then 5–10 mL of local anesthetic or other injectate (e.g., D5W) is then injected to “open up” a space for the catheter to advance freely without resistance. The catheter is threaded through the needle until approximately 3–5 cm is protruding beyond the tip of the needle. The needle is withdrawn, the catheter secured, and the remaining local anesthetic injected via the catheter. With stimulating catheters, after obtaining desired motor response with stimulation through the needle, the catheter is advanced with the nerve stimulator connected until the anesthesiologist is satisfied with the quality of the motor response. If the response is lost, the catheter can be withdrawn until it reappears and the catheter readvanced. This method requires that no conducting solution be injected through the needle (i.e., local anesthetic, saline) prior to catheter advancement, or difficulty obtaining a motor response will result.

Landmarks and Patient Positioning

The patient is positioned in the supine position with the feet extending beyond the table to facilitate monitoring of foot or toe responses to nerve stimulation.

The landmarks for a continuous popliteal block with the lateral approach are essentially the same as for the single-injection technique and include the following:

1. Popliteal fossa crease

2. Vastus lateralis

3. Biceps femoris

The needle insertion site is marked at 8 cm proximal to the popliteal fossa crease in the groove between the vastus lateralis and biceps femoris.

Technique

The continuous popliteal block technique is similar to the single-injection technique. The patient is in the supine position. Using a 25-gauge needle, infiltrate the skin with local anesthetic at the injection site 7–8 cm proximal to the popliteal crease in the groove between the biceps femoris and vastus lateralis muscles. An 8- to 10-cm needle with a Tuohy-style tip for a continuous nerve block is connected to the nerve stimulator (1.5 mA) and inserted to contact the femur (Figure 20.2-11). A slight cephalad orientation to the needle with the opening facing proximally will aid in catheter threading. Once the femur is contacted, the needle is withdrawn to the skin level and redirected in a slightly posterior direction 30°. Then it is advanced slowly while observing the patient for plantar flexion or dorsiflexion of the foot or toes. After obtaining the appropriate twitches, continue manipulating the needle until the desired response is still seen or felt using a current of 0.2–0.5 mA. The catheter should be advanced no more than 5 cm beyond the needle tip. The needle is withdrawn back to the skin level, and the catheter advanced simultaneously to prevent inadvertent removal of the catheter.

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FIGURE 20.2-11. Catheter insertion technique for popliteal sciatic block. Technique is similar to that of the single-injection technique. Catheter is inserted 3–5 cm beyond the needle tip.

The catheter is checked for inadvertent intravascular placement and secured to the lateral thigh using an adhesive skin preparation such as benzoin, followed by application of a clear dressing. The infusion port should be clearly marked “continuous nerve block.”

TIPS

• With a popliteal catheter, a response at 0.5–1.0 mA should be accepted as long as the motor response is specific and clearly seen or felt.

• A very small (e.g., 1 mm) movement of the needle often results in a change in the motor response from that of the tibial nerve (plantar flexion of the foot) to that of the common peroneal nerve (dorsiflexion of the foot). This indicates an intimate needle to nerve relationship at a level before divergence of the sciatic nerve.

• When catheter insertion proves difficult, rotate the needle slightly and try reinserting it again. When these maneuvers do not facilitate insertion of the catheter, angle the needle in a cephalad direction before reattempting to insert the catheter. With this maneuver, care should be taken not to dislodge the needle.

Continuous Infusion

Continuous infusion is initiated after an initial bolus of dilute local anesthetic is administered through the catheter or needle. For this purpose, we routinely use 0.2% ropivacaine 15–20 mL. Diluted bupivacaine or levobupivacaine are suitable but can result in additional motor blockade. The infusion is maintained at 5 mL/h with 5-mL/h patient-controlled regional analgesia.

Complications and How to Avoid Them

Table 20.2-2 provides specific instructions on some complications and how to avoid them.

TABLE 20.2-2 Complications of Popliteal Block Through the Lateral Approach and Preventive Techniques

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TIPS

• Breakthrough pain in patients undergoing a continuous infusion is always managed by administering a bolus of local anesthetic. Simply increasing the rate of infusion is not adequate.

• When the bolus injection through the catheter fails to result in blockade after 30 minutes, the catheter should be considered dislodged and should be removed.

• All patients with continuous nerve block infusion should be prescribed an alternative pain management protocol because incomplete analgesia and/or catheter dislodgment can occur.

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SUGGESTED READINGS

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