Ultrasound Techniques

The patient is the prone position. The premedication of an adult, average size patient typically consists of 2-4 mg of midazolam; 250mg -500mg of alfentanyl is administered just before insertion of the needle.

TIP: Make sure the foot on the side of the block is protruding off the table. This will allow for detection and accurate interpretation of the motor response of the foot and toes.

Equipment:

Ultrasound machine with 6-8 MHz probe

Nerve block tray with following equipment items:

1. Betadine soaked sponges
2. 4”x4” gauze packs
3. Sterile gel for the ultrasound probe
4. Local anesthetic (30-40ml)
   
5. Insulated nerve stimulator needle (50 mm)
   
6. Injection pressure monitor (optional)
7. Sterile gloves
8. Surface electrode (optional)
9. Nerve stimulator (optional)
10. Ruler and skin marker
11. Remote controller (foot pedal) for the nerve stimulator (optional)

The landmarks for this block include:

1. Popliteal crease
2. Tendon of biceps femoris (laterally)
3. Tendon of semitendinosus (medially)

The landmarks are labeled  with a marking pen.

Orient yourself as to which part of the probe corresponds to medial, lateral, superior and inferior part of the screen. After applying a large, sterile “Tegaderm” and sterile gel to the probe, touch the end of the probe with the finger and observe the position of the shadow that occurs with the touch.

TIP: Make sure that there is no air trapping between the “Tegaderm” and the probe. Any trapped air will results in “shadowing” due to the poor acoustic impedance of the air.

The ultrasound probe (6-8MHz) is applied above the popliteal crease and angled to visualize popliteal artery.

The popliteal artery (PA) is easily identified as a hollow, hypoechoic, pulsating viscus. When in doubt, a color Doppler mode can be used to demonstrate the blood flow. Pulse-wave Doppler (PWD; Figure 8) mode can also be used to distinguish between the pulsatile (arterial flow) and the laminar (venous) flow. Popliteal vein is typically slightly lateral and superficial to the artery. The vein is often missed on the exam as it collapses with minimal probe pressure. The nerve(s) is see superficial and lateral to the artery. The tibial (TN) and common peronneal (CPN) nerves are typically separated closer to the popliteal fossa crease and can be seen as two slightly hyperechoic, oval structures lateral to the popliteal artery. As the probe is moved closer to the popliteal fossa crease, the common peroneal nerve is usually lost in the view; tracking the tibial nerve proximally brings both divisions into the view until they eventually form a single nerve at 6-8 cm above the crease (Figures 9 & 10).

This image shows the typical pulsatile pattern when pulse wave Doppler (PWD) is used across the popliteal artery. Pulse wave Doppler can be most useful to discern between arterial and venous flow, particularly when image quality does not allow for color flow imaging of sufficient quality.

The needle is connected to the nerve stimulator (1.0 mA; optional) and inserted at a steep angle to the long axis of the ultrasound probe (Figure 11) and its path is visualized usually by the shadowing and its effect on the neighboring structures.

Once the nerve is localized (using US or obtaining a motor response to nerve stimulation), local anesthetic is injected while paying attention to avoid excessive resistance (<20 psi).

The local anesthetic (LA) is shown layering around the sciatic nerve. Injections are best made lateral and medial to the artery to complete the spread of the local anesthetic. After the injection of local anesthetic, nerve stimulation can not be relied upon for any consecutive injections; reliance on the resistance to injection is the only protection against injection into a potentially anesthetized nerve.

TIP: If the resistance to injection exceeds 20 psi, maintain the syringe pressure to 20 psi and slightly rotate the needle. This almost always leads to dislodgment of the needle bevel from the low-compliant space the allows the injection <20 psi.