Ultrasound-Guided Nerve Blocks

The use of ultrasound guidance to perform nerve blocks has been the most exciting development in the field of regional anesthesia in recent years. Instead of blindly advancing the needle toward the target nerve using anatomic landmarks, nerve stimulator, or paresthesia technique, the needle can now be advanced toward its target while imaging both the nerve and the advancing needle. This helps the operator appreciate variations in anatomy and identify structures such as blood vessels or the pleura that should be avoided while advancing the needle. Furthermore, local anesthetic spread around the nerves can be imaged in real time during performance of the block, which can help improve block quality ( Fig. 24-1 ). Also, intraneural and intravascular positioning of the block needle can be identified early, thereby avoiding severe injury.

Figure 24-1

Circumferential spread of local anesthetic around the sciatic nerve “doughnut sign.”

To successfully use this technique, the detailed knowledge of cross-sectional anatomy, significant skill in advancing the needle, and experience in interpreting the ultrasound images is required. This requires the knowledge of artifacts of ultrasound-guided regional anesthesia such as acoustic enhancement and shadowing ( Fig. 24-2 ). In addition, an understanding of basic ultrasound technology is needed to successfully practice ultrasound-guided regional anesthesia.

Figure 24-2

Ultrasound scan of the femoral nerve using a high-frequency linear array transducer. The needle should penetrate the fascia lata and the fascia iliaca for the block to be successful.

The objective of this chapter is to introduce pain management physicians to ultrasound-guided nerve blocks. Although ultrasound guidance for pain management has only recently gained popularity, pain physicians are using this modality more frequently, sometimes in combination with fluoroscopic guidance. Although the block descriptions in this chapter are for regional anesthesia, we believe that the principles are the same when performing these or other peripheral nerve blocks for pain management. Also, we do not recommend abandoning older approaches such as nerve stimulator or paresthesia techniques; instead, we suggest using the ultrasound image as additional information to safely guide your needle toward the target nerve. Clinical practice has shown that intraneural positioning of a needle may not always cause a paresthesia or even nerve stimulation, particularly in diabetic patients. A paresthesia or the presence of motor stimulation using a nerve stimulator may alert you that your needle is too close to the nerve when the needle is poorly imaged with ultrasound. In this chapter, we will therefore also describe the role of nerve stimulation and eliciting paresthesias while performing the various nerve blocks ( Fig. 24-3 ).

Figure 24-3

Combining a nerve stimulator, as shown, with ultrasound guidance is recommended particularly when the nerve is poorly visualized.

We recommend reading specific books on ultrasound and gaining a detailed introduction on the specific ultrasound scanner you are using because this topic goes well beyond the scope of this chapter.

Some Brief Facts

Ultrasound is a name given to high-frequency sound waves; 20,000 cycles per second (20 kHz) or higher, inaudible to human ears, that can be transmitted in beams and can be used to scan tissues of the body. Transducers emit ultrasound waves of different frequencies. Frequencies used for nerve blocks usually range from 5 to 14 MHz.

The higher the frequency, the shorter the wavelength, the better the resolution.

However, higher frequencies are more readily scattered and absorbed and therefore have poorer penetration of the tissue. Thus, for imaging of superficial structures (e.g., axillary nerve block, interscalene nerve block, vascular access) a high frequency transducer should be used. Lower frequency transducers will have better penetration of the tissue but have poorer resolution and should be used for nerves that are embedded deeper in the tissue (e.g., popliteal, infraclavicular, sciatic nerve block).

This general rule is not written in stone, for example, you may be better off using a lower frequency when doing an interscalene nerve block in an obese patient and a high frequency could be used when doing a popliteal nerve block in a thin patient. In other words, the frequency should be adjusted according to the type of nerve block being performed and the patient’s body habitus.

The transducers both transmit and receive ultrasound beams. The reflected ultrasound beams are received by the transducer and are amplified in the scanner. Different tissues alter the waves in different ways. Some scatter, whereas others reflect the waves directly before they return to the transducer as echoes. Bone and gas significantly reflect and scatter the ultrasound waves, whereas fluids produce a weak echo. Strong echoes appear as bright dots or lines on the screen and weak echoes appear as dark areas. The echoes that return from deeper structures are weaker than those from superficial structures and need to be amplified, which is done by the time-gain-compensation amplifier. After picking the appropriate frequency and depth setting you may be able to improve your image prior to starting the nerve block by adjusting the gain. Another useful function of the scanner is Doppler ultrasound. It can be used to detect blood flow in peripheral vessels and help avoid vascular injury while advancing the needle toward the target nerve. For this reason we recommend using the Doppler function to identify vessels close to the nerve before beginning any nerve block.

The two types of transducers typically used for nerve blocks are linear array ( Fig. 24-4 ) and microconvex (curvilinear) transducers ( Fig. 24-5 ). The linear array transducer emits high frequency ultrasound beams as the name states in a linear array and produces scans of rectangular shape. These transducers have a larger footprint than the microconvex probes and are better for scanning superficial structures (e.g., axillary nerve block, interscalene nerve block).

Figure 24-4

Linear array ultrasound transducer with rectangular scan.

Figure 24-5

Microconvex ultrasound transducer with fan-shaped scan.

The microconvex transducer has a small footprint and produces scans that are fan-shaped. Thus, larger sections of deeper tissue can be scanned compared to the linear array transducer. The microconvex probes usually also emit lower frequency ultrasound waves. It is therefore used to scan deeper structures (e.g., popliteal nerve block, infraclavicular nerve block) or in cases where only a small space is available for scanning (due to its smaller footprint).

To successfully perform an ultrasound-guided nerve block, the nerve structures that are to be blocked and the structures that should be avoided, such as pleura and blood vessel, need to be clearly identified. The ultrasound probe is typically placed where the needle insertion point would be in a conventional block technique. The needle is then inserted several centimeters from the probe site and advanced in plane of the ultrasound beam ( Fig. 24-6 ). This allows continuous visualization of the needle as a hyperechoic (white) line. This is sometimes challenging particularly when blocking nerves in deeper tissues. However, it is of paramount importance for safely performing the block to image the needle clearly, particularly the needle tip. The needle can easily veer out of plane while advancing, which can lead to significant misjudgment of the location of the needle tip ( Fig. 24-7 ).

Figure 24-6

Demonstration of the in-line needle advancement technique using a linear array transducer and the “blue phantom gel model”. The needle is imaged in its entirety with the needle tip in close proximity to the nerve.

Figure 24-7

Using the in plane needle advancement technique as shown in Figure 24-6 . The needle is placed intravascularly. A, intravascular position of the needle in the “blue phantom” model. B, same needle position as on the right, however, the ultrasound probe is placed out of plane. The virtual needle tip appears far away from the vessel. The real needle tip is, in fact, intravascular.

If this should occur we recommend pulling the needle back to just under the skin and then re-advancing it. Optimal imaging of the needle may also require moving the ultrasound transducer from side to side, tilting it, or rotating the probe using a slow and steady motion. In addition, the flatter the angle of approach, the more easily the needle can be visualized. It is sometimes helpful to image the needle at a flat angle before advancing the needle at the appropriate angle for the target nerve ( Fig. 24-8 ).

Figure 24-8

The needle can be imaged more easily at a flatter angle. We recommend imaging the needle at a flat angle before advancing the it toward the target nerve at a steep angle, particularly when blocking nerves in deep locations.

Role of the Nerve Stimulator During Ultrasound-Guided Nerve Blocks

Even under ultrasound guidance the needle may inadvertently make contact with the nerve causing a paresthesia or even nerve injury. This is particularly true for the novice or when the there is poor visualization of the needle or the nerve—such as in the case of nerve blocks in deeper tissues. Contact of the needle with the nerve or even intraneural positioning may not always cause a paresthesia. Special care must be taken when blocking larger nerves such as the sciatic nerve and in diabetic patients. We recommend using the nerve stimulator response as an additional signal that the needle is approaching the nerve. The nerve stimulator can be used to identify specific nerves when ultrasound imaging of the nerve and the surrounding anatomy is less then optimal. We recommend using the nerve stimulator set at a low mA (0.4 mA to 0.5 mA) to minimize unpleasant muscle stimulation while advancing the needle under ultrasound guidance.

Preparing for Ultrasound-Guided Nerve Block

Before performing an ultrasound-guided nerve block the skin needs to be prepped with an alcohol solution and the ultrasound probe should be covered with a sterile drape. For teaching purposes, the sterile cover is not shown in the following images because this would obscure the transducer position. Then, an ultrasound transducer needs to be selected. For very deep structures, a microconvex or a low-frequency linear transducer should be used and for superficial structures, a linear transducer should be chosen. Next, the frequency on the scanner needs to be adjusted (e.g., 10 to 14 mHz for superficial structures, or 5 to 9 mHz for deep scanning). Subsequently the depth on the scanner needs to be set to about 1 to 2 cm deeper than the suspected nerve depth because the image size of the area of interest will be amplified if your depth setting is optimized. Now the gain (amplification) of your ultrasound signal should be adjusted which will frequently improve the image quality and give you better contrast. Always perform the blocks in a comfortable position for the patient and yourself (e.g., sitting position). Adjust the bed height so that your back is straight and your arms are bent at an approximately 90-degree angle at the elbows with both hands resting slightly at the wrist level on the patient. Hold the ultrasound probe at its base in your nondominant hand. Rest the ultrasound probe only very slightly on the body of the patient because compression of the tissue above the nerve may obscure the image of vessels close to the nerve, with possible accidental intravascular injection of local anesthetic.

Upper Extremity Nerve Blocks

Interscalene Nerve Block

This nerve block is used for shoulder surgery as nerve roots C5, C6, C7 are blocked, including the suprascapular nerve, which supplies the glenohumeral joint of the shoulder (this is not blocked in supra-and infraclavicular nerve blocks.) Also nerve roots C8 and T1 are usually not blocked leading to ulnar sparing. It is performed with the patient in the half-lateral position with the patient’s head turned to the opposite side of the bed ( Fig. 24-9 ). The ultrasound scanner is placed on the contralateral side of the bed. We recommend using an intermediate-to-high frequency (10 to 12 MHz) linear array probe set to a depth of 3 to 4 cm (2.5 cm in thin patients). It is placed above the clavicular head of the sternocleidomastoid muscle at the level of the cricoid cartilage (C6) where the internal jugular vein and carotid artery can easily be identified. The probe is now moved laterally and the anterior and middle scalene muscles are identified. The roots appear as round to oval hypoechoic structures located between the scalene muscles ( Fig. 24-10 ). If you cannot identify the roots, move the transducer along an imaginary line made by the interscalene groove in a caudal direction and identify the nerve trunks or divisions in the supraclavicular area. Then follow the nerves back up to the C6 level. At this point we recommend using Doppler ultrasound to identify vascular structures close to the nerve roots because nerve roots and blood vessels appear very similar on the ultrasound image. After centering the nerve roots on the screen and infiltration of the skin with local anesthetic, a 22-gauge, 50 mm needle is inserted 1 to 2 cm lateral to the transducer. The needle is then advanced in plane to the probe through the middle scalene muscle. A distinct pop may be felt and tissue recoil may be seen when the needle enters and exits the middle scalene muscle. The needle tip should end up either between the C5 and C6 or the C6 and C7 nerve roots. Nerve stimulation can be used to confirm that the needle is not too close to the roots. After negative aspiration, 15 to 20 mL of local anesthetic is injected and the hypoechoic spread of local anesthetic around the roots should be visualized on the ultrasound image ( Fig. 24-11 ).

Apr 13, 2019 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Ultrasound-Guided Nerve Blocks

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