Fig. 4.1
Effects of sterile transparent dressing on image quality and ability to use Doppler ultrasound with curved probes. The Tegaderm dressing maintains image quality and ability to use Doppler as long as it is stretched (left). The IV3000 dressing can be used (middle), but multiple small adhesive wells containing air may form under the dressing, leading to poor imaging and limited ability to use Doppler. A complete commercial sterile cover (right) would be necessary if performing catheter insertion for continuous anesthesia/analgesia (Adapted from Tsui et al. [1]. With permission from Wolters Kluwer Health)
We also use individual sterile packs of gel.
For continuous blocks:
Complete sterile preparation is required; a complete sterile cover is used for the ultrasound probe. A mask, sterile gown, and gloves should be worn by the operator.
After completion of the procedure, the probe and related equipment (including all surfaces and cables) should be thoroughly cleaned.
4.3 Image Optimization
One of the most important factors in ultrasound-guided localization is patient positioning. The patient should be placed in a position such that the target area is well exposed. In the subsequent clinical chapters, patient positioning will be described for each individual block prior to discussing the ultrasound imaging technique.
The operator position should be optimized to enable both a good view of the ultrasound screen and comfortable hand positioning for needle insertion.
The reader is referred to Chap. 3 to choose the appropriate frequency of transducer at each peripheral block location; the frequency will partly determine the type of array (linear versus curved) that will be suitable. Often, a high-frequency linear array transducer (10 MHz or more) is the most appropriate for the pediatric population. If the area of interest is relatively deep, such as the gluteal region in teenagers, a lower-frequency transducer (5–7 MHz) may be advantageous with greater penetration. Whether the probe array is curved or linear depends on the region and field of view required (e.g., blocks in the lumbar region benefit from larger fields of view and lower frequencies, which curved array probes offer).
4.3.1 Probe Alignment
The probe will have a marker or groove to show which way is “up.” The marker corresponds to the top of the ultrasound image, and if you slide the probe toward the marker, the image will move in the direction that the probe is moved. Similarly, if you slide the probe away from the marker side, the image will move in the direction opposite the marker.
In most circumstances, and particularly with transverse planes of viewing, the plane of the transducer beam should intersect the axis of the nerve structures at a perpendicular position. The lateral resolution will be optimal in this situation, and artifacts such as anisotropy (Chap. 3, Sect. 3.6) will be minimized.
To obtain the best short-axis view in a coronal plane, such as scanning the supraclavicular region, follow these important steps for probe handling (Fig. 4.2):
Fig. 4.2
Probe handling during supraclavicular blockade with landmark identification of the subclavian artery. Scan across the relevant area to obtain a transverse view of the vascular structure (in this case the subclavian artery; top). To sharpen the image, rotate the probe (clockwise or anticlockwise) or tilt the probe forward and backward to achieve a perpendicular beam through the target structure (bottom)
Scan across the relevant area to obtain a transverse view of the vascular structure (e.g., subclavian artery) or nerve since it is easier to capture these structures with an ultrasound beam “transecting” in a short axis rather than obtaining a longitudinal view.
The image can be refined by rotating the probe (turn slightly clockwise or anticlockwise) or tilting the probe forward and backward to achieve a perpendicular beam through the target structure in order to sharpen the image.
Adjust the time gain compensation (TGC) so that the visualized area is of uniform echotexture. Always adjust the TGC to the center when changing transducers during a study.
Appropriate depth should be adjusted such that the target nerve is in the center of the screen with other relevant structures in view (e.g., in supraclavicular block, one should be able to visualize the subclavian artery, the first rib, and the pleura).
The focal point should be adjusted to the where the target nerve is.
A systematic approach allows the best image to be obtained in a timely fashion, increasing the success rate of ultrasound-guided peripheral nerve blocks.
Most neural structures are accompanied by blood vessels (pulsatile arteries or compressible veins) or bony landmarks, which are readily identifiable using ultrasound. Color Doppler is useful in this situation to identify blood vessels. The color convention is for red to represent blood flowing toward the transducer and blue to represent blood flowing away from the transducer. Therefore, the knowledge of anatomy is always important to identify nerves around these landmarks.
The “traceback” approach (see below) also aids the identification of a nerve, especially when differentiating between similar-looking structures such as tendons or artifacts.
4.3.2 Practical Approach: Traceback Method
As with the conventional “blind” approach, knowledge of anatomy is of upmost importance when performing peripheral nerve blocks under ultrasound guidance. The spatial appreciation in relation to the surrounding structures is also relevant. In our experience, neural structures are not always easy to identify under ultrasound; they can appear hyper- or hypoechoic and are sometimes confused with artifacts. Nerves are continuous structures, which lend themselves to be “traced” proximally and distally. This enables the differentiation from structures with similar appearance (e.g., tendons). In this section, we will describe a systematic “traceback” approach [2] to help identify the target nerves (within various regions) of commonly used nerve blocks.
Instead of immediately focusing on locating the target nerve at the commonly used block site, the goal of this exercise is to obtain a clear image of an obvious anatomic landmark (i.e., a blood vessel or bony landmark) not too far removed from one point along the target nerve’s path.
If suitable, it is generally preferred to perform the block at this location due to the dependable anatomical relations.
Otherwise, the operator focuses on the nerve (often in short axis by adjusting the transducer as described above in probe alignment) and “traces” it toward the block target area by moving the ultrasound probe in a proximal or distal direction along the nerve.
The appearance of surrounding structures (e.g., muscle and other soft tissues) changes consistently as the probe moves, whereas the appearance of the target nerve does not change in any significant manner and is traceable.
In this way, we can more easily and reliably identify the corresponding nerve or plexus.
For illustration purposes, see Fig. 4.3, which describes a traceback practice for facilitating identification of the sciatic nerve at the popliteal fossa (using vascular landmark identification). This approach allows the operator to gain confidence in their ability to recognize and locate the nerves. During the training process, we found that the traceback approach is an easy and reliable way to become proficient at identifying neural structures prior to performing regional anesthesia. However, the traceback approach may not be necessary in some locations where the larger nerves are more easily identifiable, for example, the median nerve in the axilla and antecubital fossa and the femoral nerve in the inguinal region.
Fig. 4.3
Traceback approach in the posterior thigh for identification of the sciatic nerve at the popliteal fossa (Adapted from Tsui and Finucane [2]. With permission from Wolters Kluwer Health)
We strongly recommend that the initial step in performing ultrasound-assisted regional anesthesia is identification of obvious landmarks (usually blood vessels or bony landmarks) in the vicinity of the target nerve. Table 4.1 lists the numerous nerve blocks that can benefit from identification of highly visible and dependable structures (i.e., blood vessels (especially with color Doppler) and bone) for accurate nerve identification. Nonetheless, the traceback technique is useful for popliteal nerve for identification of the bifurcation, identifying the brachial plexus at the interscalene region, or for revealing anatomic anomalies.
Table 4.1
Useful landmarks for identification of nerves using ultrasound; many can be used in “traceback” approaches
Block | Ultrasound landmark | Comments |
|---|---|---|
Interscalene | Subclavian artery | Trace nerve proximally from the distal supraclavicular location where the artery lies medial to the nerve |
Supraclavicular | Subclavian artery | Brachial plexus lies lateral and often superior to the artery |
Infraclavicular | Subclavian artery and vein | Brachial plexus cords surround the artery |
Axillary | Axillary artery | Terminal nerves surround the artery
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