block needle inline with the long axis of the ultrasound transducer, so the needle stays within the path of the ultrasound beam. In this manner, the needle shaft and tip can be clearly visualized. This approach is preferred when it is important to track the needle tip at all times (e.g., during supraclavicular block to minimize inadvertent pleural puncture). The second approach places the needle perpendicular to the probe. In this case, the ultrasound image captures a transverse view of the needle, which is shown as a hyperechoic ‘dot’ on the screen. Accurate moment-to-moment tracking of the needle tip location can be difficult, and needle tip position is often inferred indirectly by tissue movement. This approach, however, is particularly useful for continuous catheter placement along the long axis of the nerve.
OUTCOME STUDIES Ultrasound-guided techniques may improve the accuracy, success, and safety of regional anaesthesia. However, few prospective randomized outcome studies have been
conducted and published so far. Williams and coworkers suggested that the addition of ultrasound guidance improves the quality of supraclavicular block when compared to neurostimulator guidance alone. Marhofer and associates also suggest that ultrasound guidance speeds the onset, improves the quality, and reduces the incidence of vascular puncture during three-in-one blocks. No study to date, however, has examined the impact of ultrasound on nerve injury. In summary, although preliminary experience has been encouraging, more outcome data are required to define the success and safety profile of ultrasound-guided peripheral nerve blocks.
ULTRASONOGRAPHY AND NEUROAXIAL BLOCKS Neuroaxial anesthetic techniques can be challenging because of inter-individual anatomic variability and imprecise determination of the level of the vertebral interspace by physical examination alone (inaccurate 70-80% of the time). Spinal
Ultrasonography has many applications in clinical anaesthesia
needle insertion and local anesthetic injection at the wrong lumbar interspace (ie, too cephalad) may have been implicated in previously reported injuries to the conus medularis. Potentially, imaging guidance may improve accuracy and safety of needle placement during neuroaxial blocks. Over two decades ago, attempts were
made to image the ligamentum flavum using ultrasonography. Because the epidural and subarachoid spaces are surrounded by bones, anatomic assessment in this region is difficult since the majority of the ultrasound beam is reflected upon contacting the bony spinous processes. With a linear or curved 4-7 MHz probe, limited ultrasound beam passage is possible only through the interspinous space , especially in the paramedian region. The ligamentum flavum and the dura mater are dense tissues that appear hyperechoic on ultrasound while the low-density epidural space and the cerebrospinal fluid in the intrathecal space appear hypoechoic.
Ultrasound determination of the
spinal level is more accurate than clinical examination. This has been confirmed in two recent studies showing accurate ultrasound determination in over 70% of patients when compared to MRI examination. The markers were always placed within one interspace of the intended level. Ultrasonography can also determine the depth of needle penetration to reach the epidural space and can help reduce the number of needle puncture attempts. The paramedian region has been suggested by some to be the optimal window for ultrasound imaging, especially in the thoracic spine, because of a higher soft tissue to bone ratio. In contrast to peripheral nerve blocks, real- time image-guided neuroaxial techniques have not been reported. Ultrasonography has been used primarily to help define the anatomy, depth, and angle of needle penetration immediately prior to performing the technique. ■
ID
REFERENCES References available on request. (
magazine@informa.com)
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