Chapter 127. Ultrasound Guidance for Regional Anesthesia

Always scan the whole area of interest to locate structures and identify anatomical variations.

• Visualize nerves or plexus
• Visualize structures to avoid: blood vessels, pleura
• Visualize needle
• Visualize local anesthetic spread:
  • Limit volume, as injection can be stopped when spread deemed adequate
  • Avoid intravascular injection; risk of complication when needle tip in a compressed, poorly visualized vein, and when no spread is seen when injecting as a result
• Confirm position of catheter:
  • Inject fluid, agitated fluid, or air
  • Color Doppler can be used

• Sterile conductive gel and sterile protective cover for ultrasound (US) probe should be used
• Ideally, a telescopic probe cover, fixed in place by an elastic band, should be employed
• After use, probe should be sterilized or disinfected, rinsed, dried, and kept in a clean environment

Transducer shape (Figure 127-1):

• Straight (linear) transducers yield an image as wide as the probe
• Curvilinear transducers yield a semicircular picture, allowing visualization of structures not directly underlying the probe
• Probes with special shapes are used to minimize probe “footprint,” especially in small patients or children (e.g., “hockey stick” probe)

Frequency:

• Pick a transducer based on frequency (usually between 3 and 15 MHz)
• High-frequency transducer (8–12 or 15 MHz):
  • Less penetration into deeper structures
  • Best for target depth up to 3–4 cm
  • Higher resolution (picture quality)
• Low-frequency transducer (3–5 MHz):
  • More penetration into deeper structures
  • Best for targets deeper than 5 cm
  • Lower resolution (picture quality)
• After picking the appropriate transducer, fine-tune the frequency of the wave by selecting the upper, mid, or lower frequency range of that specific transducer

Transducer orientation:

Identify which side of transducer corresponds to which side of the image on the US monitor:

• Most transducers have a fixed label (i.e., palpable notch/groove) corresponding to an indicator on the screen
• If in doubt, tap on side of transducer to identify orientation

Gain:

• Affects brightness (hyperechoic) or darkness (hypoechoic) characteristics of the displayed image
• Adjust gain to obtain clear image

Time-gain compensation: