Ultrasound Imaging for Nerve Block:

1
Ultrasound Imaging for Nerve Block

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Location, location, location!

• The success of regional anesthesia follows a
  simple principle.
• Accurate placement of local anesthetic in
  sufficient amounts around the target nerve will
  result in successful blockade.
• Because we cannot see nerves that we wish to
  anesthetize, regional anesthesiology remains an
  art that is difficult to master.
• Even for the experienced, nerve block is an
  exercise of seeking the nerve by trial and error
  and relying on indirect cues (e.g., nerve
  stimulation) for guidance.
• One would hope that the art of placing a needle
  perineurally in regional anesthesia can be as
  simple as putting a needle intravenously for
  general anesthesia.

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Turning Art Into Science

• Ultrasound imaging may transform the art of
  regional anesthesiology into a science.
• Although ultrasound-assisted nerve block was
  described more than two decades ago, it did not
  receive worthy attention until recent
  technological advancement showing nerves in high
  resolution.
• Ultrasound is a preferred imaging modality over
  others (e.g., computed tomography, magnetic
  resonance imaging and fluoroscopy) because it is
  portable, affordable and accessible in the
  operating room without radiation risk.
• Not only can it visualize peripheral nerves and
  their neighboring structures, it also can
  visually track needle movement in real time and
  assess adequacy of local anesthetic spread at the
  time of injection.
• The ability to see the nerves, the needle and
  local anesthetic spread may be the key to a
  consistently successful block.

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Modern ultrasound systems

• They should be equipped with software to
  visualize both superficial tissues and
  musculoskeletal structures.
• High-resolution ultrasound (HRUS) systems come
  with software that allows optimized visualization
  of tissue contrast.
• Colour and pulsed-wave Doppler imaging helps to
  identify vessels in proximity to nerves.
• If the equipment includes a high-capacity hard
  disk the images can be stored for future
  reference.
• Short film sequences can then be stored and
  transferred to disc via a CD or DVD burner. Files
  can be stored in RAW-data, JPG, BMP or MPG4
  formats.

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Anatomical structures under Ultrasound

• These include blood vessels, muscles, tendons and
  our target for regional anesthesia, nerves.
• All of these structures are visible using HRUS
  and they can be followed over nearly their entire
  course.
• Even for structures not accessible or poorly
  accessible with ultrasound because of their depth
  or structural interference from bone (e.g., psoas
  compartment block, epidural space), the anatomy
  can be visualized to such an extent that
  important clues are obtained that help facilitate
  performance.

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Practical guideline

• Ultrasound for musculoskeletal and nerve imaging
  is commonly in the 2-15 MHz frequency range
  depending on the depth of penetration required.
• . On transverse (cross-sectional) view, a nerve
  is round or oval shaped, and this view is
  generally the best for nerve block.
• Peripheral nerves appear hypoechoic (dark) in the
  interscalene and supraclavicular regions but
  hyperechoic (bright) in most other upper- and
  lower-limb locations.
• A scan prior to needle insertion will show the
  exact nerve location, its size and depth from the
  skin, and is thus helpful in defining the desired
  site, angle and path of needle penetration.

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Real-Time Navigation

• Another benefit of ultrasound is real-time visual
  navigation guidance at the time of needle
  advancement.
• When the needle and the ultrasound beam are
  aligned with (parallel to) each other, the needle
  shows up as a hyperechoic (bright) line on the
  screen.
• Needle movement toward the target can be tracked
  in real time, thus random needle movement and the
  number of needle attempts is minimized.
• Tracking needle movement in real-time also can
  prevent the risk of inadvertent needle entry into
  the spinal canal that cannot be recognized by the
  nerve stimulator technique.

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Visualizing Pattern Spread

• Another attractive feature of ultrasound is its
  ability to show the pattern of local anesthetic
  spread at the time of injection.
• This is valuable because an incomplete block may
  result from an asymmetric or partial spread
  around the nerve.
• Under ultrasound guidance, it is now possible to
  adjust needle position half-way during local
  anesthetic injection to ensure complete
  circumferential spread.

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A Standard Practice for the Future?

• The use of ultrasound in regional anesthesia
  requires a commitment to improving patient care.
• That commitment will necessitate an investment in
  equipment, personnel, and time.