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In recent years interest has been growing in the practice of regional
anesthesia and, in particular, in peripheral nerve blocks for surgical
anesthesia and postoperative analgesia. Peripheral nerve blocks have been
found to be superior to general anesthesia1 because they
provide effective analgesia with few side effects2 and can
hasten patient recovery.3 Unfortunately, the practice of
regional anesthesia does not enjoy widespread endorsement because of
inconsistent success, varying from one anesthesiologist to another. Current
methods of nerve localization (eg, paresthesia and nerve stimulation) are
essentially “blind” procedures, since they both rely on indirect evidence
of needle-to-nerve contact.4,5 Seeking nerves by trial and
error and random needle movement can cause complications. Although uncommon,
complications such as intravascular local anesthetic injection resulting in
systemic toxicity, inadvertent spinal cord injury following interscalene
block, pneumothorax following supraclavicular block, and nerve injury have
all been reported.6,7
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Imaging guidance for nerve localization holds the promise of improving
block success and decreasing complications. Among imaging modalities
currently available, ultrasonography seems to be the one most suitable for
regional anesthesia. Perhaps the most significant advantage of ultrasound
technology is the ability to provide anatomic examination of the area of
interest in real time.8 Ultrasound imaging allows one to
visualize neural structures (plexus and peripheral nerves) and the
surrounding structures (eg, blood vessels and pleura), navigate the needle
toward the target nerves, and visualize the pattern of local anesthetic
spread.9
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An ultrasound probe (transducer) has dual functions. It emits and
receives sound waves, thus functioning both as a speaker and a microphone.
As the name implies, ultrasound waves are high-frequency sound waves
(≥20,000 cycles/sec, 20 kHz) that are not audible to the human
ear. Ultrasound frequencies useful in clinical medicine are in the megahertz
(MHz) range.10 When an electric current is applied to an
array of piezoelectric crystals (quartz) within the ultrasound transducer,
mechanical energy, in the form of vibration, is generated, resulting in
ultrasound waves. As the ultrasound waves move through body tissues of
different acoustic impedances, they are attenuated (lose amplitude with
depth), reflected, or scattered. Waves reflected to the transducer are then
transformed back into an electrical signal that is then processed by the
ultrasound machine to generate an image on the screen.
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Depending on the amount of wave returned, anatomic structures take on
different degrees of echogenicity. Structures with high water content, such
as blood vessels and cysts, appear hypoechoic (black or dark) because
ultrasound waves are transmitted through the structures easily with little
reflection. On the other hand, bone and tendons block ultrasound wave
transmission, and the strong signal returned to the transducer gives these
structures a hyperechoic appearance (bright, white) on the screen.
Structures of intermediate density and acoustic impedance, such as the liver
parenchyma or the thyroid gland, appear gray on the screen. Knowing the
speed of sound in tissue (1540 m/sec on average) and the time of ...