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Ultrasound Guided Regional Anesthesia

Terms in this set (21)

Ultrasound-guided regional anesthesia (UGRA) has become a routine technique and may become the "gold standard for performing regional anesthetic nerve blocks. List 9 advantages of UGRA. Of these advantages, which two are the"best things " about UGRA?
Nine advantages of using ultrasound guidance for regional anesthesia are:

(1) direct visualization of nerves and adjacent anatomic structures;

(2) observing local anesthetic spread in real time;

(3) detecting variations in anatomy;

(4) faster onset times;

(5) improvement in block quality;

(6) use of lower, more precise doses of local aesthetics;

(7) possible increase in safety;

(8) less painful administration compared with nerve stimulator techniques;

(9) higher patient satisfaction.

Of these, knowing where the tip of the needle is in relation to other anatomical structures and being able to see the spread of local anesthetic are the "best things about UGRA."

The key requirement for successful regional anesthetic block is to ensure optimal distribution of local anesthetic around nerve structures ' (Nagelhout and Plaus).

[Barash, Clin. Anes, 7th. 2013 p940; Authors.; Nagelhout, NA, 5th. 2013 p1106]
Define ultrasound. What is sound?
The term ultrasound refers to high-frequency sound waves above the range of human hearing; the upper limit of human hearing is about 20 KHz (20,000 Hz), therefore ultrasound is mechanical vibrations greater than 20 KHz.

Sound is a mechanical wave in which the wave motion transports energy and momentum from one point in space to another without the transport of matter.

Sound waves are longitudinal waves of compression and rarefaction (relaxation) of a medium such as air or soft tissues.

Compression refers to high-pressure zones and rarefaction refers to low-pressure zones.

[Gray, Atlas of UGRA, 2nd. 2013 p3; Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p324; Brown, Atlas of Regional Anes., 4th. 2010 p10]
Medical ultrasound machines operate in what frequency range?
Medical ultrasound machines operate at frequencies in the range between 2 and 15 MHz (megahertz, million cycles per second).

Note: different textbooks state different frequency ranges for medical devices; 2-15 MHz is inclusive of all textbook ranges.
A sound wave is modeled as a sine wave defined by three key parameters; list these three parameters.
A sound wave is defined by a specific wavelength (x) measured in units of distance, amplitude (h) measured in decibels (dB, a logarithmic measure), and frequency (f) measured in hertz (Hz) or cycles per second.

[Brown, Atlas of Regional Anes., 4th. 2010 p10, 11f]
What is the velocity of sound in soft tissues?
The velocity of sound in soft tissues is about 1540 m/s, therefore 6.5 microseconds (ms) elapse for each centimeter of soft tissue traversed by the sound wave.

[Ehrenfeld, MGH Text of Anes. Equip., 1 st. 2011 p207; Gray, Atlas of UGRA, 2nd. 2013 p4]
Describe the relationship between ultrasound frequency and depth of tissue penetration. Correlate the specific ultrasound wavelength range with soft tissue penetration depth.
There is an inverse relationship between ultrasound frequency and depth of tissue penetration.

The longer the wavelength → lower frequency, the deeper the tissue penetration.

In general, higher-frequency ultrasound probes (10-13 MHz) are best suited for visualizing structures less than 4 cm deep from the skin.

For structures deeper than 4 cm from the skin, lower frequency probes are more useful (2-5 MHz).

[Ehrenwerth, et. al, Anesthesia Equipment, 2nd. 2013 p373; Nagelhout, NA, 5th. 2013 p1106]
Construct a table correlating (1) field depth, (2) transducer frequency, and (3) common peripheral nerve blocks in ultrasound guided regional anesthesia.
Transducer Field Depth (cm) Wavelength (MHz) Peripheral Blockades

< 2.0 10-13 Wrist,

ankle block 2.0-3.0 10-13

Interscalene, axillary, brachial plexus block 3.0-4.0 10-13

Femoral, supraclavicular, transversus abdominal plane (TAP) 4.0-7.0 6-10

Infraclavicular, popliteal, subgluteal sciatic blocks 7.0-10.0 2-5

Pudendal, gluteal sciatic nerve, lumbar plexus block >10.0 cm 2-5

Anterior approach to sciatic nerve.

[Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p336f, 338t]
Sounds above 100 KHz do not occur naturally; how then are ultrasound waves (2-15 MHz) generated?
Ultrasound is generated by the piezoelectric effect. The piezoelectric effect is a phenomenon exhibited by the generation of an electric charge in response to a mechanical force (squeeze or stretch) applied on certain naturally occurring or man-made materials.

Conversely, mechanical deformation can be produced when an electric field is applied to such a material, also known as the piezo- electric effect. Ultrasound transducers consist of linear or curvilinear arrays of synthetic piezoelectric crystals (PZT, lead zirconium titanate) that produce high-frequency sound waves in response to an electric signal.

[Gray, Atlas of UGRA, 2nd. 2013 p22; Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p324
List four interactions as an ultrasound wave travels through tissues. Which of these fates is critical for ultrasound imaging?
As an ultrasound wave travels through tissues, the wave may be:

(1) transmitted (passes through),

(2) reflected,

(3) scattered (redirection of sound in any direction), or

(4) absorbed (converted to heat), depending upon the echodensity of the tissue.

To generate a clinically useful image, ultrasound waves must reflect of tissues and return to the transducer.

Reflection requires an interface between two media with different acoustic impedances, for example the interface between soft tissues and bone.

The acoustic impedance of bone is 8, water, blood, and soft tissue have similar acoustic impedances of approximately 1.6, whereas air has an acoustic impedance of 0.0001.

[Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p326-327; Brown, Atlas of Regional Anes., 4th. 2010 p10; Gray, Atlas of UGRA, 2nd. 2013 p8]
Define ultrasonography.
Ultrasonography measures the amplitude of the reflected return echo as a function of time, how long it takes for the reflected sound wave to travel back to the transducer. The time function information is then encoded as a grayscale image.

[Gray, Atlas of UGRA, 2nd. 2013 p8]
Do substances that conduct sound well and thus reflect sound poorly appear darker or brighter on ultrasonography? Give examples of types of biological substances that conduct sound well.
Substances with high water content, such as blood, cerebrospinal fluid and local aesthetics, conduct sound very well and reflect very poorly and thus are called echolucent.

Because they reflect very little sound they appear as darker areas.

[Barash, Clin. Anes, 7th. 2013 p941; Gray, Atlas of UGRA, 2nd. 2013 p3; Brown, Atlas of Regional Anes., 4th. 2010 p12; Nagelhout, NA, 5th. 2013 p1106]
What is the term for darker regions in the ultrasound image?
By convention, darker regions are called hypoechoic images and represent less (hypo) reflection (echo) and weaker signal intensities.

Black regions are called anechoic.

[Brown, Atlas of Regional Anes., 4th. 2010 p12; Nagelhout, NA, 5th. 2013 p1106; Barash, Clin. Anes, 7th. 2013 p941; Gray, Atlas of UGRA, 2nd. 2013 p3|
Do substances that conduct sound poorly and thus reflect almost all the sound appear darker or brighter on ultrasonography? Give examples of biological substances that reflect most of the sound back to the transducer.
Substances low in water content or high in materials, such as bone, fascia, vessel walls and air, are poor sound conductors and thus reflect almost all the sound.

Substances that reflect most of the sound are called echogenic and appear as brighter or white areas.

[Brown, Atlas of Regional Anes., 4th. 2010 p12; Barash, Clin. Anes, 7th. 2013 p941; Gray, Atlas of UGRA, 2nd. 2013 p3; Nagelhout, NA, 5th. 2013 p1106]
What is the term for brighter (white) regions in the ultrasound image?
By convention, brighter or white regions are called hyperechoic images and represent more (hyper) reflection (echo) and stronger signal intensities.

[Brown, Atlas of Regional Anes., 4th. 2010 p12; Gray, Atlas of UGRA, 2nd. 2013 p3; Barash, Clin. Anes, 7th. 2013 p941; Nagelhout, NA, 5th. 2013 p1106
What phenomenon allows for detection of blood vessels in an ultrasound image?
The Doppler shift is the change in frequency when a sound wave strikes a moving object.

This means the frequency of the original emitted and subsequently reflected sound waves are not the same.

Red blood cells are the primary reflectors that produce Doppler shifts.

Doppler shifts in clinical imaging are in the audible range (10 KHz).

Ultrasound machines can color-encode the mean velocity (color Doppler) and power spectrum of the frequency shift (power Doppler).

[Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p328; Ehrenfeld, MGH Text of Anes. Equip., 1st. 2011 p211; Gray, Atlas of UGRA, 2nd. 2013 p20; Barash, Clin. Anes, 7th. 2013 p941]
When viewing the cross (transverse) section of an object, are you viewing the short axis or long axis of the object?
A cross (transverse) section of the object is viewed along the short axis of the object. (Think of sliced salume.)

[Barash, Clin. Anes, 7th. 2013 p941; Authors.]
When viewing the length (longitudinal view) of an object, are you viewing the short axis or long axis of the object?
A lengthwise (longitudinal) presentation of an object is viewed along the long axis of the object. (Think of looking at a hot dog nestled in a bun from the view of the mustard bottle.)

[Authors.; Barash, Clin. Anes, 7th. 2013 p941]
How are most nerves imaged in ultrasound guided regional anesthesia (along which axis)?
Most nerves are imaged in cross section in ultrasound guided regional anesthesia.

[Barash, Clin. Anes, 7th. 2013 p941]
Describe the two techniques of orienting the needle with respect to the ultrasound beam during ultrasound guidance.
The in-plane (IP) approach generates a long-axis (hot dog) view of the needle, allowing full visualization of the shaft and tip of the needle.

The out-of-plane (00P) approach generates a short-axis view of the needle shaft, which appears as a bright (echogenic) dot in the image.

[Gray, Atlas of UGRA, 2nd. 2013 p20; Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p328; Brown, Atlas of Regional Anes., 4th. 2010 p14; Barash, Clin. Anes, 7th. 2013 p941]
List some advantages and disadvantages of the out-of-plane (00P) approach to ultrasound guidance.
The out-of-plane (OOP) approach is most similar to the traditional approaches to regional block guided by nerve stimulation or palpation, providing a natural transition from one form of guidance to the other.

The OOP uses a shorter needle path than in-plane approaches.

If short-axis views of the nerve are used, an OOP results in catheter placement that is guided along the path of the nerve.

A limitation of the out-of- plane view is that it generates a short-axis view of the block needle, which may be hard to visualize.

With an OOP view, the operator cannot confirm that the needle tip (rather than the shaft) is being imaged, and therefore needle location is often inferred from tissue movement or small injections of solution.

[Gray, Atlas of UGRA, 2nd. 2013 p20; Brown, Atlas of Regional Anes., 4th. 2010 p14; Barash, Clin. Anes, 7th. 2013 p941; Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p328|
Describe an advantage and some disadvantages of the in-plane (IP) approach to ultrasound guidance.
In-plane guidance allows the most direct visualization of the needle and injection.

A disadvantage of the IP technique is the long needle path, which results in more tissue/structures for the needle to cross.

It may be challenging to keep the needle within the thin imaging ultrasound beam in the in-plane technique.

Finally, partial line-ups (visualization of the needle shaft without visualization of the needle tip into the scan plane) create a false sense of security and therefore compromise the safety of the technique.

[Barash, Clin. Anes, 7th. 2013 p941; Gray, Atlas of UGRA, 2nd. 2013 p20; Hadzic & NYSORA, Hadzic's Peripheral Blocks, 2nd. 2012 p328; Brown, Atlas of Regional Anes., 4th. 2010 p14
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