Ultrasonography (US) as a means to guide peripheral nerve blockade (PNB)

was �rst explored by anesthesiologists at the University of Vienna in the mid-

1990s. Although radiologists had made use of ultrasound technology to guide

needles for biopsy, the application of this imaging modality for PNB was novel

at that time. The utility of ultrasound to facilitate a range regional anesthesia

techniques including brachial plexus and femoral blocks was demonstrated. A

decade later, colleagues from the University of Toronto, Canada, began to

embrace this technology, further demonstrating its utility and describing in

detail the sonoanatomy of the brachial plexus. A number of advances in

technology took place in the meantime, including smaller and more mobile

ultrasound platforms, improved resolution, and needle recognition software,

all cumulatively leading to increased bedside utility of ultrasound by

anesthesiologists.

The previously used surface anatomy-based techniques, such as nerve

stimulation, palpation of landmarks, fascial “clicks,” paresthesias, and

transarterial approaches, did not allow for the monitoring of the disposition of

the local anesthetic injectate. Ultrasound guidance, however, offers a number

of important practical advantages for nerve blockade. Ultrasound allows

visualization of the anatomy of the region of interest. This allows more

informed guidance for the needle pathway to the target while avoiding

structures that might be damaged by the needle. Ultrasound also allows

visualization of the needle tip as it is passed through the tissues, con�rming

alignment with the intended path, again reducing the likelihood of

inadvertent needle trauma to unintended structures. Perhaps most important,

real-time ultrasound imaging permits continual visualization of local

anesthetic solution delivery to ensure proper distribution, with the potential

for adjustment of the needle tip position as necessary to optimize local

anesthetic distribution.

Introduction of ultrasound guidance in regional anesthesia has led to

re�nement of many nerve block techniques, expanded use of PNB, and

greater acceptance by surgical colleagues and patients.

INTRODUCTION

ADVANTAGES OF ULTRASOUNDGUIDANCE

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Courses

Nov 28 - Nov 29

Advanced Ultrasound-GuidedTechniquesBrussels, Belgium

Jul 26 - Jul 27

Advanced Ultrasound-GuidedTechniquesLeuven, Belgium

Jun 6 - Jun 7

Advanced Ultrasound-GuidedTechniquesSan Diego, CA

Oct 10 - Oct 11

Building Blocks of Ultrasound-Guided Regional AnesthesiaBrussels, Belgium

Jul 4 - Jul 5

Building Blocks of Ultrasound-Guided Regional AnesthesiaLeuven, Belgium

Introduction to Ultrasound-Guided RegionalAnesthesia

Ultrasound-guided PNB may be broken down into two fundamental aspects:

imaging structures in the plane of section, including the target nerve, and

guiding the needle. Understanding and recognition of three-dimensional

anatomic structures on a twodimensional image requires training in the

technology and sonoanatomy pattern recognition (Table 1).

TABLE 1. Optimizing sonoanatomy visualization.

Choose appropriate transducer/frequencyUnderstand underlying anatomic relationshipsApply varying degree of pressure with transducerAlign transducer with underlying nerve targetRotate transducer to �ne-tune imageTilt the transducer to optimize image

As anatomic recognition remains essential to placing blocks, even with

realtime visual guidance, specialty society guidelines for training residents and

fellows continue to stress the importance of anatomical dissection and gross

anatomy training as an inherent component of learning ultrasound-guided

regional anesthesia (UGRA). In a study conducted over a 1-month regional

anesthesia rotation, residents demonstrated markedly improved recognition

of relevant structures at the sites of several different PNBs, using ultrasound

imaging. In an evaluation of ultrasound-guided interscalene block instruction,

residents demonstrated increasing ef�ciency of sonoanatomy recognition as

their experience over the course of the rotation increased.

More innovative methods of training have shown promise as well. Integrating

an anatomic program into the software of a bedside ultrasound machine has

been shown to improve scores on a written test of anatomy. After exposure to

a multimedia anatomy presentation, residents and community

anesthesiologists demonstrated increased knowledge of ultrasound anatomy

on a posttest, although they were not able to improve scores on a practical

examination of sonoanatomy on live models. However, the optimal link

between anatomic knowledge and recognition of two-dimensional anatomic

patterns on ultrasound has not yet been adequately explored.

Certain basic tenets of optimizing an ultrasound image are applicable to all

nerve blocks. For instance, sonography requires an understanding of

mechanics and ergonomics. Novices are subject to errors such as probe

fatigue, reversing probe orientation, and inadequate equipment preparation.

To optimize the ultrasound image, the mnemonic PART (pressure, alignment,

rotation, tilting) has been recommended. Pressure is necessary to minimize

the distance to the target and compress underlying subcutaneous adipose

tissues. Alignment refers to placing the transducer in a position over the

extremity (or trunk) at which the underlying nerve is expected to be in the

�eld of view. Rotation allows �ne-tuning of the view of the target structure.

Tilting helps to bring the face of the probe into a perpendicular arrangement

with the underlying target to maximize the number of returning echoes and

thus provide the best image (Figure 1). In-depth discussion on optimizing

ultrasound imaging is discussed in “Optimizing an Ultrasound Image“.

ULTRASOUND AND SONOANATOMY

FIGURE 1. Fine adjustment of the probe tilt is necessary to optimize echo return from the targetstructure and enhance image resolution (yellow arrowheads indicate sciatic nerve at the popliteal fossa).

NYSORA Tips

To optimize the ultrasound image, the mnemonic PART has beenrecommended: pressure, alignment, rotation, tilting.Recognition and understanding of sonoanatomy requires knowledgeof the underlying three-dimensional anatomy.Optimum visualization of the target nerve requires appropriatetransducer pressure, alignment with nerve, and rotation and tilting ofthe probe to �ne-tune the image.

Nerve imaging may be performed in either short-axis (probe face

perpendicular to axis of nerve) or long-axis (probe face parallel to axis of nerve)

position (Figure 2).

OPTIMIZING NERVE AND NEEDLEIMAGING WITH ULTRASOUND-CLINICAL SCENARIOS

FIGURE 2. The median nerve. A: Cross-section (target structure out of plane to ultrasound beam;yellow arrowhead). B: Longitudinal section (target structure in plane to ultrasound beam; redarrowheads).

It is frequently easier to recognize the round, often-hyperechoic, neural

element with short-axis imaging, especially for a beginner. Because most

nerve blocks are conducted in the extremities, this orientation results in a

transducer position that is transverse, across the long axis of the arm or leg. In

general, understanding the course of the nerves, based on knowledge of gross

anatomy, allows one to align and rotate the transducer perpendicular to the

course of the nerve subsequently adjusting the tilt as described previously to

optimize the image.

Once the nerve and surrounding anatomy are identi�ed, a needle path may

be chosen so that it is imaged either in plane (needle parallel to long axis of

probe) or out of plane (needle perpendicular to long axis of probe) to the

ultrasound beam. While neither method has been shown superior for either

block success or patient safety, the preferred approach may vary with

anatomic or technical considerations. However, with in-plane imaging, it is

possible to maintain an image of the entire needle, including the tip, although

it can be challenging to keep the needle entirely in the viewing plane of the

transducer. This method is especially bene�cial during instruction, as the

supervisor has continual visualization of the needle tip as it is advanced

through the tissues.

During out-of-plane imaging, the observer is able to see only the cross section

of the needle, which appears as a small hyperechoic dot, at any plane along its

entire length, so that distinguishing the tip from the shaft is much more

dif�cult.

Guiding the needle tip to the target while maintaining the entire needle in the

plane of imaging, however, can be challenging (Table 2).

TABLE 2. Optimizing needle imaging with ultrasound.

Utilize a shallow angle of approach, if possible“Heel” the transducer to make the face more parallel tothe needleRotate the transducer to ensure the entire needle is seenTilt the transducer as necessaryChoose an “echogenic” needleApply needle recognition software, if available“Hydrolocation” may help ascertain needle tip location

Appropriate adjustment of the bed height and ergonomic placement of the

ultrasound so that operator’s eyes can easily and rapidly shift from the image

to the �eld (Figure 3), where needle alignment with the long axis of the probe

can be ensured, is bene�cial. It is surprisingly easy for the transducer to

wander away from the plane of the needle while one’s vision is �xed on the

ultrasound screen. This is more likely if the operator has the probe and needle

aligned perpendicular to his or her own axis of viewing, as opposed to aligning

the needle and probe with the viewing axis.

FIGURE 3. Ergonomic positioning for bed height and ultrasound position.

In a study of novice medical students learning the basics of UGRA, Speer et al

found that the subjects required less time to locate the target, and were better

able to keep the needle visualized in plane on the ultrasound image, when

eyes, needle, probe, and viewing screen were aligned. Needle guides may also

permit improved imaging of the needle during approach to the target,

although more work has been done in vascular access. One disadvantage of

needle guides is that they restrict needle motion to one plane, which may not

be always desirable.

Nerves in short axis have an appearance that is to some extent determined by

their proximity to the neuraxis. Although in most areas nerves are round, they

may appear fusiform, such as the musculocutaneous nerve in the proximal

arm, or ovalshaped, such as the sciatic nerve in the infragluteal region. In close

association with the spine, nerves and nerve roots are comprised primarily of

neural tissue, with minimal connective tissue. Because neural tissue appears

hypoechoic on ultrasound imaging, while the connective tissue between

fascicles is hyperechoic, nerves near the neuraxis appear as dark nodules.

As nerves course peripherally, the number of fascicles increases, although they

diminish in size, while the amount of connective tissue also increases. These

changes lead to an increasingly complex “honeycomb” appearance on

ultrasound in short-axis viewing (Figure 4). Unfortunately, because of the

technology limitations of the current ultrasound machines, the number and

arrangement of fascicles within a peripheral nerve may not be accurately

portrayed.

FIGURE 4. A: Proximal nerve appearance in the interscalene groove (yellow arrows indicate nerveroots) with little echogenic connective tissue. B: More distal in the supraclavicular fossa (red arrowsindicate brachial plexus trunks) with “honeycomb” appearance.

While different tissues have characteristic appearances on ultrasound, nerve

may not be easily be distinguished from tendon when both are viewed in

short axis. However, using knowledge of anatomy, the operator can follow the

course of the structure caudad-cephalad to determine the nature of the

structure imaged. The tendons will eventually disappear into the muscle of

origin or insert into bones. A good example is the median nerve at the wrist,

where it is dif�cult to discern the neural structure from the many tendons in

the carpal tunnel, versus at the midforearm, where the nerve is much more

visually distinct, as it is situated between two layers of muscle, with no

surrounding tendons (Figure 5).

An important aspect of preparing for a block is to obtain the preferred

imaging plane while planning the route for needle path. The operator should

make certain that no vulnerable structures are in the projected course, such as

a blood vessel, the pleura, or sensitive structures such as periosteum.

FIGURE 5. A: The median nerve at the wrist among many tendons within the carpal tunnel. B: Themedian nerve more proximal in the forearm surrounded by muscle.

This process is referred to as a “preblock scan,” which can contribute to patient

safety and block success. In addition to two-dimensional imaging, the color

Doppler setting should be utilized to identify small vessels, which may readily

be confused with nerve structures (particularly roots) when viewed in short

axis (Figure 6).

FIGURE 6. The supraclavicular brachial plexus with surrounding vasculature. The subclavian artery isindicated by the multicolor area, with the transverse cervical artery indicated by the red area.

To maintain the view of the needle tip and shaft, several techniques can be

used. The more parallel the needle is to the face of the probe, the more echoes

are transmitted back to the transducer, resulting in a superior image. This can

be accomplished by gently indenting the skin at the needle insertion site or by

moving the insertion site further away from the probe, resulting in a less-acute

angle of insertion (Figure 7). The limitation of this approach is that a longer

needle may be required, and more tissue is traversed en route to the target.

FIGURE 7. Needle insertion directly beside the ultrasound probe may result in difficult visualization.Insertion at a distance from the probe permits a shallower approach, allowing for stronger echo returnand better visualization of the needle (green arrow) although traversing a longer tissue path.

Another technique, referred to as heeling, involves pressing in on the edge of

the transducer opposite the side of needle insertion, which results in a more

parallel alignment of the probe face with the needle. In addition, the needle

itself may be structurally altered to increase its echogenicity; commercially

available versions of these “echogenic needles” usually have been etched on

the surface of the shaft with crosshatches to create a greater degree of scatter

of the ultrasound beam.

As noted, needle guides may be utilized to improve needle imaging, though at

the cost of constraint of movement. Laser guidance systems have also been

created to improve alignment, with some success. One novel, alternative

method of targeted needle placement and local anesthetic delivery utilizes a

GPS guidance system, which may be especially useful when imaging is made

dif�cult by steep needle angles. Proprietary software for needle localization at

steep angles makes use of spatial compound imaging, which combines

images of different angles of insonation. This results in enhanced needle

imaging with both standard and echogenic block needles. Finally, localization

of the needle tip may be accomplished with “hydrolocation,” in which small

volumes of either dextrose solution or local anesthetic are injected to visualize

spread within tissues, which typically reveals the position of the needle tip.

NYSORA Tips

• Several different techniques are useful to maintain visualization of the

needle with ultrasound imaging, including use of a shallow angle of

approach, “heeling” the transducer, commercially available echogenic

needles, and physical measures such as rotation and tilting of the

transducer.

• In addition, hydrolocation with a small injection of �uid can be utilized

to facilitate localization of the needle in dif�cult situations.

• The needle should be advanced with continuous visualization to avoid

injury to anatomic structures.

• A preblock scan, including use of the color Doppler function, helps plan

the course of the needle.

• Passage of the needle tip through fascial planes that abut a nerve

should be conducted in a tangential fashion to avoid impaling the nerve

when the fascia “releases” the needle.

In advancing the needle tip toward the targeted nerve with in-plane imaging,

one should be cautious and deliberate, attempting to maintain the needle in

plane at all times (Table 3). The in-plane needle tip is characterized by a

double-echo return generated from the beveled surface.

TABLE 3. Safety tips during ultrasound-guided nerve blocks.

Perform a “preblock scan” to ascertain anatomyUtilize the color Doppler setting to identify blood vesselsDo not advance needle if tip is not localized“Hydrodissection” can be utilized to delineate anatomyWhen pushing through fascia toward a nerve, approachtangentiallyPass through fascia slowly, awaiting a “pop” or suddenreleaseReoptimize image of needle tip after passing throughfasciaWhen in doubt about needle-nerve interface, gentlymove needle to ascertain that the nerve does notmove with it (indicating that tip is embedded withinepineurium)

SAFE NEEDLE GUIDANCEWITH ULTRASOUND

Ultrasound is re�ected from both the super�cial and the deep walls of the

needle, resulting in a step appearance that can be distinguished from the

single return of the needle shaft. A subtle sliding motion of the ultrasound

probe can aid in con�rming the location of the tip as the beam walks up and

down the needle shaft.

Commonly, fascial planes will be encountered that resist advance of the

needle. These tough layers of connective tissue may be seen to “tent” as the tip

pushes against them, suddenly giving way and snapping back to their original

position. This abrupt change may have two consequences: First, the needle

may advance quickly and inadvertently beyond the intent of the operator

(unless this is anticipated); second, the needle may move out of plane. At this

point, the forward motion of the needle should be stopped until the in-plane

image is once again optimized. It is common for such fascial planes to lie just

super�cial or adjacent to the nerve target, as at the interscalene groove, the

axillary neurovascular bundle, or the femoral nerve. This motion may actually

result in the needle thrusting forward and encountering the nerve if the

sudden give of the fascial plane is not anticipated. For this reason, it is

recommended to approach nerves tangentially, projecting the advance of the

needle so that its tip will lie adjacent to the nerve, but not aiming for its center.

The resistance encountered by these tough facial planes may also

inadvertently redirect a needle when approached at a shallow angle.

Temporarily steepening the needle angle may permit an easier and more

controlled passage. Unfortunately, ultrasound guidance does not always

produce clear images that allow one to distinguish the nerve tissue from

surrounding tissue. In such situations, as the needle is advanced,

“hydrodissection” (deliberate injection of �uid into tissue planes) can be

utilized to separate structures, allowing better clarity in imaging, with either

dextrose or local anesthetic solution.

In addition, the behavior of tissues can be observed as the needle is advanced

to help localize the needle tip in relation to neural tissue.

While it was once held that contacting a nerve with a needle tip would likely

result in paresthesia, and, indeed, this was considered an appropriate nerve

localization technique, we now know that paresthesia is not consistently

elicited with needlenerve contact. This emphasizes the need to accurately

localize the needle tip with ultrasound imaging as well as using additional

monitoring during PNBs to detect hazardous needle-nerve relationships, such

as nerve stimulation and injection pressure monitoring.

NYSORA Tips

Deposition of local anesthetic solution should be optimized by takingadvantage of fascial planes or sheaths that can contain or channel thedrug around the nerveand longitudinally along its course.For nerves without such local fascial containment, the solution shouldbe injected in a circumferential manner to hasten onset of the block.

The peripheral nerve stimulator (PNS) has been a standard tool in nerve

localization during PNBs for several decades, with a high degree of success

and low complication rate. However, the widespread adoption of ultrasound

imaging has called into question its ongoing role in PNB. Over a decade ago,

Perlas et al evaluated the sensitivity of upper extremity peripheral nerves to

peripheral nerve stimulation during ultrasound imaging of needle to nerve

contact. The authors reported that, despite visualizing the needle tip

indenting the surface of the nerve, with the stimulator set to deliver a current

of 0.5 mA or less, no motor stimulation occurred over 25% of the time.

Several studies, with a variety of different blocks, have been performed to

assess the utility of this localization tool in association with UGRA. Whether for

supraclavicular block, for axillary block, or for femoral block authors have

shown that addition of the nerve stimulator as a nerve localization tool during

ultrasound-guided PNB was not contributory to success.

Moreover, Robards et al found that absence of a motor response to PNS

between 0.2 and 0.5 mA during popliteal block did not always exclude

placement of the needle within the nerve, and that stimulation might actually

lead to unnecessary manipulation of the needle into the nerve.

However, the stimulator may be useful as an adjunct to UGRA for reasons

other than ensuring block ef�cacy. Because it has been well established that a

threshold of nerve stimulation lower than 0.2 mA indicates high likelihood of

needle tip placement within the nerve, the stimulator may be employed

during UGRA as a safety monitor. The nerve stimulator is particularly necessary

during US-guided block of the deep nerves, or when the ultrasound image is

less precise than desired. In this setting, an evoked motor response could warn

against intrafascicular injection of local anesthetic.

Moreover, in some circumstances, it may be desirable to identify different

nerves with more precision, as during axillary block, for which the PNS serves

to delineate the nerves by their speci�c motor response to electrical

stimulation. There may be, in some anatomic locations, neural structures that

can be challenging to identify by visualization alone, whether they are the

target of blockade or one simply wishes to avoid them with the needle; in

these cases, a PNS may be invaluable to provide this identi�cation.

Finally, there are nerves that do not lend themselves readily to ultrasound

visualization, primarily because of depth or osseous interference with

ultrasound transmission. The most common example of this is the posterior

approach to the lumbar plexus, in which ultrasound can be used to identify

local osseous structures to guide the block, but for which the PNS remains a

valuable tool for guidance of the needle tip into proximity with the nerves of

the plexus.

Taken overall, a plethora of data indicate that routine use of a nerve stimulator

during ultrasound-guided nerve blocks yields clinically relevant safety

information that can in�uence the clinical decision making and positively

affect patient safety.

However, the primary purpose of the suggested routine use of nerve

stimulation with UGRA is for safety monitoring, rather nerve localization

(Figure 8). In this capacity, the nerve stimulator can be simply set at 0.5 mA (0.1

USE OF PERIPHERAL NERVESTIMULATION WITH ULTRASOUND

ms), 2 Hz, without changing the current intensity throughout the procedure.

While the motor response is not sought, occurrence of the motor response

should necessitate cessation of the needle advancement and slight

withdrawal of the needle as a motor response at this current delivery setting

almost always indicates needle-nerve contact or intraneural needle

placement.

FIGURE 8. Algorithm: The primary purpose of the suggested routine use of nerve stimulation with UGRAis for the purpose of safety monitoring, rather than nerve localization.

After accurate needle placement near the target nerve, and after ascertaining

that aspiration is negative for intravascular needle placement, injection of the

local anesthetic is made in the tissue plane that contains the nerve(s) to be

anesthetized (Table 4).

TABLE 4. Optimizing local anesthetic deposition.

Inject local anesthetic solution in small aliquotsObserve for pain or high pressure during injectionMake certain that spread of �uid is observed at needle tipduring injectionAspirate between injectionsBe aware of intervening fascial planes that may sequesteror channel the solutionAvoid deposition of local anesthetic into muscleFor solitary nerves in extremities, seek to create a “donut”or “halo” around nerveFor nerves within a fascial enclosure, seek to “�ll” thefascial con�nes with solutiona

Brull et al evaluated the long-held notion that the local anesthetic solution

should be directed in a circumferential manner around the visible nerve, with

change of needle position if necessary, in comparison to simply allowing the

solution to accumulate along one aspect of the nerve with one needle

position.

OPTIMIZING THE DELIVERY OFLOCAL ANESTHETIC NEAR THETARGET NERVE

They found that the resultant block set up 33% more rapidly with the former

than with the latter. While the creation of a “donut” or “halo” around the nerve

may be suggested as a general recommendation, some nerves, by virtue of

their anatomical situation, may not require such deliberate circumferential

placement. This is typically dictated by the location and con�guration of

overlying or surrounding fascial planes, such as in the interscalene groove and

at the femoral triangle. Optimal delivery of local anesthetic around each nerve

is described in subsequent speci�c UGRA sections.

Real-time imaging of local anesthetic injection permits assessment of correct

disposition of the �uid. The injection phase should be carried out with small

aliquots of local anesthetic (3–5 mL), with a short period allowed to elapse

between each, to allow evidence of any symptoms of local anesthetic systemic

toxicity (LAST) to be manifest before continuing to administer the drug, as

recommended by the American Society of Regional Anesthesia and Pain

Medicine (ASRA) guidelines.

In addition, delivery of each aliquot should be preceded by aspiration and

should progress with attention to opening injection pressures or complaints of

pain or paresthesia in the distribution of the target nerve.

While ultrasound has been shown to produce a lower likelihood of

intravascular needle placement, intravascular injection with LAST may still

occur. It is thus imperative to be aware of the location of vessels, which have a

distending pressure so low that ordinary pressure at the body surface with a

transducer obliterates their lumen entirely. Therefore, it is helpful to screen for

the presence of vessels using color Doppler during the pre block scan.

However, small vessels may be missed, and Doppler function deteriorates at

greater depths.

Thus, it is imperative to observe the ultrasound image throughout the

injection for evidence of tissue spread by the local anesthetic solution at the

tip of the needle. Failure to visualize such spread suggests that the tip of the

needle either is out of plane or is in the lumen of a vessel.

Errant needle placement has been described both into vessels and into

nerves. Moayeri et al in a cadaver-based study, have shown that ultrasound

imaging is sensitive to injection into the peripheral nerve, with as little as 0.5

mL causing visible evidence of nerve distension. Such visualization allows

immediate withdrawal of the needle, which may reduce the chance for nerve

injury, compared to injection of a large volume of local anesthetic.

Ultrasonography has revolutionized the �eld of regional anesthesia. The

effective application of this technology requires understanding of two-

dimensional anatomy, optimal imaging of the nerves and anatomical

structures, accurate real-time needle guidance, and precise local anesthetic

delivery. The combination of these elements ensures that the most bene�t

can be derived from this powerful imaging modality, ensuring high nerve

block success and improved patient safety, particularly with regard to LAST.

Kapral S, Krafft P, Eibenberger K, et al: Ultrasound-guided supraclavicularapproach for regional anesthesia of the brachial plexus. Anesth Analg1994;78:507–513.

CONCLUSION

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