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Print version ISSN 0034-7094
Rev. Bras. Anestesiol. vol.57 no.1 Campinas Jan./Feb. 2007
Ultrasound-guided nerve blocks*
Bloqueos nerviosos guiados por ultrasonido
Pablo Escovedo Helayel, TSAI; Diogo Brüggemann da ConceiçãoII; Getúlio Rodrigues de Oliveira Filho, TSAIII
IInstrutor Co-Responsável do CET, Coordenador do
Núcleo de Ensino e Pesquisa em Anestesia Regional (NEPAR)
do CET/SBA Integrado de Anestesiologia da SES-SC
IIPesquisador do NEPAR do CET/SBA Integrado de Anestesiologia da SES-SC
IIIResponsável do CET, Pesquisador do NEPAR do CET/SBA Integrado de Anestesiologia da SES-SC
OBJECTIVES: Ultrasound-guided nerve blocks are based on the direct visualization
of nerve structures, needle, and adjacent anatomic structures. Thus, it is possible
to place the local anesthetic precisely around the nerves and follow its dispersion
in real time, obtaining, therefore, more effective blockades, reduced dependency
on anatomic references, decreased anesthetic volume, and increased safety.
CONTENTS: The aim of this paper was to review the physical mechanisms of image formation, ultrasound anatomy of the neuro axis and of the brachial and lumbosacral plexuses, equipment and materials used in the blockades, settings of the ultrasound equipment to improve the image, planes of visualization of the needles, the techniques, and training in ultrasound-guided nerve blocks.
CONCLUSIONS: The steps for a successful regional block include the identification of the exact position of the nerves, the precise localization of the needle, without causing injuries to adjacent structures, and, finally, the careful administration of the local anesthetic close to the nerves. Although neurostimulation is very useful in identifying nerves, it does not fulfill all those requirements. Therefore, it is believed that ultrasound-guided nerve blocks will be the technique of choice in regional anesthesia in a not too distant future.
Key Words: ANESTHESIA, Regional; EQUIPMENT, Ultrasound; ANESTHETIC TECHNIQUES, Regional block.
Y OBJETIVOS: Las técnicas de bloqueos nerviosos guiados por ultrasonido
se basan en la visualización directa de las estructuras nerviosas, de
la aguja de bloqueo y de las estructuras anatómicas adyacentes. De esa
manera, se puede depositar la solución de anestésico local precisamente
en torno de los nervios y acompañar su dispersión en tiempo real,
obteniéndose así, un bloqueo más eficaz, de menor latencia,
menor dependencia de referencias anatómicas, menor volumenn de solución
anestésica y una mayor seguridad.
CONCLUSION: El artículo revisa los aspectos relativos a los mecanismos físicos para la formación de imágenes, la anatomía ultra sonográfica del neuro eje y de los plexos braquial y lumbo sacral, los equipos y materiales empleados en los bloqueos, los ajustes del aparato de ultrasonido para mejorar las imágenes, los planos de visualización de las agujas de bloqueo y las técnicas y el entrenamiento en bloqueos guiados por ultrasonido.
CONCLUSIONES: Los pasos para obtener el éxito en anestesia regional incluyen la identificación exacta de la posición de los nervios, la localización precisa de la aguja, sin lesiones en las estructuras adyacentes y, finalmente, la inyección cuidadosa de anestésico local junto a los nervios. Aunque la neuro estimulación sea de gran ayuda en la identificación de los nervios, ella no logra, aisladamente, rellenar todas esas exigencias. A causa de eso, se cree que los bloqueos guiados por ultrasonido serán la técnica de elección para la anestesia regional en un futuro no muy distante.
The first report on the use of ultrasound in regional anesthesia dates back to 1978 1, in a supraclavicular brachial plexus block. The blockade was done after identifying only the subclavian vessels and injecting of the local anesthetic around them. In the beginning of the 1980s, neuroaxial ultrasound was introduced as a tool to localize and measure the depth of the epidural space 2,3. However, despite the high index of success in brachial plexus blocks and in the identification and determination of the depth of the epidural space, the technological impossibility of visualizing non-vascular structures adjacent to the brachial plexus and of filtrating artifacts generated in the images of the neuroaxis, withheld the popularization of ultrasound assistance to regional anesthesia 4,5. The last 10 years have seen great progress in the generation and resolution of ultrasound images, allowing not only the visualization of the vessels, but also of nerve roots, peripheral nerves, dura mater, pleura, and fascias 6-16. The technological evolution of ultrasound equipment made possible the reduction in the size of the equipment and the production of portable machines with high quality image, decreased cost, and greater versatility. Thus, the use of the ultrasound is increasingly more frequent in regional anesthesia. The use of ultrasound images to guide needles in nerve blocks, promoting anesthesia and analgesia, has been described in adults and children, as well as in the treatment of chronic pain (stellate ganglion block, celiac ganglion block, third occipital nerve block, and periradicular injections). However, this technique is used more often in brachial plexus block, femoral nerve block, and sciatic nerve block 12,17-28. Besides, ultrasound guided techniques have been described in neuroaxial blocks, paravertebral blocks, intercostal nerve blocks, iliohyopogastric nerve block, ilioinguinal nerve block, saphenous nerve block, and pudendal nerve block 29-34.
Ultrasound guided techniques are based on direct ultrasound visualization of nerves, needle, and adjacent anatomic structures, making it possible to apply the local anesthetic precisely around nerves and to follow its dispersion in real time, achieving a more effective blockade, with reduced latency, decreased dependency of anatomic landmarks, reduced volume of local anesthetics, and increased safety 8,22,25,28,35-44. Therefore, the use of ultrasound in regional anesthesia is capable of offering several potential advantages when compared to the "blind" techniques, which can be seen in Table I.
PHYSICAL MECHANISMS OF ULTRASOUND INVOLVED IN IMAGE GENERATION
By definition, ultrasound is produced by sound waves with frequencies above 20,000 Hz. A special display of piezoelectric crystals (quartz) in the transducers produces ultrasound from electric energy. Sound waves are transmitted as oscillating waves with alternating pressures of 0.5 to 5 megapascal (Mpa) 45. The structures reached by ultrasound waves are said to be insonated. Sound waves are reflected by tissues and transformed in electrical energy by transducers and then in bidimensional images represented in a gray scale by the software of the ultrasound equipment. In clinical practice, transducers capable of producing ultrasound frequencies from 2 to 20 MHz are available. There are no reports of harmful effects caused by ultrasound waves in those frequencies, which are considered safe. The ultrasound wavelength is obtained by dividing the velocity of propagation by the frequency, which determines the axial and lateral resolution of the image. Ultrasound frequency is inversely related to its wavelength and also determines the depth of tissue penetration.
High frequency transducers (10 to 15 MHz) visualize superficial structures, up to 3 cm deep, such as the brachial plexus in the interscalenic, supraclavicular, and axillary regions 46. Transducers with frequencies ranging between 4 and 7 MHz are ideal for structures located at a depth of up to 5 cm, such as the sciatic nerve in the popliteal fossa 27,47,48, and the brachial plexus in the infraclavicular region 49,50. To identify structures located more deeply, like the sciatic nerve in the gluteal region, or the epidural space in adults, 2 to 5 MHz transducers are used 4,51.
Thus, increasing the ultrasound frequency increases the quality of superficial images while decreasing the visualization of deeper structures. Decreasing the frequency of the ultrasound decreases image resolution, but the penetration of the sound waves is increased, enabling the visualization of deeper structures.
The velocity of propagation of ultrasound waves is specific for each tissue. Sound waves are propagated in tissues rich in water at a mean speed of 1,540 m.sec-1, while in air and in the bones they are propagated at 440 and 4,080 m.sec-1, respectively. This generates a difference in the reflection of these waves (echogenicity), resulting in white, black, and gray contrasts (interfaces) delimitating anatomic structures. The acoustic impedance of a tissue is the result of its density multiplied by the velocity of propagation 45. The capacity to reflect ultrasound waves is determined by the difference in acoustic impedance among the different tissues and the angle of the ultrasound beam to the structure in question. In air-tissue and bone-tissue interfaces the differences are so great that almost all the energy transmitted is reflected, forming "acoustic shadows" 52. Hyperechoic structures reflect more sound waves, being represented by white areas, such as bones, tendons, and fat. Hypoechoic structures are represented by black areas where the sound waves are attenuated, such as tissues rich in water. The attenuation of reflected ultrasound waves occurs with loss of energy to tissues during their propagation, producing heat (absorption), and by the lateral dispersion of the ultrasound beam. Attenuation is specific for each tissue and proportional to the frequency, being expressed in dB/cm/MHz units. In soft tissues, the attenuation constant corresponds to 0.75 dB/cm/MHz. Despite the high absorption index of these tissues, a significant loss in the capacity to generate the image only occurs with frequencies above 15 MHz 45. The Doppler effect is a consequence of the difference in echogenicity between the original frequency emitted and the frequency received, generated by the movement of the source of the echo (blood), getting closer or farther away, in relation to the receptor unit. So, it is possible to measure the speed of the blood flow in the vessel and, when necessary, differentiate it from nerves 0,53.
The quality of the images depends on the quality of the equipment, transducer used, ability of the operator to perform and read the exam, and of the adjustments made in the equipment that maximize image resolution.
Peripheral nerves can have oval, triangular, or round morphologies, and some present the three forms along their extension 48,54. Besides, depending on localization, nerve size, transducer frequency, and angulation of the ultrasound beam, they can have specific echogenic characteristics (hypoechogenic or hyperechogenic). Nerve structures can be visualized in longitudinal or transversal cuts. The transversal cut of cervical nerve roots shows hypoechogenic nodules, while the longitudinal cut demonstrates tubular hypoechogenic areas 16 characteristic of a monofascicular pattern. The cervical roots from C4 to C7 are commonly seen in a neck exam; however, the roots of C8 to T1 are not 16,53. In the interscalenic sulcus, the superior, medial, and inferior cords of the brachial plexus can be identified in the transversal cut as three hypoechogenic nodules aligned between the anterior and middle scalene muscles, but the visualization of the inferior cord is more difficult, since it is posterior to the subclavian artery 53. The cords of the brachial plexus are more easily visualized then the roots that give rise to the plexus, because they are thicker 12. In the supraclavicular fossa, the cords and divisions of the brachial plexus can be seen as a group of multiple hypoechogenic nodules above and lateral to the subclavian artery 12 (Figure 1). The cords of the brachial plexus are formed immediately distal to this region, and the posterior cord is above the medial and lateral cords 53. Depending on the anatomic configuration of the supraclavicular fossa, the area to manipulate the transducer and the needle might be limited 50,53,55. Conventional linear transducers have a rectangular surface of contact measuring about 3.8 to 5 cm in length. However, linear transducers similar to hockey sticks, measuring 2.5 cm, can reduce this restriction 56. In the infraclavicular area, the cords of the brachial plexus are identified as hyperechogenic nodules that form a triangle around the axillary vessels, the lateral cord is anterior to the other cords, and the axillary vein is located between the medial cord and axillary artery 46,53,56.
In the axilla, the terminal branches of the brachial plexus are arranged around the axillary artery, showing great mobility and variation in its location 46,57 (Figure 2). The longitudinal cut of peripheral nerves demonstrates multiple, parallel, discontinued, hypoechogenic areas (nervous tissue) separated by hyperechogenic bands (connective tissue). The transversal view of peripheral nerves shows hypoechogenic nodules (nerve tissue) surrounded by a hyperechogenic background (connective tissue), configuring a fascicular or "honeycomb" pattern (Figure 3). However, this fascicular echoic texture does not have an exact histologic correlation, being capable of generating an image of 1/3 of the existing fascicles 9. The possible reasons for this phenomenon are: inability to visualize fascicles, unless they are perpendicular to the ultrasound beam, associated with poor quality resolution, which condenses adjacent structures in the same echogenicity. However, it has been demonstrated that relatively small nerves, such as the recurrent laryngeal nerve, do not have a fascicular pattern, which is present only in larger peripheral nerves, such as the median and sciatic 11. In some cases, the nerve structure should be visualized in two views and be followed distally for a positive identification. This tracking of peripheral nerves is hindered by their mobility, being easier to accomplish in the transversal cut. Some structures, like tendons and small vessels, can be mistaken for peripheral nerves. However, using transducers with a frequency greater than 10 MHz (high-resolution ultrasound) it is possible to observe a fibrillar pattern with thin, continuous hyperechogenic (similar to fibrillas) and hypoechogenic (less prominent than in nerves) bands. One can also use passive mobilization of the peripheral nerves in the forearm to differentiate between nerves and tendons 58. The angle of the ultrasound beam influences the echogenicity of peripheral nerves because they are formed by nerve tissue (hypoechogenic) and connective tissue, like the epineuro and perineuro (hyperchogenic). Optimal echogenicity is obtained when the beam is perpendicular to the nerve, forming an image with a fascicular pattern. As the angle changes, the image presents ambiguous characteristics of sound wave reflection, decreasing its echogenicity (anisotropy) 59.
Vessels are differentiated from small nerves by the compression caused by the transducer and using color Doppler. Larger peripheral nerves of the lower limbs (femoral and sciatic) have an elliptical or triangular shape and are characteristically more hyperechogenic and anisotropic than the nerves in the upper limbs 60,61. Thus, they are more difficult to visualize than the nerves in the brachial pleexus7. However, in 87% of the patients the sciatic nerve could be identified in the gluteal region and below it as a solitary hyperechogenic, elliptical structure between the ischial tuberosity and the greater trochanter of the femur, using a 2 to 5 MHz transducer 51 (Figure 4). In the middle third of the thigh, the nerve has a triangular shape and close to the popliteal fossa it becomes round and the formation of its terminal branch can be observed. In this region the nerve is more superficial and a higher frequency (4 to 7 MHz) transducer should be used 8,48,62. The factor that contributes for the visualization of the sciatic nerve is its relatively high echogenicity, contrasting with the low echogenicity of the surrounding muscles. However, patients undergoing surgical procedures in the area the nerve is looked for and in the elderly, these interfaces are decreased, compromising its identification 8. Curiously, the tibial nerve has a typical fascicular pattern, while the common peroneal nerve has fewer and wider fascicles 9. In the popliteal fossa, the position of these nerves may vary, which might hinder their identification by "blind" techniques 48. The femoral nerve is consistently visualized in its retroperitoneal trajectory 63. Below the inguinal ligament, it might be divided in its anterior and posterior branches. In this region, the femoral nerve has an oval shape and is located in a triangular space lateral to the femoral artery, superior to the iliopsoas muscle and inferior to the iliac fascia 64. In the femoral trigone, its oblique trajectory might make its insonation and, consequently, its visualization at a 90°-angle difficult 13,61.
The visualization of the noble structures of the neuro axis is hindered by the presence of the calcified osteo-ligamentary structures of the vertebral column. Besides, due to the depth of the nerve structures to be blocked, it is necessary to use ultrasound of smaller frequencies, obtaining images that are less clear. Image resolution is maintained up to a depth of 6 to 8 cm, using transducer frequencies of 3.5 to 8 MHz 4,65. The ultrasound anatomy of the neuroaxis can be identified in the longitudinal and transverse planes, and in the median and paramedian regions of the vertebral column 31,65-69. In the transversal cut, the intervertebral space can be observed between the spinous processes. The transverse processes and lateral articular facets are easily identified. The ligamentum flavum and dura mater, located in the midline, are aligned with the transverse processes, being hyperechogenic. The echogenicity of the ligamentum flavum and dura mater is very similar, hindering their visualization, as well as of the epidural space (non-echogenic). The subarachnoid space is hypoechogenic, limited posteriorly by the dura mater-ligamentum flavum complex, and anteriorly by the vertebral body (hyperchogenic) (Figure 5).
In the paramedian longitudinal cut, the intervertebral space can be delimited by the cephalad and caudal spinous processes. Thus, the ligamentum flavum and the dura mater can be identified between them, anterior to the subarachnoid space and to the vertebral body, consecutively.
The paramedian longitudinal cut allows the visualization of the structures in the same disposition of the longitudinal view. However, the number of bone structures is smaller, reducing the loss of sound energy to those surfaces, with the consequent reduction in acoustic shadows. Thus, this approach favors the visualization of ligaments, meninges, and nerve structures 66,69. Besides, the quality of the ultrasound images of the neuroaxis is inversely proportional to the age of the patient 69. In children younger than three months, ligaments and bones are not completely calcified, and the depth of nerve structures and epidural space is smaller, allowing the use of high-frequency transducers, providing for high quality images. However, with body growth and increased bone calcification, these characteristics are attenuated, as well as the quality of the images generated 70.
EQUIPMENT AND MATERIAL USED IN ULTRASOUND-GUIDED BLOCKS
Ideally, a high-resolution device (capable of emitting ultrasound frequencies above 10 MHz), with color and pulsatile Doppler, is necessary for peripheral nerve blocks. Ultrasound machines can be portable or placed in stations. Portable devices have technology capable of generating and storing high-resolution images. Large machines have greater processing and storing capacities and can be equipped with printers and CD/DVD recorder.
Images generated by portable devices fulfill the needs of regional anesthesia with reduced costs. The higher quality of the images produced by larger machines has a cost-benefit ratio favorable only for scientific purposes. Besides the ultrasound machine, two wide-band transducers or three fixed frequency transducers are used. Transducers can have different ultrasound frequencies, shapes and contact surface size. In clinical practice, transducers capable of producing ultrasound frequencies from 2 to 20 MHz are available. Transducers used to guide nerve blocks can be convex or linear. The lateral divergence of the sound waves of convex transducers is greater, with greater field of vision and decreased image definition. Linear transducers are used to identify superficial structures, such as nerves, muscles, tendons, and vessels because of the greater image resolution 59. The contact surface of linear transducers is rectangular, 3.8 to 5 cm long, while hockey stick-shaped linear transducers measure 2.5 cm. A sterile, gelatinous solution is used to decrease the air interface between the skin and the transducer. Besides, in simple peripheral nerve blocks, the contact surface of the transducer with the skin is covered with an adhesive, sterile, plastic material. In continuous peripheral nerve blocks and in neuroaxial blocks, the transducer and its cable are completely covered with a plastic, sterile wrap.
In ultrasound-guided nerve blocks one can use those needles commonly used in regional anesthesia, such as Tuohy needles, isolated neurostimulation needles, and blunt needles. The caliber of the needle influences its visualization 71. Wider needles are easily identified because of the larger area on transversal cuts and do not deviate as much from the alignment plane of the image 72. Some needles used in ultrasound-guided biopsies are made of a material capable of reflecting more the ultrasound waves (hyperechhogenic), being easily identified during the procedure. They cost more, and this expenditure would only be justified for deep blocks in which the needles have small insonation angles, decreasing considerably the visualization 71,73. There are no hyperechogenic needles designed specifically for nerve blocks. Thus, wider needles have been used in deeper blocks 74,75.
ADJUSTMENTS TO OPTIMIZE THE IMAGE AND TECHNOLOGICAL ADVANCES IN ULTRASOUND
Ultrasound machines have commands to adjust and improve image definition according to their technological configuration, to fulfill the needs of each patient, creating a high quality image.
On the menu there are several modes of visualization for the different types of structures, obeying a pre-established program of ultrasound characteristics, capable of producing the best image possible of the structures studied. The "small parts" mode offers optimal conditions for image generation and for the identification of peripheral nerves and muscle-skeletal strucutres 9. This program enhances the ultrasonographic characteristics of superficial structures, favoring the visualization of nerve structures. Some of the latest generation equipment offers a specific mode to visualize peripheral nerves, enhancing their characteristics. The depth of the images can be increased to allow a wider visualization of the region studied and be reduced afterwards for more details of the dynamics of the blockade. "Image gain" can be regulated to increase contrast as a whole or separately, in superficial and deep levels. Thus, the glow of adjacent structures can be regulated for better definition. The zoom is used to amplify the details of an area of the image, but does not necessarily maintain its definition. In wide band transducers, the ultrasound frequency can be regulate to obtain the best resolution possible for the depth of the nerve. Modern ultrasound equipment has several resources to filter artifacts in the image. These mechanisms can be used both before and after image processing. Some of them are common to every ultrasound machine, while others are innovations exclusive to certain brands. Spatial image composition is made of an array of multiple lines of crystals in the transducer capable of emitting and receiving ultrasound waves in several angles, while conventional transducers have just one line of crystals 76. Thus, the superposition of echoes from different insonation angles is processed digitally, forming a higher quality image with fewer artifacts than conventional high-resolution ultrasound 77. Sometimes, the identification of the tip of the needle is hindered by the presence of small insonation angles. It has been demonstrated that the spatial composition of the image is capable of improving the visualization of the tip of the needle in those angles. Real time processing involving adaptive analysis and increased image (XRES imaging) is a consequence of the digital signal processing that adapts to a target image, considering its echotexture and structure 78. With a multiresolution algorithm, the echotexture of the structure is recognized, accentuated, and their interfaces are equalized, decreasing artifacts. This adaptative process is capable of improving the resolution of the image generated by the spatial composition of the image 78. Tissue harmonic image is composed by harmonic waves (ultrasound waves produced by tissue vibration and propagated through the tissue non-linearly) 79. Those harmonic waves are multiple integrals of the frequency emitted and their higher frequencies are used to form the images. Suppressing reflected waves of lower frequencies decreases artifacts, improving contrast and the lateral resolution of the image. Due to the low axial resolution, it is advisable to start the exam in the conventional bidimmensional mode, followed by harmonic waves for better visualization of the details of the structures 76. The concept of coded excitation is based on the encoding of the ultrasound beam through the creation of a repetitive pattern of 1s and 0s. Codes are emitted and received, being recognized to form the image. Based on them, it is possible to produce longer waves, and to visualize deep structures with high quality resolution 76.
PLANES OF NEEDLE VISUALIZATION
Peripheral nerves can be visualized on longitudinal or transversal cuts. However, transversal cuts are more appropriate for peripheral nerve blocks. The main reasons include: easier technique for capturing and maintaining the image during the block, better visualization of adjacent structures, and capacity to evaluate the distribution of the local anesthetic around the nerve 72. There are two techniques to visualize the position of the needle relative to the transducer when using transversal cuts. The first to be described was the transversal alignment to the ultrasound beam, in which the needle is introduced transversally to the transducer, and only the tip of the needle and its acoustic shadow, along with tissue dislocation when the needle goes through, can be visualized. Test injections are often necessary to help visualize the tip of the needle 72. The second technique is the longitudinal alignment to the ultrasound beam in which it is possible to visualize the tip and the shaft of the needle while it is introduced. This technique demands more precise movements to maintain the alignment and increases the distance between the skin and the nerve (Figure 6). The choice of the plane of introduction of the needle can be affected by the anatomical characteristics of the region. For example, below the gluteal region, the longitudinal alignment implies a painful introduction through the posterior thigh musculature, while in the transversal technique the needle can be introduced among these muscles, being less uncomfortable. Besides, the visualization of the entire needle while it is being introduced might not be necessary due to the absence of noble anatomical structures adjacent to the nerve in this region. However, in supraclavicular blocks, the longitudinal alignment technique allows the needle to be tracked and, possibly, decreases the morbidity of this block 25.
TRAINING IN ULTRASOUND GUIDED REGIONAL BLOCKS
Using ultrasound to teach the techniques of regional anesthesia provides dynamic anatomical information during the blockade and allows direct and safer supervision 37. For this reason, ultrasound assistance can be considered an invaluable tool for teaching and training institutions 80. However, some manual abilities and theoretical notions should be developed before performing nerve blocks in patients 35. Coupling ultrasound generated images and regional anesthesia requires experience in basic ultrasound techniques and keen knowledge of the images of nerves and adjacent structures. Thus, training on ultrasound-guided nerve blocks should begin with a theoretical-practical teaching of the images of the anatomical structures present in the different approaches to the nerve plexuses and neuroaxis, as well as the patterns of dispersion of the local anesthetic.
Learning curves to acquire the necessary manual skills on ultrasound examination and in the longitudinal and transversal alignment of the needle to the ultrasound beam, allowing the visualization of the shaft and tip of the needle or just the tip, respectively, should be established in models (pork shoulder, turkey breasts, or gelatin molds) 35,81. Such ability is indispensable to ultrasound-guided regional anesthesia, guaranteeing greater precision and safety 25. These steps can be concisely developed in workshops 5.
After these requirements are fulfilled, training in patients can be started with greater safety and quality.
TECHNIQUE OF ULTRASOUND-GUIDED NERVE BLOCK
The techniques of ultrasound-guided nerve blocks, preferentially the transversal cut to identify the nerve, can be easily achieved because they provide the visualization of the dispersion of the anesthetic around the nerve.
Initially, an inventory of the ultrasound anatomy of the region is done, identifying structures like vessels, bones, pleura, and the target nerve 25. In this first moment it is essential to obtain the best visualization plane possible of the structures and to perform the necessary adjustments in the ultrasound equipment, because the success of this technique can be related to the quality of the images obtained 56. Afterwards, the tip of the needle is guided up to the nerve, the anesthetic is injected and its diffusion around the nerve is observed as a black halo (doughnut effect) 36,47.
This diffusion pattern assures that the nerve was involved by the local anesthetic, guaranteeing short latency and a high success rate of the nerve block 36,48. If the distribution of the local anesthetic around the nerve is incomplete, the tip of the needle should be repositioned to guarantee the local anesthetic gets in touch with the region of the nerve that was not surrounded by it initially 25,72. The intraneural injection of local anesthetics can be detected by the increased volume of the nerve 39. Thus, the success of ultrasound-guided nerve blocks depends on the visualization of an "ideal" dispersion pattern of the local anesthetic and not on the proximity of the tip of the needle to the nerve, as is the case of neurostimulation techniques and paresthesia 41,42,66,82. The presence of air bubbles in the anesthetic solution gives rise to acoustic shadows, hindering the identification of the structures. Therefore, the needle should be filled with anesthetic, avoiding the accumulation of air and fuzzy images. When the dispersion is not visualized during the administration of 1 o 2 mL of the solution (test-injection) it should be interrupted immediately because it might configure an intravascular administration 55. Test-injections can also be used to facilitate visualization of the tip of the needle and improve the resolution of the interfaces among the nerves and surrounding structures, working as an inverted contrast 72. However, the use of neurostimulation associated with ultrasound visualization is hindered when local anesthetics are used (conducting solutions) as test injections. In this case, the administration of 5% DW (non-conducting solution) should be used to maintain the neurostimulation capacity of the needle by decreasing the conducting surface and increasing the density of the current at the tip of the needle 83.
Neurostimulation identifies peripheral nerves by triggering distal motor responses. The stimulation is capable of producing a functional response in peripheral nerves. In situations a motor response cannot be produced, such as amputees or patients with severe peripheral neuropathies, the direct visualization of the nerve or plexus allows the local anesthetic to be deposited close to it 47,84. Besides, neurostimulation is a unidimensional technique that is neither capable of localizing the tip of the needle and its relation to vessels and pleura nor showing the real time dispersion of the local anesthetic 42. Thus, the risk of pneumothorax, accidental vascular administration, and nerve block failure is increased. Another remarkable characteristic of ultrasound-guided nerve blocks is its capacity to detect the presence of anatomical variations 48,85 that many times are responsible for failures or make it impossible to make a nerve block based on neurostimulation alone 54. Since ultrasound is a bidimensional technique, on transversal cut, or even tridimensional, if longitudinal and transversal cuts are used alternately, of neural identification, it is capable of providing real time structural information on the interaction of nerves, vessels, needle, and local anesthetic 15. Finally, the dependence on anatomical parameters for nerves blocks, especially in obese patients, is decreased with ultrasound direct visualization when compared to neurostimulation 86. So, ultrasound can contribute to decrease the failure rate of "blind" techniques in this population 87,88. Some centers chose the association of direct ultrasound visualization and neurostimulation (peripheral nerves) as a back up technique 55. Some researchers defend the isolated use of ultrasound-guided nerve blocks after obtaining proficiency in this modality, but there is no scientific evidence capable of determining such status. Others consider that those techniques are synergic agonists, not antagonistic, and prefer to take advantage of the qualities of both techniques for safer, precise nerve blocks with a short latency and a reduction in the time necessary to perform the block.
Real time ultrasound-guided neuroaxial blocks are technically more complex, due to the presence of the vertebral column (bone and calcified ligaments) that create an extensive area of acoustic shadow, providing small windows for visualization and manipulation of the transducer and needle 4. The longitudinal paramedian approach is an excellent option to visualize the lumbar and, especially, the thoracic epidural space 67,69. Real time ultrasound-guided neuroaxial blocks have been proposed for children and adults 40,66. However, the use of neuroaxial ultrasound in adults has been used more frequently to determine difficult cases, measure the depth of the epidural space, identify precisely the intervertebral space, and project the trajectory of the needle, optimizing the success and training of the epidural block 3,32,65,80,89. Besides, ultrasound images of the neuroaxis help monitor the dispersion of local anesthetics, during blocks, and blood, during blood tamponade, as well as the migration of catheters in the epidural space 90-92.
The steps for success in regional anesthesia include the exact identification of the nerves, the precise location of the needle without damaging adjacent structures, and the careful administration of the local anesthetic close to the nerves 22,42. Although neurostimulation is very helpful in identifying nerves, it cannot fulfill all those requirements.
Besides, ultrasound is capable of disseminating the teaching of regional anesthesia because it is easy to learn and to supervise, with an excellent safety profile and high success rate 37, encouraging anesthesiologists with little experience in regional blocks to choose this technique 42.93. For all the reasons exposed here, it is believed that ultrasound-guided nerve blocks will be the technique of choice in a not too distant future.
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Dr. Getúlio Rodrigues de Oliveira Filho
Rua Luiz Delfino, 111/902
88015-360 Florianópolis, SC
Submitted em 13
de março de 2006
Accepted para publicação em 15 de setembro de 2006
* Received from Hospital Governador Celso Ramos, CET/SBA Integrado de Anestesiologia da Secretaria de Estado de Saúde de Santa Catarina (SES-SC), Florianópolis, SC