(1)
Anesthesiology and Intensive Care, Regina Margherita Children Hospital, Turin, Italy
Keywords
UltrasoundRegional anesthesiaTechniquesPediatricThere has been a recent increase in the use of regional anesthesia in pediatric patients; this explosive growth, particularly in the use of truncal blocks, can be attributed in part to the refinement of anatomically based ultrasound imaging to facilitate nerve localization. Historically, pediatric regional anesthesia has posed a significant challenge due to the close proximity of nerves to critical structures and the need for limiting the local anesthetic volume below toxic levels in children. Ultrasound guidance, however, allows the visualization of important anatomy and can help overcome many of these traditional obstacles [1].
This technique, in fact, has brought pediatric regional anesthesia to new levels improving the quality of anesthetic blockade with faster onset time, longer duration of blocks, and lower dose of local anesthetics [2].
Although ultrasound may be useful for nerve localization, one of the main benefits is to provide visualization of the dispersion of the local anesthetic within the desired tissue plains. Ultrasound has been shown to provide adequate landmarks for determining the location of nerves in children along with a discriminatory approach to evaluating nerve location and anatomical variations in infants and children. This technology however requires a significant training and skill for its successful implementation.
Ultrasound guidance is therefore strongly recommended when performing peripheral nerve blocks in infants and children [3, 6–8].
This chapter will review a variety of common peripheral nerve and central neuraxial blocks that can be performed using ultrasound guidance in children. We have to emphasize that ultrasound guidance is a relative recent innovation to the field of regional anesthesia; most of the current literature is not evidence based. As a result, much of the data comes from case reports and case series.
Our goal is to provide the pediatric anesthesiologist with a comprehensive summary of the relevant sonoanatomy, techniques, and outcomes of ultrasound guidance for peripheral nerve blocks of the extremities and trunk as well as neuraxial blocks in pediatric patients based on currently available literature.
20.1 Ultrasound Equipment
Mobile, usually cart-based, echographs consist primarily of a probe (transducer), a computer that controls the transducer, sends the impulse, receives the echo, and processes the signal, a visualization system, and a storage device for later digital editing and printing (Fig. 20.1a, b). The most important part of the echograph is the transducer which can present different forms depending on its specific use. The important technical specifications are its axial and lateral resolution and the frequency achieved. Resolution is the ability to discern two dots as being distinct from each other on the x and y plane: on the (x) axis – which is parallel to the propagation line of the ultrasonic beam – and on the (y) axis – which is the axis vertical to the ultrasonic beam. Today, due to technological advancement in signal processing software, it is also possible to improve data quality for imaging and to influence the definition: to reduce the phenomenon of signal attenuation, the reflected signals are amplified taking into account the delay with which the signal is received, thus allowing statements as to their depth. Finally, it should be kept in mind that the ultrasonic beam transmitted from the transducer results in the two-dimensional representation of a three-dimensional object. Therefore, recordings from at least two sets of planes are required to be able to reconstruct the object in its original shape [4, 5].
Fig. 20.1
Cart-based echographs
20.2 Minimum Requirements for Regional Anesthesia Ultrasound Equipment
The ultrasound equipment used in regional anesthesia should be transportable. Portable units as well as compact scanner systems mounted on a cart are commercially available.
These should meet at least the following requirements: color Doppler to identify blood vessels and distinguish them from the surrounding tissues, contrast images (gain), and sufficient storage capacity for images and preferably also for films.
For any ultrasound-guided block technique, a sterile preparation of the probe with an adhesive sheet and the block area also using sterile gel is recommended (Fig. 20.2a, b). When a catheter is introduced, it is recommended to completely wrap the probe with a proper sterile cover (Fig. 20.2c).
Fig. 20.2
(a) Adhesive sheet, (b) Sterile gel, (c) Sterile cover
20.2.1 Ultrasound Probe
Transducer characteristics, such as frequency and shape, determine ultrasound image quality. The transducer frequencies used for peripheral nerve blocks range from 3 to 15 MHz. Linear transducers are most useful for nerve imaging to provide high-resolution images. For superficial structures (e.g., nerves in the interscalene, supraclavicular, and axillary regions), it is ideal to use high-frequency transducers in the range of 10–15 MHz remembering that depth of penetration is often limited to 2–3 cm below the skin surface.
For visualization of deeper structures (e.g., in the infraclavicular and popliteal regions), it may be necessary to use a lower frequency transducer (less than or equal to 7 MHz) because it offers ultrasound penetration of 4–5 cm or more below the skin surface. However, the image resolution is often inferior to that obtained with a higher frequency transducer.
Two types of probes are commonly used in children (Fig. 20.3):
Fig. 20.3
Probes commonly used in children
Linear probes (6–13 MHz): the resulting imagine is square, with good resolution in the near field, but narrow depth; probe frequency is selected according to nerve depth, 7 MHz for structures deeper than 5 cm, 10 MHz for structures between 3 and 5 cm deep, 12 MHz for structures maximally 3 cm deep, and shorter probes such as the Hockey stick probe with a length of 2.5 cm for pediatric use.
Sector probes (2–5 MHz): the resulting image is trapezoidal, with good resolution and depth, but structures in the near field are poor imaged.
To correctly perform a nerve block under ultrasound guidance, it is important to proceed as follows:
1.
Localize the nerve
2.
Move the ultrasound probe
3.
Move the block needle
20.2.2 Identification of the Structures in Ultrasonography
The term isoechogenic refers to a structure displayed in an ultrasound image with a certain intensity on a gray scale.
Formations with low echogenicity are called hypoechoic, formations that do not deliver any echo at all are anechoic, and those formations with high echogenicity are called hyperechoic. Normally the echogenicity of an investigated structure corresponds to its type and composition; however, not every anechoic structure is fluid nor must every hyperechoic structure be solid.
The terms shadow cone and acoustic enhancement refer to structures that inhibit the travel of ultrasound, or, respectively, increase its speed. Such situations occur typically in investigations of bone or calcium structures (e.g., gallstones) and formations containing liquid (e.g., liver cysts).
Vessels: their identification is of particular importance because it guides to the neurovascular bundle. Vessels appear hypoechoic; discrimination between arteries and veins and also applying pressure with the probe are possible with color Doppler: veins are compressible while arteries remain pulsatile; gentle pressure is needed because during venous compression, intravenous needle tip placement may be missed.
Muscle: this has a fibrolamellar ultrasonographic appearance with either heterogeneous structures with bandlike hyperechoic intramuscular septae or homogenous structures.
Bones: the cortex of the bone is hyperechoic and the areas behind or deep to the cortex are completely anechoic.
Fatty tissue: hypoechoic.
Tendons, pleura: hyperechoic.
Local anesthetic: direct observation of the spread of local anesthetic during injection is mandatory; local anesthetic solution appears anechoic.
Neuronal structures: every nerve has a particular appearance in ultrasonography with regard to the shape, the echogenicity, and the quantity of connective tissue present between the nerve fibers.
The proximal parts of the brachial plexus appear as hypoechoic round to oval structures; by contrast more peripheral nerve structures appear hyperechoic. In the lower extremity, most of the neural structures appear hyperechoic (Fig. 20.4a, b).
Fig. 20.4
(a) Scanning of the axilla. (b) A artery, N nerve, V vein
20.2.3 Needle Handling
Proper needle handling skills are required for accurate and smooth needle insertion during ultrasound-guided nerve blocks. If the operator is not ambidextrous and prefers to use the dominant hand to handle the needle and inject local anesthetic, then the operator must choose a proper body location and orientation in relationship to the patient.
There are two ways to align the transducer with the needle:
In plane (IP): The transducer is exactly parallel to the needle. In this case it is possible to see the whole length of the needle and the needle tip. Advantages: Because the needle tip is visualized, it is possible to position it correctly without the risk of injuring nerves or vessels. Disadvantages: Since ultrasound has an extremely narrow beam width, it can be difficult to keep the needle constantly in view (Fig. 20.5a, b).
Fig. 20.5
In plane approach
Out of plane (OOP): The needle and the transducer are perpendicular to each other. In this case it is possible to see the cross section of the needle as a hyperechoic dot, which however can result from any other needle segment and not the tip. Advantages: The needle cross section is easily identified. Disadvantages: One is not sure where the tip of the needle is. This potentially carries the risk of injury to nerves and blood vessels (Fig. 20.6a, b).
Fig. 20.6
Out of plane approach
20.3 Ultrasound-Guided Regional Anesthetic Techniques
20.3.1 Upper Limb Blocks
Peripheral regional anesthesia is of great utility in children undergoing surgery on the upper extremities. Many approaches to the brachial plexus have been described; in children the most commonly performed and reported brachial plexus blockade is the axillary block. This may be due to the fact that other block sites are situated near critical structures such as the cervical pleura (supraclavicular and infraclavicular) and the spinal cord (interscalene). Introduction of ultrasound imaging will likely greatly increase the performance of brachial plexus blocks in infants and children at locations besides the commonly described axillary approach by allowing for real-time visualization of anatomical structures.
20.3.1.1 Interscalene Approach
The interscalene block is indicated for all upper extremity surgery, but particularly if the shoulder is involved.
Ultrasound imaging allows visualization of the C5, C6, and C7 nerve roots between the anterior and middle scalene muscles. In a transverse oblique plane at the level of the cricoid cartilage and at the posterolateral aspect of the sternocleidomastoid muscle, the muscle appears as a triangular-shaped structure overlying the internal jugular vein and common carotid artery (Fig. 20.7).
Fig. 20.7
ca carotid artery, jv jugular vein
The scalene muscles serve as useful landmarks; the anterior scalene lies deep to the sternocleidomastoid and lateral to the subclavian artery, while the middle and posterior scalenes are located more posterolaterally. The neurovascular (interscalene) sheath appears as a hyperechoic structure within the interscalene groove. The trunks and/or roots of the brachial plexus may be visible as round- or oval-shaped hypoechoic structures. The roots or trunks lie between the scalenus anterior and the scalenus medius at this level. The prominent internal jugular (anechoic) lies medially (Fig 20.8a, b).
Fig. 20.8
(a) Scanning of the interscalene groove. (b) SCM sternocleidomastoid muscle, ASM anterior scalene muscle, MSM middle scalene muscle, VA vertebral artery, arrows roots of brachial plexus
Interscalenic approach is performed with a high-frequency linear probe; the transducer position is transverse on neck, 3–4 cm superior to clavicle, over external jugular vein (Fig. 20.9).
Fig. 20.9
IP interscalene approach
A combined ultrasound-guided nerve stimulating technique may facilitate nerve localization. Using an in-plane approach and slight redirections to advance the needle close to the brachial plexus, local anesthetic spread around the nerve roots or trunks may be visualized. Precise needle placement may limit the dose of local anesthetic required.
The goal, in fact, is local anesthetic spread around superior and middle trunks of brachial plexus, between anterior and middle scalene muscle (Fig. 20.10). Ultrasound guidance allows multiple injections around the brachial plexus, therefore eliminating the reliance on a single large injection of local anesthetic for block success as is the case with non-ultrasound-guided techniques. Ability to inject multiple aliquots of local anesthetic also may allow for the reduction in the volume of local anesthetic required to accomplish the block.
Fig. 20.10
arrows roots of brachial plexus, yellow line needle, blue: local anesthetic, red line spread of local anesthetic
Due to potential adverse effects including pneumothorax, vertebral artery injection, and intrathecal injection, the intrascalene block is not common in pediatrics. Palpation of the interscalene grove often proves challenging in children under general anesthesia, and as a result, a recent report states that this block is not recommended for any heavily sedated or anesthetized patient. However, the improvements in nerve localization made possible due to ultrasound guidance have the potential to increase the use of this block in children [9–11].
20.3.1.2 Supraclavicular Approach
The supraclavicular block is indicated for all upper extremity surgery, but particularly if the shoulder is not involved; in general indications are arm, elbow, forearm, and hand surgery.
It is a popular technique for surgery below the shoulder because the onset and quality of anesthesia are fast and complete.
The proximity of the brachial plexus at this location to the chest cavity and pleura has been of concern to many practitioners; however, ultrasound guidance has resulted in a resurgence of interest in the supraclavicular approach to the brachial plexus. The ability to image the plexus, rib, pleura, and subclavian artery with ultrasound guidance has increased safety due to better monitoring of anatomy and needle placement [12].
The subclavian artery crosses over the first rib between the insertions of the anterior and middle scalene muscles, at approximately the midpoint of the clavicle. The pulsating subclavian artery is readily apparent, whereas the parietal pleura and the first rib can be seen as a linear hyperechoic structure immediately lateral and deep to it, respectively. The rib, as an osseous structure, casts an acoustic shadow, so that the image field deep to the rib appears anechoic or dark. A reverberation artifact often occurs, mimicking a second subclavian artery beneath the rib.
The trunks and divisions of the plexus appear as hypoechoic grapelike clusters laterally and cranially the artery, while a hyperechoic line with dorsal shadowing indicates the first rib (Fig. 20.11).
Fig. 20.11
FR first rib, SA subclavian artery, yellow line brachial plexus
With a high-frequency probe in the coronal oblique plane, the plexus divisions and/or roots are visible lateral to the subclavian artery (Fig. 20.12). Using an in-plane approach, directing the needle from lateral to medial avoids vascular structures in contact with the plexus.
Fig. 20.12
IP suprclavicular approach
The goal is local anesthetic spread around brachial plexus, lateral, and superficial to subclavian artery.
This approach lends itself to a continuous catheter technique because nerve structures are close proximity to another.
When compared to other brachial plexus blocks, there is an increased risk of pneumothorax due to the proximity of the lung parenchyma at the level of this block. By using an in-plane approach, ultrasound guidance may reduce this risk by providing clear visibility of the needle shaft and tip, making the supraclavicular approach one of the most reliable and effective blocks of the brachial plexus (Fig. 20.13).
Fig. 20.13
SA subclavian artery, FR first rib, Arrows needle, red line spread of local anesthetic
20.3.1.3 Infraclavicular Approach
The indications for the infraclavicular approach to the brachial plexus are arm, elbow, forearm, and hand surgery.
Identification of the arterial pulse on the sonographic image is an easy primary goal in establishing the landmark. The axillary artery and vein are located deep and medial to the cords, with the vein positioned medial and caudal to the artery.
The cords of the infraclavicular portion of the brachial plexus appear hyperechoic and lateral and/or below the subclavian artery; the pleura is hyperechoic and medial. Although all the cords surround the artery, they are not visualized with equal clarity. The lateral cord is most easily viewed and appears as a hyperechoic oval structure. In contrast, the posterior and medial cords may be difficult to visualize, in part because the view may be obstructed by the axillary vasculature; the medial cord lies between the artery and vein while the posterior cord is deep to the artery. The pectoralis major and minor muscles lie superficial to the neurovascular bundle and are separated by a hyperechoic lining (perimysium) (Figs. 20.14 and 20.15).
Fig. 20.14
AA axillary artery, AV axillary vein, LC lateral cord, PC posterior cord, MC medial cord
Fig. 20.15
Color Doppler of the infraclavicular area
The block is typically performed with the patient in supine position with the head turned away from the side to be blocked. The arm is abducted to 90° and the elbow flexed.
The probe is positioned immediately medial to the coracoid process of the scapula under the clavicle in a parasagittal plane so that the plexus can be scanned transversely (the marker on the probe is directed toward the patient’s head). The transducer is moved in the superior-inferior direction until the artery is identified in cross section. While the plexus lies quite deep in adults, the structure is much more superficial in children, making a higher frequency probe optimal (Fig. 20.16).
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