Nội dung

11 Ultrasound Imaging for Pediatric Anesthesia: A Review Santhanam Suresh MD, Lauren J. Taylor BA and Amar S. Minhas Department of Pediatric Anesthesiology, Children’s Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, IL USA 1. Introduction There has been a recent increase in the use of regional anesthesia in pediatric patients (Lacroix, 2008). 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. This chapter will review a variety of common peripheral nerve and central neuraxial blocks that can be performed using ultrasound guidance in children. This aspect of regional anesthesia has the potential for significant contributions to evidence-based medicine. Because ultrasound guidance is a relative newcomer 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. This chapter is based on an extensive review of the literature using MEDLINE and EMBASE from 1980 to May 28, 2009. 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. 2. Peripheral nerve blocks 2.1 Upper extremity blocks Peripheral regional anesthesia is of great utility in children undergoing surgery on the upper extremities. Although many approaches to the brachial plexus have been described, the axillary block using conventional methods is still the most commonly performed and reported brachial plexus blockade in children. 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). Although there is a paucity of related literature, the 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. www.intechopen.com 190 Ultrasound Imaging 2.1.1 Axillary block 2.1.1.1 Sonoanatomy The axillary approach blocks the radial, median and ulnar nerves. A probe placed perpendicular to the anterior axillary fold provides a short-axis view of the neurovascular bundle with the biceps brachii and coracobrachialis muscles lateral and the triceps brachii medial and deep to the biceps. The axillary artery is a useful ultrasound landmark and appears as a circular anechoic structure adjacent to the biceps and coracobrachialis muscles. Terminal nerves surround the artery with the radial nerve located deep and along the midline of the artery. The median and ulnar nerves are both superficial to the artery, with median nerve visible between the artery and biceps muscle while the ulnar nerve is found more medial (Figure 1). It is important to be aware of anatomic variation, particularly in the location of the median nerve. The musculocutaneous nerve is located between the coracobrachialis and biceps muscles, but it is difficult to detect in children (Rapp & Grau, 2004). In contrast to the textbook anatomical description, in an ultrasound image the radial, median, and ulnar nerves often appear the same size as the axillary artery (Rapp & Grau, 2004). Fig. 1. Sonoanatomy of the axillary block location using a linear hockey stick probe (SLA 613 MHz, 25-mm footprint) placed transversely to the humerus. 2.1.1.2 Technique An axillary block may be used for procedures on the elbow, forearm, or hand. The use of ultrasound guidance for this block in children, however, is not well described in the literature; a recent search produced no original reports (Fleischmann et al., 2003). This block may be performed in children using techniques borrowed from adult patients. With a hockey stick or linear small footprint probe placed proximally in the axilla and transverse to the humerus, an in-plane technique may be used. It is best to use multiple injections with needle redirections to ensure that the local anesthetic surrounds all the terminal nerves of the plexus. Due to the close proximity of the axillary vein and artery, multiple punctures may be necessary to avoid intravascular injection. www.intechopen.com Ultrasound Imaging for Pediatric Anesthesia: A Review 191 2.1.1.3 Outcome and discussion In children, the axillary block is one of the most popular approaches to the brachial plexus (Cregg et al., 1996). Due to the abundance of vessels in the axillary region, caution must be taken when performing this block to avoid intravascular injection. This is particularly important when multiple punctures are required to achieve the desired circumferential local anesthetic spread. The use of ultrasound for real-time visualization may reduce this risk. Color Doppler is also a helpful tool to identify the axillary vasculature. Due to the superficial location of the plexus, ultrasound guidance may help the anesthesiologist guide the needle advancement (Tsui & Suresh, 2010a). 2.1.2 Interscalene block 2.1.2.1 Sonoanatomy Ultrasound imaging allows visualization of the C5, C6, and C7 nerve roots between the anterior and middle scalene muscles (Figure 2). In a transverse oblique plane at the level of the cricoid cartilage and at the posteriolateral aspect of the sternocleidomastoid muscle, the muscle appears as a triangular shaped structure overlying the internal jugular vein and common carotid artery. 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. Fig. 2. Sonoanatomy at the interscalene groove using a linear hockey stick probe (SLA, 6-13 MHz, 25-mm footprint). 2.1.2.2 Technique For optimal imaging, the probe is placed in a transverse oblique plane at the level of the cricoid cartilage. 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 www.intechopen.com 192 Ultrasound Imaging visualized. Precise needle placement may limit the dose of local anesthetic required (Tsui & Suresh, 2010a). This combined ultrasound guided-nerve stimulation technique has also been described in the literature using an out-of plane approach and displacement of the middle scalene muscle tissue as a means of tracking the needle tip (Fredrickson, 2007). 2.1.2.3 Outcome and discussion An interscalene approach is a means of providing analgesia for patients undergoing surgery of the shoulder or proximal arm that can last 6-12 hours. This block may also be used for catheter placement for continuous analgesia (Marhofer et al, 2005). While it may be advantageous to use nerve stimulation in conjunction with ultrasound to confirm the identity of the localized nerve root, a report documenting the performance of an interscalene block in a 7-year-old child with femur fibula ulna syndrome suggests that ultrasound guidance may be of special value for patients in whom the use of nerve stimulation is impossible (Van Geffen et al., 2006). 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 (Bernards et al., 2008). However, the improvements in nerve localization made possible due to ultrasound guidance have the potential to increase the use of this block in children. 2.1.3 Supraclavicular Block 2.1.3.1 Sonoanatomy At the level of a supraclavicular approach, the trunks and divisions of the brachial plexus are superiolateral to the subclavian artery and above the first rib. This relationship is most clearly visualized with the probe in a coronal oblique plane. The subclavian artery may be used as a primary ultrasound landmark and appears as an anechoic, hypodense, pulsatile structure. Color Doppler may be useful to confirm vascular identification. The trunks and divisions of the plexus appear as three hypoechoic grape-like clusters around the artery while a hyperechoic line with dorsal shadowing indicates the first rib (Figure 3). 2.1.3.2 Technique Literature describing the supraclavicular technique in children is limited (Suresh et al., 2009). A recent report describes an approach similar to that used in adult patients (De Jose Maria et al, 2008). With a high frequency probe in the coronal oblique plane, the plexus divisions and/ or roots are visible lateral to the subclavian artery. Using an in-plane approach, directing the needle from lateral to medial avoids vascular structures in contact with the plexus. An out-of plane approach may also be successful. 2.1.3.3 Outcome and discussion The supraclavicular block may be performed for patients undergoing most procedures of the upper extremities. However, 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 (Tsui & Suresh, 2010a). www.intechopen.com Ultrasound Imaging for Pediatric Anesthesia: A Review 193 Fig. 3. Sonoanatomy at the supraclavicular block location using a linear hockey stick probe (SLA, 6-13 MHz, 25-mm footprint) placed in the coronal oblique plane. 2.1.4 Infraclavicular block 2.1.4.1 Sonoanatomy In the infraclavicular approach to the brachial plexus, the cords are located inferior and medial to the coracoid process. Both the axillary artery and vein serve as useful ultrasound landmarks for the infraclavicular approach. These vessels are located deep and medial to the cords, with the vein positioned medial and caudal to the artery (Figure 4). Although all the cords surround the artery, they are not visualized with equal clarity. The lateral cord is most Fig. 4. Sonoanatomy at the infraclavicular block location using a linear hockey stick probe (SLA, 6-13 MHz, 25-mm footprint) placed below the clavicle in a parasagittal plane. www.intechopen.com 194 Ultrasound Imaging 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). 2.1.4.2 Technique While the plexus lies quite deep in adults, the structure is much more superficial in children, making a higher frequency probe optimal. Marhofer et al. described a lateral approach to this block in which the probe was placed transversely below the clavicle with the patient in the supine position (Marhofer et al, 2004). The authors achieved sufficient visualization of the plexus in all 40 patients studied. With the needle inserted using an out-of-plane approach 1 cm inferior to the probe, the local anesthetic dispersion was also clearly visualized. A recent report by De Jose Maria et al described a different technique in which the probe was placed either parallel or in a slightly parasagittal plane to the clavicle (De Jose Maria et al., 2008). The authors recommended using a medium frequency probe to maximize visibility of deeper structures in older children. The needle was inserted cephalad to the probe and redirected when necessary to ensure optimal spread of the local anesthetic around the cords. 2.1.4.3 Outcome and discussion The infraclavicular block is commonly used for patients undergoing surgery of the upper arm and elbow. It may also be a useful approach for placing a catheter in the posterior cord during more extensive procedures. The risks of this block are similar to the supraclavicular approach, with the danger of pneumothorax being most serious. Just as with the supraclavicular block, an in-plane needle insertion under ultrasound guidance may minimize the risk by allowing clear visualization of the needle tip and shaft (Chin et al., 2008). In addition, due to the closer proximity of the cervical pleura to the plexus cords medially, a lateral puncture site is recommended (Greher et al., 2002). Ultrasound imaging may also be advantageous in avoiding multiple puncture sites and visualizing underdeveloped structures like the coracoid process that may be difficult to palpate in children using “ blind” techniques (Marhofer et al., 2004; de Jose Maria & Tielens, 2004). The first controlled study by Marhofer et al in 2004 compared ultrasound guidance to traditional nerve stimulation technique for performance of the infraclavicular block in children. The authors reported ultrasound guidance to be associated with superior visual analog scores during block puncture, better sensory onset times (mean times of onset were 9 vs. 15 mins), longer duration of sensory block, and better sensory and motor block scores 10 mins after the block was performed. The authors explained the faster onset and longer duration of the blocks by the more accurate deposition of local anesthetic possible under ultrasound guidance (Marhofer et al, 2004). Although this study was a prospective, randomized controlled blinded study, the authors did not comment on the power necessary to determine statistical significance. 2.2 Lower extremity blocks Multiple regional anesthesia techniques may be used for children undergoing surgery of the lower extremities, including lumbar plexus, femoral, and sciatic nerve blockades. These www.intechopen.com Ultrasound Imaging for Pediatric Anesthesia: A Review 195 procedures can be performed successfully with the aid of ultrasound, which allows clearer localization of structures as well as visual confirmation of the local anesthetic spread. 2.2.1 Lumbar plexus block 2.2.1.1 Sonoanatomy Lumbar plexus blocks provide analgesia to the major branches of the lumbar plexus, including the femoral, genitofemoral, lateral femoral cutaneous, and obturator nerves and are therefore indicated for surgical procedures involving the hip, knee, or foot. In infants, a transverse (axial) view using a linear probe reveals several bony structures including spinous processes, laminae, and transverse processes, as well as musculature such as the erector spinae and quadratus lumborum. The plexus is located deep to these structures and is embedded in the psoas muscle (Figure 5). In a transverse scan, the spinous processes appear hypoechoic (likely due to dorsal shadowing effect) and extend superficially, whereas the transverse processes are hyperechoic masses or lines at the lateral edge of each vertebra. In the longitudinal scan, the lateral edges of the transverse processes can be identified at the most lateral point near multiple hyperechoic nodules (Tsui & Suresh, 2010a). In this view the plexus lies between and just deep to the lateral tips of the processes. Fig. 5. Sonoanatomy at the lumbar plexus block location using a linear probe (HFL38, 6-13 MHz, 38-mm footprint) placed transversely at L4/ 5. 2.2.1.2 Technique With the patient in the lateral decubitus position, ultrasound guidance may be used to identify the transverse processes of L4 or L5 and visualize the psoas major muscle and the plexus in relation to the paravertebral musculature. To locate the bony landmarks, it is important to alternate between transverse and longitudinal planes to survey the region of between the processes. Because of the depth and the similar echogenicity of the plexus to psoas major, however, ultrasound visualization of the nerves may not be possible in older children. In these patients, instead of real-time guidance the block is usually ultrasound www.intechopen.com 196 Ultrasound Imaging aided, meaning that location and depth of the transverse processes are determined using ultrasound but the actual needle placement is not imaged. In younger children the plexus is easier to identify because the muscle tissue appears relatively more hypoechoic than the nerve structures, which contain hyperechoic connective tissue layers (Kirchmair et al., 2004). It is best to insert the needle in a cephalad or caudad direction rather than medially towards the spinal canal. 2.2.1.3 Outcome and discussion Visualization of the lumbar paravertebral region using sonographic imaging has been described in the literature, with the authors reporting successful intra and postoperative analgesia (Johr, 2005). Although ultrasound may enable visualization of the transverse processes or the plexus itself, use of a nerve stimulator, maintaining a needle puncture relatively lateral and cranial (Schuepfer & Johr, 2005), and performing a regular aspiration during injection are recommended to increase safety of the block. Without clear tracking of the needle tip and it may be difficult to achieve an adequate view of the local anesthetic spread, especially if the needle is inserted in a relatively medial and caudal location (Tsui & Suresh, 2010a). Therefore, a risk-benefit analysis is necessary before performing this block. There is a lack of data in the pediatric literature describing the use of ultrasound imaging for other blocks of the nerves of the lumbar plexus, notably the fascia iliaca block, 3-in-1 block, or individual blocks of the lateral femoral cutaneous or obturator nerves. However, these blocks could all benefit from the advantages offered by ultrasound guidance. In particular, visualization of local anesthetic spread will likely reduce reliance on injecting larger than necessary volumes of local anesthetic in an attempt to block all of the related nerves (Simion & Suresh, 2007). 2.2.2 Femoral nerve block 2.2.2.1 Sonoanatomy The femoral nerve originates from nerve roots L2, L3, and L4 and when blocked provides surgical anesthesia and analgesia for the anterior thigh extending to the knee. The femoral artery is easily visualized and serves as the principle ultrasound landmark (Figure 6). With the probe placed perpendicular to the nerve axis (i.e., coronal oblique) at the level of and parallel to the inguinal crease, the nerve appears lateral to the large, circular, and anechoic femoral artery. Color Doppler may be helpful to identify the femoral vasculature. The nerve typically assumes a triangular shape but may be of variable size. The fascia lata (most superficial) and iliaca (separating the nerve from the artery) are visible superficial to the femoral nerve and most often appear as bright and longitudinally angled echogenic signals (Tsui & Suresh, 2010a). 2.2.2.2 Technique A femoral nerve block may be used for surgical procedures on the anterior thigh and knee, including anterior thigh biopsy and knee arthroscopy. With the patient supine, the femoral artery, vein, and nerve can all be visualized with a linear probe placed along the inguinalfemoral crease. When inserted using an in-plane approach, the needle tip can be visualized as it enters the fascia iliaca. It is important that the needle be placed inside the fascia iliaca compartment. In a study comparing ultrasound guidance to nerve stimulator technique, the nerve was visible in all children studied when the probe was placed parallel and inferior to the inguinal ligament and lateral to the femoral artery. Ultrasound was also used effectively to visualize the needle tip and facilitate needle redirections (Oberndorger et al., 2007). www.intechopen.com Ultrasound Imaging for Pediatric Anesthesia: A Review 197 Fig. 6. Sonoanatomy at the femoral nerve block location using a linear hockey stick probe (SLA, 6-36 MHz, 25-mm footprint) placed parallel and inferior to the inguinal ligament. 2.2.2.3 Outcome and discussion The proximity of the femoral nerve to major vasculature increases the potential for inadvertent vessel puncture and hematoma formation. Although there is no direct evidence to prove that ultrasound could reduce this risk, it is the authors’ experience that fewer adverse events occurred when this block was performed under ultrasound guidance. In addition, precise ultrasound imaging of the local anesthetic spread may reduce the need for larger volumes used when injecting blindly. 2.2.3 Sciatic nerve block 2.2.3.1 Sonoanatomy The sciatic nerve is formed by nerve roots L4 to S3 and innervates the posterior thigh and leg below the knee, with the exception of the medial portion. As it leaves the pelvis through the greater sciatic foramen, the sciatic nerve can be found inferior to the gluteus maximus muscle. The nerve lies between the ischial tuberosity and the greater trochanter, however typically only the greater trochanter can be seen when using a small foot print linear probe (Figure 7). Curved probes (curvilinear) with moderate-low frequency (e.g.2-5MHz) allow deeper ultrasound beam penetration and are often necessary in older children. Although the greater trochanter and ischial tuberosity serve as the principle bony landmarks for the subgluteal approach, they must be palpated with great care as these structures do not reach adult morphology until puberty (Scheuer et al., 2000). Thus, since the size of the greater trochanter is dependent on the age of the child, it is only highly recognizable at 6 to 8 years of age (Scheuer, 2000). While the medial aspect of the greater trochanter appears largely hypoechoic, the sciatic nerve is predominantly hyperechoic and is typically elliptical in a short-axis view. The pathway of the sciatic nerve continues through the posterior popliteal fossa before bifurcating to form the common peroneal and tibial nerves. A linear probe transversely oriented in the popliteal crease captures both the tibial and common peroneal nerves located www.intechopen.com 198 Ultrasound Imaging Fig. 7. Sonoanatomy at the subgluteal location using a linear probe (HFL38, 6-13 MHz, 38mm footprint). Image shows where the sciatic nerve lies at the midpoint between the ischial tuberosity and the greater trochanter of the femur. medially and laterally, respectively, to the popliteal vessels (Figure 8). This vasculature, particularly the popliteal artery, serves as useful ultrasound landmarks (color Doppler may be useful). At the popliteal crease, the tibial nerve is found more superficial and most adjacent to the artery. However, as the probe is moved cephalad, the artery becomes deeper and more distant as the tibial nerve moves laterally to join the common peroneal nerve. At and cephalad to the bifurcation, the sciatic nerve appears as a large round to flat-oval hyperechoic structure. The biceps femoris muscle lies superficial to the bifurcating nerves and appears as a large oval shaped structure with less internal punctuate areas (hyperechoic spots) than the nerves. 2.2.3.2 Technique A sciatic block is commonly used for surgical procedures of the foot and ankle, as well as in combination with a femoral nerve block for patients undergoing knee surgery (De Jose Maria et al., 2008). In children, subgluteal and popliteal approaches are most commonly employed. For the subgluteal approach, the child should be in the Sim’s position with the hip and knee flexed (van Geffen & Gielen, 2006). To visualize the sciatic nerve in cross section, the probe should be oriented transversely at the mid point of the ischial tuberosity and the greater trochanter. The nerve can be seen deep to and at the junction where the gluteus maximus muscle meets the biceps femoris-semitendinosus muscle complex. Both inand out-of-plane approaches have been described with success. For a more distal block of the sciatic nerve at the popliteal fossa, it is best to place the patient in the prone position with the probe above the popliteal crease. The probe is moved cephalad until the nerves merge together. Using an in-plane approach, the needle is placed in close proximity to the sciatic nerve and the local anesthetic spread surrounds the nerve. To achieve an ideal image, a probe appropriate to the size of the child must be used. Deeper structures in older children require lower frequency probes for sufficient visualization. Such a low frequency probe might be necessary for the subgluteal approach as there is www.intechopen.com