Techniques for peripheral nerve blocks have evolved with the potential for improved safety and efficacy. For axillary blocks, the progression has been from paresthesia and trans-arterial techniques to electro-stimulation and ultrasound guidance. Debate continues over the recommended number of injections required for successful performance of axillary blocks. Central to this debate is the question of whether there are anatomic impediments to local anesthetic spread within the axillary sheath. Winnie et al.1 proposed that the axillary sheath was essentially a tubular structure formed by the prevertebral fascia, and thus a single injection into the sheath would successfully bathe all the terminal branches of the brachial plexus. Contrarily, Thompson and Rorie2 examined the brachial plexus on cadavers by using anatomic dissection and computed tomography dye studies on surgical patients. With their findings, they proposed the existence of septae within the sheath, which they felt precluded local anesthetic spread and thus required a multiple injection technique for successful brachial plexus anesthesia. Franco et al.3 demonstrated a sheath around the neurovascular bundle of the brachial plexus in cadavers. The sheath had a fibrous external appearance and was filled with loose connective tissue (Fig. 1). Other authors using magnetic resonance imaging have observed septae but felt that, while these thin layers of connective tissue might delay spread of local anesthetics, they did not prevent ultimate spread throughout the sheath after single injection.4 Magnetic resonance imaging studies by Klaastad et al.5 demonstrated incomplete cross-sectional spread of local anesthesia and incomplete axillary block with a single catheter injection.
All previous imaging techniques used to define the presence of connective tissue impediments to local anesthetic spread used delayed images.3,4 As the use of ultrasound-guided regional anesthesia has increased in popularity, real-time observation of the spread of local anesthetic is now possible.6 Three-dimensional (3-D) ultrasound may offer further improvements in imaging and reveal possible connective tissue impediments to flow in more detail.
We present a case of a patient receiving an axillary brachial plexus block guided by 3D ultrasound (Philips iU22 Live 3D ultrasound System; Andover, MA), which revealed initial non-contiguous spread of local anesthetic due to distinct linear hyperechoic connective tissue structures.
A 52-yr-old man with a medical history significant for diabetes and well-controlled hypertension presented for elective repair of Dupuytren’s contracture and carpometacarpal arthroplasty of his right hand. The planned anesthetic was general anesthesia with a laryngeal mask airway and postoperative pain control with an axillary block.
With the patient in the supine position, the right arm was abducted approximately 90° and the elbow flexed. The hand was resting on a pillow lateral to the patient’s head. The axillary artery was palpated and marked with a skin marker. After sterile skin preparation with chlorhexidine, an ultrasound probe X-7-2 Matrix (Philips; Andover, MA) was draped with a sterile sheath and placed over the brachial plexus in the axilla. After obtaining a cross-sectional view of the axillary artery and brachial plexus and placement of a lidocaine skin wheel, we guided a 22-gauge, 30° bevel, 40-mm insulated needle (Stimuplex; B. Braun) under direct visualization superior to the artery in a longitudinal approach until a median nerve motor response was demonstrated in the hand at 0.45 mA at 2 Hz. Twenty milliliter of 0.5% ropivacaine was then administered by incremental dosing (Fig. 2). Local anesthetic was seen on ultrasound as a hypoechoic shadow increasing in size throughout injection and surrounding the median nerve as well as the needle (Fig. 3). After the injection, the needle was redirected inferior and deep to the axillary artery to the radial nerve until an extensor response was seen at 0.4 mA. An additional 20 mL of local anesthetic was injected and ultrasound imaging revealed that the radial, ulnar and medial nerves were all surrounded with local anesthetic. After completion of the injection, the image was rotated to a cross-sectional view where we visualized the expansion and contraction of the hypoechoic spaces between the hyperechoic connective tissue with every arterial pulsation (Fig. 4). An additional 5 mL of local anesthesia was injected around the musculocutaneous nerve. Thirty minutes after the injection, complete sensory and motor block of the right arm was confirmed.
Real-time 3D ultrasound imaging offers additional insight into the functional anatomy of the brachial plexus sheath. In agreement with some recent studies, the local anesthetic spread appears to be impeded by distinct soft tissue barriers between the nerves of the brachial plexus. After completing the injections all nerves were surrounded by local anesthetic resulting in a complete motor/sensory block.
After completion of the injection, we were able to visualize the influence of the arterial pulsation on the spread of local anesthetics (Video 1; please see video clip available at www.anesthesia-analgesia.org). The local anesthetic solution is in a dynamic environment influenced by the arterial pulsation. Indeed, if there were discrete compartments between the nerves of the brachial plexus, one would expect that the multiple injection techniques would have emerged to be clearly significant to the single injection technique, which remains controversial in the literature. Our real-time 3D ultrasound-guided experience of 10 axillary blocks, including 5 with catheters, has consistently demonstrated the appearance of thin tissue barriers between the nerves. Injection of 30 mL of local anesthetic through the axillary catheters demonstrated communication channels among the compartments. Further research with real-time continuous observation of local anesthetic spread after single injection may provide further insight into the dynamics of local anesthetic spread in the brachial plexus sheath as well as other nerve blocks.
The Matrix X-7-2 probe was designed for 3D ultrasound of the pediatric heart. Current limitations of 3D ultrasound technology include an upper frequency limit of 7 MHz which decreases the resolution of superficial scanning. Three-dimensional acquisition is based on matrix array transducer with more than 2500 piezoelectric elements. All of the elements of the matrix array transducer transmit and receive data and, with signal processing within the transducer head, funnels this information into 128 channels of a standard ultrasound system. The differences between 2D and 3D ultrasound are similar to the differences between a plain radiograph and computer tomography. Unlike 2D ultrasound that captures a planar image, 3D ultrasound technology captures a 3D volume that enables multiple planes of view by manipulating the image without movement of the ultrasound probe. Real-time acquisition with the Matrix X-7-2 ultrasound probe has a volume scan of 30° to 50° (Fig. 5). The X-7-2 probe and iU22 live 3D ultrasound technology with a touch of a button changes the standard 2D ultrasound view to the less familiar real-time 3D image, allowing the practitioner to compare sonoanatomy. The hyperechoic cord structures seen in the 3D ultrasound in the long-axis are peripheral nerves, confirmed by nerve stimulation and appropriate motor response. In the 3D short-axis, brachial plexus nerves appear as bright hyperechoic structures as compared to 2D ultrasound image (Fig. 6).
Further development of 3D ultrasound probes with higher frequency capabilities is needed to further enhance image quality and linear probe designs which would aid in needle tracking. With the X-7-2 probe held stationary, the large 3D volume of ultrasound data can be manipulated and real-time and observation of the needle can be seen in multiple planes and may enhance localization of the needle in relationship to the nerves (Video 2; please see video clip available at www.anesthesia-analgesia.org).
In summary, we present 3D ultrasound images of local anesthetic spread within the brachial plexus sheath. We anticipate that further research with advanced ultrasound technology will help unravel some of the mysteries of the dynamics of local anesthetic spread among the nerves of various body plexii.
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