**2.1. A brief history**

The first published account of ultrasound use with peripheral nerve blockade occurred in 1978 when Doppler sonography assisted blood flow detection during supraclavicular brachial plexus block [5]. Although the initial technology did not allow for direct nerve visualization, this was later rectified in 1994, when advancements in technology allowed the first document‐ ed use of ultrasound to visually facilitate supraclavicular brachial plexus block [5]. Since this time, ultrasound use for regional anesthesia has shown increasing popularity, and ultrasound

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technology has mirrored practitioner demand with machines possessing greater portability, simplicity, and image resolution [5]. Literature regarding the utility of ultrasound for a variety of peripheral nerve blocks continues to emerge.

appropriate local anesthetic spread, as well as an understanding of novel probe operating mechanics and regular needle tip visualization [7], [17], [18]. As a result, images may appear ambiguous to the novice operator [19], and identifying the intricate neurovascular anatomy of a common PNB structure as the brachial plexus may prove formidable [20]. Inexperience leading to inability to recognize common on-screen artifacts stemming from image processing may also skew interpretation [21]. In contrast to a definitive motor response end-point elicited with nerve stimulator, the optimal pattern of local anesthetic deposition and distribution

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Ultrasonography may also prove challenging as a result of current technological limitations. For example, discriminating neuronal tissue and its epineurium from that of connective tissue or tendons may prove difficult due to the similar hyperechoicity, or echotexture [7], [20]. Furthermore, ultrasound imaging has been shown to underrepresent the total number of neuronal fascicles as compared to light microscopy, and the possibility of intraneural injection

Upper extremity peripheral nerve blocks account for the majority of performed regional anesthesia techniques in most anesthesia practices [24]. Of the upper extremity PNBs, the interscalene block (ISB) is the most commonly applied block for patients undergoing shoulder surgery [25], [26], [8], imparting both anesthesia and analgesia with adequate coverage of the shoulder, lateral arm, and lateral forearm [27]. The ISB was first described in 1970 by Winnie, who noted based on anatomic and radiographic imaging that the interscalene space allowed for a novel, percutaneous approach to anesthetizing the proximal brachial plexus [28]. This approach allowed for brachial plexus anesthesia of similar quality to that of thoracic epidural anesthesia [28]. Compared to the previously described axillary and subclavian approaches prior to this time, the ISB was quickly favored for its ease of execution due to readily palpable landmarks in patients with large body habitus, no requirement for unique upper extremity positioning, and ability to readily repeat the block during protracted surgical procedures [28]. Both single-shot and continuous catheter placement have been successfully performed with

ISB via landmark-paresthesia, nerve stimulator, or ultrasound-guided technique [8].

With the exception of the supraclavicular nerves, the brachial plexus is responsible for all motor and sensory innervation to the shoulder area [8]. The brachial plexus is an intricate neuronal network originating as ventral rami from cervical nerve roots, C5-8, and initial thoracic nerve root, T1 [24]. Together, these roots within the neck further subdivide into trunks, divisions, cords, and, ultimately, peripheral branches traveling distally into the upper arm [29]. After exiting the vertebral column, the roots become trunks as they traverse through the apposition of the anterior and middle scalene muscles, or interscalene groove [24]. Beyond the distal first

(a topic of controversy with respect to morbidity) exists [23], [20].

**3. The interscalene brachial plexus block**

continues to be investigated [22], [18].

**3.1. Block description**

**3.2. Anatomy**

#### **2.2. Advantages**

The rising popularity of ultrasound guidance for peripheral nerve blockade (PNB) stems from numerous described advantages supporting its use [6], [7], [5]. Perhaps the principal benefit of ultrasound resides in the technology's inherent ability to directly visualize peripheral nerves and tissue planes in real-time, allowing for optimal injectate or catheter placement with the ultimate goal of optimizing neural blockade [7]. Today's ultrasound machines are equipped with high-frequency probes capable of imaging the majority of nerves necessary for a wide array of regional blocks, and also their oblique course as they traverse the body [7]. This imaging modality permits the identification of relatively diminutive 2 mm diameter digital nerves [7], as well as differentiation of complex neurovascular nuances as found within the brachial plexus [8]. Additional benefit is conveyed in the ability to reposition one's needle in assessing for adequate local anesthetic spread, fascial plane movement, or lack thereof with intravascular injection [7]. The idea of preemptively scanning patient anatomy for neurovas‐ cular variations or abnormalities has been suggested as a means of improving patient safety by preventing block complication [9].

A number of objective evaluations have supported the efficacy of ultrasound guidance during PNB. When compared with performance via peripheral nerve stimulation (PNS), PNB executed using ultrasound guidance has been shown to require less time to perform, possesses more rapid onset and longer duration of anesthesia, and is more likely to be successful (less block failure) [6]. The use of ultrasound rather than PNS has also been shown to decrease the risk of vascular puncture [6], [10], and demonstrate improved quality of sensory block [11]. The use of ultrasonography does not exclude the use of PNS for PNB, and the combination for brachial plexus block was shown to have decreased risk of central nervous system toxicity secondary to local anesthetic versus a PNS-landmark technique [12]. Another study demon‐ strated high rates of success with axillary brachial plexus block using sonography regardless of concurrent PNS use [13]. Compared with PNS for femoral nerve block, ultrasound guidance also provides a reduction in the minimum effective anesthetic volume (MEAV50) [14], and has allowed reduced dosing for many blocks, with a potential impact on local anesthetic systemic toxicity and therefore patient safety [15]. Lastly, given the steady rise in yearly surgical procedures [1], findings such as decreased time to perform PNB [6], [7] and recent demon‐ stration of cost-effectiveness in clinical practice [5] will likely support the role of ultrasound guidance in regional anesthesia's future.

#### **2.3. Disadvantages**

Despite many reported advantages to ultrasound guidance during PNB, several barriers to implementation and training have been described. One such limitation arises from peripheral nerve anatomical variation leading to difficulty in regional pattern recognition [16]. Difficulty to trainees may arise from the necessary knowledge of cross-sectional anatomy, terminology, appropriate local anesthetic spread, as well as an understanding of novel probe operating mechanics and regular needle tip visualization [7], [17], [18]. As a result, images may appear ambiguous to the novice operator [19], and identifying the intricate neurovascular anatomy of a common PNB structure as the brachial plexus may prove formidable [20]. Inexperience leading to inability to recognize common on-screen artifacts stemming from image processing may also skew interpretation [21]. In contrast to a definitive motor response end-point elicited with nerve stimulator, the optimal pattern of local anesthetic deposition and distribution continues to be investigated [22], [18].

Ultrasonography may also prove challenging as a result of current technological limitations. For example, discriminating neuronal tissue and its epineurium from that of connective tissue or tendons may prove difficult due to the similar hyperechoicity, or echotexture [7], [20]. Furthermore, ultrasound imaging has been shown to underrepresent the total number of neuronal fascicles as compared to light microscopy, and the possibility of intraneural injection (a topic of controversy with respect to morbidity) exists [23], [20].
