Blanco and colleagues ^1–3 have recently described novel ultrasound-guided thoracic interfascial nerve blocks, the pectoral nerve block (PECS) ^1,2 and serratus plane block (SPB), ³ for anesthesia and/or analgesia of the anterior/anterolateral chest wall. ^1–4 The SPB may also anesthetize the axilla via blockade of the intercostobrachial nerve. ³ These blocks were originally developed for breast surgery in an attempt to avoid some of the rare but serious complications of thoracic paravertebral and neuraxial blocks. During a PECS-I block, the local anesthetic (0.4 mL/kg or approximately 20–30 ml) ¹ is injected as a single injection into the myofascial plane between the pectoralis major and minor muscle, aiming to block the medial and lateral pectoral nerves. ¹ PECS-II block is a modification of the PECS-I block (modified PECS-I block) and involves two injections. ² The first injection is the same as that for a PECS-I block (but with 10 mL of local anesthetic), ² but the second injection is performed deep to the pectoralis minor muscle, at the level of the third and fourth rib, into the interfascial plane between the pectoralis minor and serratus anterior muscle (with 20 mL of local anesthetic). ² The aim of the PECS-II block is to anesthetize the pectoral nerves, intercostobrachial nerve, third to sixth intercostal nerves, and the long thoracic nerve. ^2,4 The PECS-II block is therefore used for more extensive breast surgery, including mastectomy with or without axillary clearance. ² The SPB ³ is a more recent addition to the family of thoracic interfascial nerve blocks and involves a single injection of 0.4 mL/kg of local anesthetic into the myofascial plane between the latissimus dorsi and the serratus anterior muscle more posteriorly and at the level of the fifth rib. ³ Local anesthetic spreads in the serratus plane, deep to the latissimus dorsi, and along the lateral chest wall to affect the lateral cutaneous branches of the second to ninth intercostal nerves and possibly the long thoracic and thoracodorsal nerves. ^3,4 A clear understanding of the sonoanatomy of the thoracic wall is a prerequisite to effectively using a PECS or SPB. The following section describes the gross anatomy, ultrasound scan technique, and sonoanatomy of the thoracic wall relevant for the thoracic interfascial nerve blocks. Because these blocks are frequently used for breast surgery, a brief description of the innervation of the breast is also included.

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   Muscles: Muscles involved with thoracic interfascial nerve blocks are pectoralis major, pectoralis minor, serratus anterior, intercostal muscles, and the latissimus dorsi.

   
   
   
   
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      Pectoralis major: The pectoralis major muscle is a triangular, fan-shaped muscle that makes up the bulk of the anterior chest wall (Figs. 10–1 and 10–2). It has two parts: the clavicular head and the sternocostal head (Fig. 10–1). The clavicular head originates from the medial half of the clavicle, and the sternocostal head arises from the anterior surface of the lateral margin of the sternum, the first seven costal cartilages, and aponeurosis of the external oblique muscle. Muscle fibers from the two heads converge laterally to form a flat tendon that is inserted into the lateral lip of the bicipital groove (intertubercular sulcus) of the humerus. It also forms the anterior fold of the axilla. The pectoralis major muscle receives its innervation from the lateral and medial pectoral nerves of the brachial plexus. The clavicular head is innervated by the lateral pectoral nerve, and the sternocostal head is innervated by both the lateral and medial pectoral nerve. It is involved with flexion, adduction, and medial rotation of the humerus; depression of the arm and shoulder; and elevation of the ribs.

      
      
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      Pectoralis minor: The pectoralis minor muscle is a thin, triangular-shaped muscle located deep to the pectoralis major muscle (Figs. 10–3 to 10–5). It is significantly smaller in size than the pectoralis major muscle and originates from the outer surface of the third to fifth ribs (Fig. 10–4). The muscle fibers converge superolaterally to form a flat tendon that is attached to the coracoid process of the scapula (Fig. 10–4). It also forms part of the anterior wall of the axilla. The pectoralis minor also receives its innervation from the lateral and medial pectoral nerves of the brachial plexus. It is involved with depression of the elevated shoulder, and along with the serratus anterior muscles, pulls the scapula forward.

      
      
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      Serratus anterior: The serratus anterior muscle covers most of the lateral thoracic wall (Fig. 10–2) and originates as 9 to 10 muscular slips from the external surface of the first to eighth or ninth ribs (Fig. 10–2). Because two slips originate from the second rib, the number of slips is usually greater than the number of ribs from which they arise. The muscle fibers converge posteriorly to be inserted into the medial border of the scapula. It contributes to forming the medial wall of the axilla. It is also called the “boxer’s muscle” because it causes protraction of the scapula around the rib cage—a movement that occurs when throwing a punch. It is also involved with upward rotation of the scapula that occurs while lifting a load overhead. The serratus anterior muscle is innervated by the long thoracic nerve, which travels caudally on the outer surface of the muscle. Injury to the long thoracic nerve can lead to a “winged scapula.”

      
      
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      Latissimus dorsi: The latissimus dorsi muscle is a large, flat muscle located on the dorsum of the trunk. It originates from the spinous processes of the last six thoracic vertebra (T7–T12), the thoracolumbar fascia, and the posterior third of the external lip of the iliac crest. The muscle fibers converge cranially to form a flattened tendon that is inserted into the floor of the bicipital (intertubercular) groove anterior to the attachment of the teres major muscle. It is involved with adduction, extension, and internal rotation of the arm at the shoulder and innervated by the thoracodorsal nerve. The thoracodorsal artery descends inferiorly with the thoracodorsal nerve and supplies the latissimus dorsi muscle.

      
      
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      Teres major: The teres major muscle is a rounded muscle that is attached between the scapula and humerus. It originates from the posterior surface of the inferior angle and lower part of the lateral border of the scapula. The fibers converge laterally to a flat tendon that is inserted into the medial lip of the bicipital groove. The teres major is located superior to the latissimus dorsi, and the muscle fibers run parallel to each other to its insertion in the humerus. It is innervated by the lower subscapular and thoracodorsal nerves, which are branches of the posterior cord of the brachial plexus, and receives spinal contributions from the C5 to C8 spinal nerves. It is involved with extension and medial rotation of the humerus.

   
   
   
   
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   Nerves: The nerves involved with thoracic interfascial nerve blocks are intercostal nerves, pectoral nerves, long thoracic nerve, and thoracodorsal nerve.

   
   
   
   
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      Intercostal nerve: The intercostal nerves are the anterior primary rami of the spinal nerves T1 to T11. The anterior primary rami of the 12th spinal nerve form the subcostal nerve. The first and second intercostal nerve, in addition to supplying the intercostal spaces, provide innervation to the upper limb. The lower five intercostal nerves (T7–T11) also supply the abdominal wall and are therefore called the thoracoabdominal nerves. The intercostal nerves. T3 to T6 are typical intercostal nerves because they only supply the thoracic wall. The anterior division of the first thoracic spinal nerve divides into two branches: a larger branch that exits the thorax close to the neck of the first rib, and a smaller branch, the first intercostal nerve, that runs through the intercostal space and ends close to the sternum as the anterior cutaneous branch of T1. The first intercostal nerve also receives a small communication from the second intercostal nerve posteriorly along the neck of the rib. This is the “nerve of Kuntz,” which is present in 40% to 80% of individuals.

      
      
      
      
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        Each typical intercostal nerve (Fig. 10–6) passes below the neck of the rib (with the same number) to enter the costal groove. At the posterior part of the costal groove, the intercostal nerve lies between the parietal pleura (with the endothoracic fascia) and the internal intercostal membrane (Fig. 10–6). Otherwise, throughout its course through the intercostal space, the intercostal nerve lies between the innermost intercostal and the internal intercostal muscle (Figs. 10–6 and 10–7). The lateral cutaneous branch pierces the intercostal and serratus anterior muscle complex at the level of the midaxillary line and gives off its anterior and posterior branches (Figs. 10–6, 10–8, and 10–9). The anterior branch (T2–T6) courses forward and supplies the skin on the lateral and anterior aspect of the chest wall (Figs. 10–1, 10–6, and 10–9). In females they form the lateral mammary branches of the intercostal nerve (same number) and supply the breast (Figs. 10–6 and 10–10). The posterior branch courses backwards and supplies the skin over the scapula and the latissimus dorsi muscle. The anterior cutaneous branch of the intercostal nerve (ie, the main intercostal nerve) courses forward through the intercostal space and emerges close to the sternum by crossing anterior to the internal thoracic (mammary) artery (Fig. 10–6). It then pierces the internal intercostal muscle, the external intercostal membrane, and the pectoralis major muscle to terminate as the anterior cutaneous nerve of the thorax and innervate the overlying skin after dividing into its medial and lateral branches (Fig. 10–6). The lateral branch supplies the medial and anterior aspect of the chest wall and in females the medial and anterior aspect of the breast and thus is referred to as the medial mammary nerves (T2–T6) (Figs. 10–3, 10–6, and 10–10). The intercostobrachial nerve, which corresponds to the lateral cutaneous branch of the second intercostal nerve (T2), emerges from the intercostal space and runs oblique towards the arm to supply the axilla and upper part of the medial aspect of the arm (Figs. 10–3, 10–8, and 10–10). The intercostobrachial nerve may also receive contributions from the first, third, and fourth intercostal nerves. ⁵

      
      
      
      
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      Pectoral nerves: The pectoral nerves are frequently described as “pure motor nerves,” but there is growing evidence that they are also involved with afferent nociception ⁶ and proprioception, similar to that with other pure motor nerves. ⁷ Afferent nociception may be transferred by the pectoral nerves from the acromioclavicular joint, coracoclavicular ligaments, subacromial bursa, articular capsule of the shoulder joint, periosteum of the clavicle, and pectoral muscles, and via cutaneous branches they may innervate the anterior chest wall and anterior margin of the deltoid muscle. ⁶

      
      
      
      
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        The pectoral nerves are also traditionally described as two nerves, the medial and lateral pectoral nerves, with the lateral pectoral nerve (LPN) being larger than the medial pectoral nerve (MPN). ^8,9 The ansa pectoralis is a loop of communication between the LPN and MPN (Figs. 10–3 and 10–4). Published data suggest that the LPN most frequently arises from the anterior divisions of the upper and middle trunk (33.8%), but it may also arise from the lateral cord (23.4%), of the brachial plexus. ⁶ The MPN also has a variable origin and may arise from the medial cord (49.3%) or anterior division of the lower trunk (43.8%) or lower trunk (4.7%). ⁶ Spinal contribution to the LPN and MPN also varies. ¹⁰ Two types of spinal origin of the LPN (C5–C7 in 50% and C6 and C7 in 50%) and three types of spinal origin of the MPN (C8 and T1 in 73,3%, C8 in 23.4%, and T1 in 3.3%) have been described. ¹⁰ After its origin the LPN crosses anterior to the axillary vessels, pierces the clavipectoral fascia, and supplies the pectoralis major muscle (Fig. 10–5). ^6,8 The LPN also shares a constant course with the thoracoacromial vessels and lies on the deep surface of the pectoralis major, beneath the muscle fascia, with the pectoral branch of the thoracoacromial artery (TAA) (Figs. 10–3, 10–4, and 10–11). ^6,8,11 After its origin, the MPN courses downwards lying anterior to the axillary artery and deep to the pectoralis minor muscle (Figs. 10–3, 10–4, and 10–11). ^8,11 It then pierces the pectoralis minor muscle from beneath at about the midclavicular line and over the third intercostal space. ⁸ A few branches of the MPN may also loop around the inferior border of the pectoralis minor muscle to enter the pectoralis major. ⁸

        
        
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        The pectoral nerves may also be present as three constant branches (Figs. 10–3, 10–4, and 10–11), ^10–12 that is, a superior branch that supplies the clavicular fibers of the pectoralis major, the middle branch that courses on the undersurface of the pectoralis major muscle (beneath its fascia) with the pectoral branch of the TAA to innervate the sternal part of the pectoralis major muscle, and the inferior branch that passes under the pectoralis minor muscle to innervate it and the costal part of the pectoralis major muscle. ¹¹ Given the variable spinal origin and formation of the pectoral nerves, a “subpectoral plexus” ¹⁰ (Fig. 10–12) of nerves with the C5–T1 nerve roots, the two pectoral nerves, and the three terminal branches has been described. ^10,12 With this arrangement the superior and middle branches are divisions of the LPN, and the inferior branch is formed by fusion of the MPN and ansa pectoralis from the C7 (Figs. 10–11 and 10–12). ^10,12

      
      
      
      
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      Long thoracic nerve: The long thoracic nerve, also known as the Bell’s nerve, originates from the ventral rami of the C5, C6, and C7 and descends to the lateral thoracic wall (Fig. 10–9) where it innervates the serratus anterior muscle.

      
      
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      Thoracodorsal nerve: The thoracodorsal nerve originates from the posterior cord of the brachial plexus with spinal contributions from the C6 to C8. As it descends along the posterior wall of the axilla, it is accompanied by the thoracodorsal artery and innervates the latissimus dorsi muscle.

   
   
   
   
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   Blood vessels: The following blood vessels are of interest while performing thoracic interfascial nerve blocks: axillary, thoracoacromial, and thoracodorsal artery.

   
   
   
   
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      Axillary artery: The axillary artery is a continuation of the subclavian artery into the axilla. It begins at the lateral border of the first rib and ends at the lower border of the teres major muscle after which it continues distally as the brachial artery. It has three parts: the first part lies between the lateral border of the first rib and the medial border of the pectoralis minor muscle and gives off the superior thoracic artery; the second part lies deep to the pectoralis minor muscle and gives off the lateral thoracic and TAA; the third part lies between the lateral border of the pectoralis minor muscle and the lower border of the teres major muscle and gives off three branches: the subscapular artery, the anterior circumflex humeral artery, and the posterior circumflex humeral artery.

      
      
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      Thoracoacromial artery: The TAA, after its origin (Figs. 10–3 and 10–4), runs a short course along the upper margin of the pectoralis minor muscle, penetrates the clavipectoral fascia (Fig. 10–5), and divides into its terminal branches: the clavicular, acromial, deltoid, and pectoral branches. The TAA is important for a PECS block because, as described earlier, the pectoral nerves and the ansa pectoralis have a constant relationship with the artery (Fig. 10–11). ^8,9 The LPN also runs parallel to the pectoral branch of the TAA in the myofascial plane between the pectoralis major and minor muscles (Figs. 10–4 and 10–11), lying deep to the muscle fascia. ^8,9 The ansa pectoralis nerve is also formed immediately distal to the origin of the TAA (Fig. 10–4). ⁶

      
      
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      Thoracodorsal artery: The thoracodorsal artery (Fig. 10–3) is a branch of the subscapular artery and travels inferiorly along the lateral chest wall (Fig. 10–3), lying deep to the latissimus dorsi muscle initially and then on the external surface of the serratus anterior muscle. It is accompanied by the thoracodorsal nerve (Fig. 10–3) and supplies the latissimus dorsi.

   
   
   
   
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   Fascia

   
   
   
   
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      Clavipectoral fascia: This is a fascial layer that is interposed between the clavicle and upper border of the pectoralis minor muscle (Fig. 10–5). The portion of the clavipectoral fascia that is attached between the first costosternal articulation and the coracoid process is usually denser than the rest and is referred to as the “costocoracoid ligament.” Inferiorly it is thin, and at the upper border of the pectoralis minor muscle it splits to invest the muscle (Fig. 10–5). Below the inferior border of the pectoralis minor muscle the clavipectoral fascia continues downwards as a single layer, the suspensory ligament of axilla, or Gerdy’s ligament, and attaches to the axillary fascia (Fig. 10–5). The clavipectoral fascia is pierced by the cephalic vein, lateral pectoral nerve, TAA, and lymphatics (Fig. 10–5).