Pectoralis and Serratus Plane Blocks

INTRODUCTION

Pectoralis nerve (Pecs) and serratus plane blocks are newer ultrasound (US)-guided regional anesthesia techniques of the thorax. The increasing use of ultrasonography to identify tissue layers and, particularly, fascial layers has led to the development of several newer interfascial injection techniques for analgesia of the chest and abdominal wall. For instance, the Pecs I block was devised to anesthetize the medial and lateral pectoral nerves, which innervate the pectoralis muscles.

This is accomplished by an injection of local anesthetic in the fascial plane between the pectoralis major and minor muscles. The Pecs II block (which also includes the Pecs I block) is an extension that involves a second injection lateral to the Pecs I injection point in the plane between the pectoralis minor and serratus anterior muscles with the intention of providing blockade of the upper intercostal nerves. A further modification is the serratus plane block, in which local anesthetic is injected between the serratus anterior and latissimus dorsi muscles.

These interfascial injections were developed as alternatives to thoracic epidural, paravertebral, intercostal, and intrapleural blocks, primarily for analgesia after surgery on the hemithorax. Initially, Pecs blocks were intended for analgesia after breast surgery; however, case reports have also described the use of Pecs and serratus plane blocks for analgesia following thoracotomy and rib fracture. Information from the currently published literature on Pecs and serratus plane blocks in peer-reviewed journals is summarized in Table 1.

TABLE 1.Summary of published controlled clinical trials and case reports.

Author, YearStudy TypeSurgery/IndicationBlock TypeNInjectateOutcome
Blanco et al., 2013
Volunteer study
Serratus plane
40.4 mL/kg levobupivacaine
0.125% and gadolinium
Mean duration of paresthesia in the intercostal nerve distribution T2–T9, was 752 minutes (injection superficial to serratus anterior)
Wahba and Kamal, 2014
Randomized controlled trial
Mastectomy
Pecs II versus PVB
60
0.25% levobupivacaine:
15–20 mL T4 PVB,
10 mL Pecs I block

Pecs blocks reduced postoperative morphine consumption (first 24 h) and pain scores (first 12 h) in comparison with PVB following mastectomy
Fujiwara et al., 2014
Case report
Insertion of cardiac resynchronization device
Intercostal at first and second interspace, Pecs I block
1
0.375% ropivacaine:
4 mL intercostal block,
10 mL Pecs I block
Surgery performed under intercostal/
Pecs I blocks and dexmedetomidine
Kunhabdulla
et al., 2014
Case report
Analgesia for rib fracture
Serratus plane
1
20 mL bolus 0.125% bupivacaine, then infusion of 0.0625% bupivacaine at 7–12 mL/h
Effective analgesia to enable physiotherapy and ambulation
Madabushi
et al., 2015
Case report
Analgesia for thoracotomy
Serratus plane
1
6 mL bolus 1% lignocaine, then infusion of bupivacaine 0.1% at 7 mL/h
Improvement in pain and ventilation
Murata et al., 2015
Case report
Breast surgery
Pecs II
2
35 mL 0.2% ropivacaine (mastectomy); 45 mL 0.2% ropivacaine (lumpectomy)
Mastectomy performed under Pecs II block and supplemental infiltration
Ueshima, 2015
Case report
Segmental breast resection
TTP combined with Pecs II
1
0.15% levobupivacaine:
15 mL TTP,
10 mL Pecs I,
20 mL Pecs II
Surgery performed under TTP and
Pecs II blocks
Bashandy and Abbas, 2015
Randomized controlled trial
Mastectomy
Pecs II
120
0.25% bupivacaine:
10 mL Pecs I,
20 mL Pecs II
Lower visual analog scale pain scores and opioid requirements in the Pecs group compared to control group
Kulhari, 2016
Randomized controlled trial
Radical mastectomy
Pecs II versus PVB
40
25 mL 0.5 % ropivacaine
Duration of analgesia increased in Pec’s block compared to PVB group (4.9 versus 3.3 hours)
Hetta, 2016
Randomized controlled trial
Radical mastectomy
Serratus plane
64
30 mL 0.25 % bupivacaine, Serratus plane; 15 mL 0.25% bupivacaine, PVB
Increased opioid consumption in the serratus plane compared to the PVB group

Pecs blocks have also been proposed in letters to the editor as alternative techniques to anesthetize operative regions such as the axilla, proximal medial upper arm, and posterior shoulder, which are not innervated by the brachial plexus (Figure 1).

Figure 1. Transducer position for Pecs blocks. Also, refer to Figures 9 and 11.

ANATOMY OF THE PECTORAL AND AXILLARY REGIONS

Pecs blocks are applied in the pectoral and axillary regions, with the muscles in both regions innervated by the brachial plexus. The pectoral region overlies the pectoralis major muscle and is limited by the axillary, mammary, and inframammary regions (Figure 2).

Figure 2. Pectoral region.

The axillary region is lateral to the pectoral region and consists of the area of the upper chest that surrounds the axilla. In both regions, there are muscles, nerves, and vessels within the fascial layers (Figure 3). In the pectoral region, there are four muscles relevant to Pecs blocks: the pectoralis major, pectoralis minor, serratus anterior, and subclavius muscles. The pectoralis major and minor muscles are innervated by the lateral and medial pectoral nerves; the serratus anterior is innervated by the long thoracic nerve (C5, C6, and C7); and the subclavius is innervated by the upper trunk of the brachial plexus (C5 and C6).

Figure 3. Axillary region.

The axillary region is a pyramidal structure with four borders:
1. The apex or axillary inlet, formed by a lateral border of the first rib, superior border of the scapula, and the posterior border of the clavicle
2. The anterior border, formed by the pectoralis major and minor muscles
3. The lateral border, formed by the humerus
4. The posterior border, formed by the teres major, latissimus dorsi, and subscapularis muscles.

Figure 2, the muscles, nerves, and vessels relevant to Pecs and serratus plane blocks are summarized in Tables 2, 3 and 4 respectively.

TABLE 2.Nerves relevant to pecs and serratus plane blocks.

Nerve
Origin
Innervation
Relevance
Long thoracic
Roots
(C5, C6, C7)
Serratus anterior muscle
Known as the nerve to the serratus anterior.
Lateral pectoral
Lateral cord
(C5, C6, C7)
Pectoralis major and pectoralis minor musclesPenetrates the clavipectoral fascia to supply the pectoralis major directly and, through communication with the medial pectoral nerve, the pectoralis minor. There is no cutaneous branch. Can be located on the deep surface of the pectoralis major.
Medial pectoralMedial cord
(C8, T1)
Pectoralis major and pectoralis minor musclesPenetrates the deep surface of the pectoralis minor to innervate this muscle before penetrating it to supply the pectoralis major muscle.
IntercostalAnterior rami of thoracic spinal nervesSegmental somatic sensory innervation to skinLateral cutaneous branches of T2-T6 innervate the lateral breast. Accessible in the mid-axillary line.
ThoracodorsalPosterior cord (C6, C7, C8)Latissimus dorsi muscleLarge nerve from the posterior cord, which has a course in the posterior axillary wall, crosses the lower border of the teres major to enter the deep surface of the latissimus dorsi muscle. The thoracodorsal nerve is adjacent to the thoracodorsal artery.

TABLE 3.Muscles relevant to pecs and serratus plane blocks.

Muscle Innervation Relevance
Pectoralis majorMedial (C8, T1) and lateral
(C5–C7) pectoral nerves
Sonographic landmark.
Pectoralis minorBoth pectoral nerves (C5–C8)Sonographic landmark.
Serratus anteriorLong thoracic nerve (C5–C7)Sonographic landmark for Pecs II and serratus plane blocks. The
intercostobrachial, long thoracic, and thoracodorsal nerves lie on
this muscle. The thoracodorsal artery is superficial to this muscle.
Teres majorSubscapular nerve (C5–C6)
(offspring of subscapularis
muscle)
Contributes to the posterior wall of the axilla.
SubscapularisUpper and lower subscapular
nerves (C5–C8)
Contributes to the posterior wall of the axilla.
Latissimus dorsiThoracodorsal nerve (C6–C8)Contributes to the posterior wall of the axilla; sonographic landmark
for serratus plane blocks.

TABLE 4.Vessels relevant to pecs and serratus plane blocks.

Vessel Relevance
AxillaryIs the continuation of the subclavian artery after it passes the lateral border of the first rib. It lies
lateral to the axillary vein. It gives of the branches listed below.
Superior thoracicBranch of the first part of the axillary artery; supplies both pectoral muscles.
ThoracoacromialArises from the second part (deep to the pectoralis minor) of the axillary artery, close to the upper
border of the pectoralis minor; pierces the clavipectoral fascia in the infraclavicular fossa; has four
branches that may arise deep or superficial to the clavipectoral fascia.
Lateral thoracicFollows the lower border of the pectoralis minor; supplies both pectoral muscles.
ThoracodorsalArises from the third part (distal to the pectoralis minor) of the axillary artery; initially known as
the subscapular artery, located in the posterior axillary wall (largest branch of the axillary artery),
before becoming the thoracodorsal artery; has a course with the thoracodorsal nerve, which
innervates the latissimus dorsi.

The pectoral and axillary regions are separated by fascias. In the pectoral region, there are two main fascias: the superficial fascia and the deep thoracic fascia. The deep thoracic fascia divides into three separate fascias: the pectoral (superficial), clavipectoral (intermediate), and exothoracic (deep). The clavipectoral fascia stretches between the clavicle and the pectoralis minor (Figure 4) and encloses the pectoralis minor with a thin layer of fascia. Between the pectoralis minor and subclavius muscles, the two layers of the clavipectoral fascia fuse.

Figure 4. The clavipectoral fascia and its continuation into the axillary fascia.

Caudal to the pectoralis minor, the clavipectoral fascial layers rejoin to form the suspensory ligament of the axilla, which is joined to the axillary fascia (Figure 5).
At the pectoral level, the fascias create four potential compartments for the injection of local anesthetic:

  1. Between the superficial and deep pectoral fascial layers
  2. Between the pectoral fascia and the clavipectoral fascia
  3. Between the clavipectoral fascia and the superficial border of the serratus anterior muscle
  4. Between the serratus anterior muscle and the exothoracic fascia
Figure 5. Section of the axilla showing the clavipectoral fascia enclosing the subclavius and the pectoralis minor muscles. Inferior to the pectoralis minor muscle, the clavipectoral fascia becomes the suspensory ligament.

The first two compartments are in the pectoral region, but the third and fourth communicate with the axillary region. The nerves and vessels in this region create communications by crossing the compartments. The nerves of the pectoral region are mainly the lateral and medial pectoral nerves, but there is also an important innervation from the supraclavicular nerve and from the lateral and anterior branches of the intercostal nerves. The lateral pectoral nerve crosses the axillary artery anteriorly and pierces the clavipectoral fascia in close relationship with the thoracoacromial artery on the undersurface of the upper portion of the pectoralis major muscle, which it supplies with lateral cord fibers from C5–C7 (Figure 6).

Figure 6. Relationship of the lateral pectoral nerve to the acromiothoracic artery.

The lateral pectoral nerve is medial to the pectoralis minor before entering the pectoralis major muscle; it communicates across the axillary artery with the medial pectoral nerve and, through this communication (via ansa pectoralis), supplies the pectoralis minor. The medial pectoral nerve arises from the medial cord fibers from C8–T1, behind the axillary artery at the level below the clavicle, and passes through the deep surface of the pectoralis minor, which is perforated and then enters and innervates pectoralis major. Both pectoral nerves enter the deep surface of the pectoralis major, and neither has a cutaneous branch. The nerves of the axillary region are the intercostobrachialis, intercostal T3–T9, long thoracic, and thoracodorsal. The intercostobrachialis nerve is the lateral cutaneous brnach of the second and third intercostal nerves in 67% and 33% of cases, respectively. It crosses the serratus anterior muscle in the midaxillary line to innervate the axilla. The intercostobrachialis nerve is a vital nerve if regional anesthesia of the axilla is required.

The intercostal nerves (T3–T9) provide motor supply to the intercostal muscles and receive sensory information from the skin and parietal pleura. The intercostal nerves have posterior, lateral, and anterior branches and an anterior accessory branch that innervates the sternum. The lateral branches innervate most of the pectoral and axillary regions, together with the posterior hemithorax, back to the scapula. They pierce the external intercostal muscle and exit between the serratus anterior digitations at the level of the midaxillary line. The long thoracic nerve is in the axillary compartment close to the lateral thoracic branch of the thoracoacromial artery and travels down the lateral aspect of the serratus anterior muscle, which it innervates.
Arising from the posterior cord, the thoracodorsal nerve, C6–C8 (nerve to the latissimus dorsi), has a course posteriorly in the axillary compartment, in close relationship with the thoracodorsal artery (Tables 2 through 4). The thoracodorsal nerve becomes prominent when the humerus is abducted and laterally rotated. It is an important and large nerve in danger during reconstructive surgery and other operations involving the lower axilla (see Table 2).

For a more comprehensive review, see Functional Regional Anesthesia Anatomy.

THORACIC WALL BLOCKS

Pecs I Block

The Pecs I block involves a hydrodissection of the plane between the pectoral muscles with local anesthetic to block the lateral and medial pectoral nerves. The main landmarks to identify the point of injection under US guidance are the pectoralis major and pectoralis minor muscles and the pectoral branch of the thoracoacromial artery. The block is performed with the patient supine, either with the arm next to the chest or abducted 90 degrees. With standard American Society of Anesthesiology (ASA) monitoring and supplemental oxygen, the operator locates the coracoid process on US in the paramedian sagittal plane. The transducer is rotated slightly to allow an in-plane needle trajectory from the proximal and medial side toward the lateral side (ie, the caudal border of the transducer is moved laterally, while the proximal border remains unchanged) (Figure 7). This rotation helps image the pectoral branch of the thoracoacromial artery. The proper fascial plane is confirmed by hydrodissection to open the space between the pectoralis muscles. The suggested volume is 0.2 mL/kg of a longacting local anesthetic (Figure 8 however, the reader should be informed that dose-ranging studies have not been conducted at the time of publication, and, therefore, there are no evidencebased recommendations currently available.

Figure 7. Transducer position for the Pecs I block.
Figure 8. Sonogram of the Pecs I injection. Left: needle placement; right: desired spread of local anesthetic.

Pecs II Block

The goal of the Pecs II block is to infiltrate two fascial compartments by dividing the dose of local anesthetic between the pectoral nerves (the pectoral fascia and clavipectoral fascia) and under the pectoralis minor muscle (between the clavipectoral fascia and the superficial border of the serratus muscle). The local anesthetic should cover two important compartments of the fascias involved: The pectoral compartment with the pectoral nerves and the intercostal branches for the axilla and chest. The block is performed with the patient supine, either with the arm abducted 90 degrees or by his or her side. The first injection is similar to Pecs I, whereas the second is made at the anterior axillary line at the level of the fourth rib. The depth is usually 1–3 cm for the first injection and 3–6 cm for the second injection. With the transducer at the midclavicular level and angled infero-laterally, the axillary artery and vein and the second rib can be identified (Figure 9).

Figure 9. Transducer position for the Pecs II block.

The transducer is then moved laterally until the pectoralis minor and serratus anterior are identified. With further lateral transducer movement, the third and fourth rib can then be identified. The local anesthetic is injected at two points: The first injection of approximately 0.2 mL/kg long-acting local anesthetic is made between the pectoral major and minor muscles, and a second injection of 0.2 mL/kg is made between the pectoralis minor and serratus anterior muscles. Figure 10 illustrates the sonographic anatomy, needle trajectory, and the desired spread of injectate.

Figure 10. Pecs II sonogram: steps to locate points of injection. A: Left: Start from the clavicle; right: Count ribs down to the axilla. B: Left: First injection between the pectoralis major and pectoralis minor; right: Angle probe to locate Gilbert’s ligament. C: Left: Above the serratus muscle; right: Underneath the serratus muscle; cl, clavicle; scm, subclavius muscle; pM, pectoralis major; pm, pectoralis minor; av, axillary vein; aa, axillary artery; pl, pleura; sm, serratus muscle.

Serratus Anterior Plane Block

The serratus plane block is performed in the axillary region, at a more lateral and posterior location than the Pecs I and II blocks. At the axillary fossa, the intercostobrachialis nerve, lateral cutaneous branches of the intercostal nerves (T3–T9), long thoracic nerve, and thoracodorsal nerve are located in a compartment between the serratus anterior and the latissimus dorsi muscles, between the posterior and midaxillary lines.

The two main anatomical landmarks are the latissimus dorsi and the serratus anterior muscles. The thoracodorsal artery runs in the fascial plane between the two. The ribs, pleura, and intercostal muscles can also be seen during the procedure. Lying on the side or supine with the arm brought forward is the preferable patient position. There are two main methods for identifying the plane for the serratus block. The first method requires counting the ribs from the clavicle while moving the transducer laterally and distally until the fourth and fifth ribs are identified (Figure 10). The transducer is orientated in the coronal plane and then tilted posteriorly until the latissimus dorsi (a superficial thick muscle) is identified (Figure 11). The serratus muscle, a thick, hypoechoic muscle deep to the latissimus dorsi is imaged over the ribs. Translating the transducer posteriorly facilitates identification of the plane between the serratus anterior and latissimus dorsi muscles. An alternative method is to place the transducer across the axilla, where the latissimus dorsi will appear more prominent (Figure 12). The location of the thoracodorsal artery is easier to identify this way. Both in-plane and out-of-plane approaches are appropriate. Following the identification of sonographic landmarks, regional anesthesia can be achieved using a 38-mm, 6–13-MHz, linear transducer set for small parts and a depth of 1–4 cm; a 50–100-mm, 22-gauge regional block needle; and an injectate of 0.4 mL/kg of long-acting local anesthetic.

Figure 11. Transducer position required for the serratus plane block.
Figure 12. Sonogram of two possible levels for the serratus plane block below (left) or above the muscle (right).

THE ANALGESIC POTENTIAL OF PECS BLOCKS

A summary of published studies is given in Table 1. At the time of writing, there were two randomized controlled trials (180 patients), five case reports (6 patients), and one volunteer study (4 patients, 8 hemithoraces). Bashandy and Abbas reported lower visual analog scale pain scores and opioid requirements in the Pecs group compared to a control group of patients following mastectomy. In this study, there was an inadequate description of allocation concealment and blinding of operating room personnel. Wahba and Kamal compared Pecs blocks to paravertebral blocks in 60 patients undergoing mastectomy. They found that Pecs blocks reduced postoperative morphine consumption (in the first 24 hours) and pain scores (in the first 12 hours) in comparison with paravertebral blockade following mastectomy. Pecs blocks have also been used for the insertion of a cardiac resynchronization device. The few remaining reports describe the utility of serratus plane blocks for analgesia following rib fracture and thoracotomy.

SUMMARY

Pecs and serratus plane blocks are newer US-guided blocks for analgesia after breast and lateral thoracic wall surgery. The key sonographic landmarks are the pectoralis major, pectoralis minor, and serratus anterior muscles and the pectoral branch of the acromiothoracic artery. As there are currently few reports on the utility of these US-guided interventional analgesia modalities, data from the imminent randomized controlled trials will be necessary to establish the analgesic benefit, indications, and safety of the Pecs and serratus plane blocks.

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Pectoralis and serratus plane block

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