Background
There is a high incidence of chronic post-surgical pain following breast cancer surgery, which has led to the development of regional techniques aimed at optimising peri-operative analgesia of the axilla and antero-lateral chest wall.1,2
Pectoralis I and II blocks (Pec I / Pec II) are modern regional anaesthetic techniques performed in combination to anaesthetise the nerves involved in breast surgery and axillary node dissection.3 The Pecs II block can be highly challenging in certain patient groups due to difficulty in delineating correct muscle and fascial planes. This can result in misplaced LA and block failure.
A variation of the Pecs II involves directly locking the needle onto the rib for safety before injecting deep to serratus anterior (SA), aiming to help eradicate uncertainty of correct plane identification.
Local anaesthetic spread and clinical efficacy is thought to be independent of whether injection occurs between pectoralis minor and serratus anterior or deep to serratus anterior, however, our local experience suggests that this approach may be less effective for axillary dissection. This study aimed to gain insight to determine if this is actually the case, or if favouring one plane over the other produces better axillary spread.
Method
Following institutional approval, ultrasound guided Pecs II blocks were performed superficial and deep to SA respectively in 2 soft-fixed GenelynTM embalmed cadavers in the Anatomy Department at the University of Aberdeen.
The needle tip was advanced from a caudal direction into the fascial planes between PM and Pm for the Pecs II block (Figure 1A) and directly onto the 3rd rib for the sub-serratus plane block using ultrasound guidance (Figure 1B). Once the correct plane had been confirmed, 10ml of 10% methylene blue dye was injected. Cadaveric dissection was performed to assess dye spread across the thoracic wall.
Figure 1. US Guided Injections
Conclusion
Results
• Pecs II (Figure 2A) and the sub-serratus plane (Figure 2B) produced comparable staining of the intercostobrachial nerve, 3rd intercostal nerve, thoracodorsal nerve and long thoracic nerve (Table).
• Dye spread was limited to the lateral cutaneous branches of the intercostal nerves and did not stain the thoracic wall below the level of the 4th intercostal space in either technique.
￼￼￼￼￼Medial Pectoral Nerve Yes Partly: one branch to the inferior surface of pectoralis minor.
Long Thoracic Nerve
This could be a place for your table.
￼￼Lateral Pectoral Nerve Yes No
￼Thoracodorsal Nerve
￼Intercostobrachial Nerve
Yes (10.7cm)
Partly: 11.5cm of 18.1cm total length
Yes
No (12.1cm)
Partly: 9.5cm of 17.2 total length
Yes
￼• Our results demonstrate that injecting into the sub-serratus plane approach produces comparable thoracic spread with Pecs II injection above SA, and therefore is an alternative for mastectomy where delineating the clavi-pectoral fascia is challenging. However, due to the reduced axillary uptake following this injection, we suggest the superficial injection above the serratus muscle may provide better anaesthesia for axillary surgery.
• The sub-serratus plane also failed to stain deeper branches of the pectoral nerves. Although primarily motor nerves, blockade of deeper pectoral branches not covered by a Pecs I maybe important in providing total analgesia, and would be more likely to be achieved with the injection above SA.
Limitations
• GenelynTM-embalmed cadavers, which aim to offer preserved real-life tissue flexibility, they may lack the ability to simulate the natural spread of LA that
would be expected in real-life. Other studies have elected to use Thiel embalming techniques, which maintain tissue planes but lose overall shape and cellular preservation.22-24 Considering the growing use of cadavers for training and research, comparison of the two would be interesting.

5 - IS THE CONVENTIONAL ULTRASOUND GUIDED PECTORALIS II OR SUB-SERRATUS PLANE SUPERIOR FOR BREAST SURGERY: A CADAVERIC STUDY