The Pecs I, Pecs II and Serratus Plane blocks are superficial thoracic wall blocks which through blockade of the pectoral, intercostal, thoracodorsal and long thoracic nerves can be used to provide analgesia for breast surgery and other procedures/surgery involving the anterior chest wall.

The innervation of the breast is supplied mainly by the anterior branches of the 4th, 5th and 6th intercostal nerves which arise from the thoracic spinal nerves (T4-6). the apex of the axilla is supplied by the intercostobrachialis nerve; this is a cutaneous branch of the second intercostal nerve (T2). the pectoral major and minor muscles are innervated by the lateral pectoral nerve (C5-7) and medial pectoral nerve (C8-T1).

Terminal branches of the supraclavicular nerves (C3-4) innervate the upper part of the breast and this should be taken into account when the surgical procedure involves this area, because Pecs blocks will not block the supraclavicular nerve. Breast surgery however, is rarely performed at this level.

The long thoracic nerve (C5-7) supplies the serratus anterior muscle. the thoracodorsal nerve (C6-8) supplies latissimus dorsi and this is relevant for more extensive procedures.

The lateral cutaneous branches of the thoracic intercostal nerves (T₂ – T₉) together with the long thoracic and the thoracodorsal nerves can be found in a compartment space between serratus anterior and latissimus dorsi between the posterior and mid axillary lines.

Local anaesthetic is placed between the muscles of the thoracic wall, using ultrasound guidance.