Final answer:
The brachial plexus passes between the anterior and middle scalene muscles at the level of the interscalene block. This is relevant to regional anesthesia as it enables precise anesthesia for upper limb surgeries but carries the risk of affecting the phrenic nerve.
Step-by-step explanation:
The brachial plexus is a nerve plexus that is formed by the lower cervical spinal nerves (C4-C8) and the first thoracic spinal nerve (T1), and it reorganizes to give rise to the nerves that control the muscles of the arms. During an interscalene block, which is a type of regional anesthesia, the brachial plexus passes between the anterior and middle scalene muscles in the neck. This block is performed at the level of these muscles and is used to provide anesthesia for upper limb surgeries, offering advantages such as a demonstrated decrease in perioperative opioid consumption and improved pain control. However, due to its proximity, there is a risk that anesthetic can affect the phrenic nerve, potentially leading to diaphragmatic paresis, especially in patients with pre-existing lung conditions, this needs to be taken into account when choosing regional anesthesia techniques.