Phrenic Nerve Infiltration in Fibromyalgia

The experimental intervention will consist of ultrasound-guided anesthetic blockade of the phrenic nerve at the laterocervical supraclavicular level with 1 ml of lidocaine without vasoconstrictor 2% to infiltrate the skin and 3ml of bupivacaine without vasoconstrictor 0.25% for neural blockade, making the local anesthetic surround the nerve between the anterior scalene muscle and the sternocleidomastoid muscle.

The placebo intervention will be similar in relation to 2% lidocaine without vasoconstrictor for the skin, but an ultrasound-guided puncture will be performed at the level of the subcutaneous cellular tissue by injecting 3 ml of physiological saline.