A 76-year-old male presented for reverse total shoulder arthroplasty (TSA) in the beach chair position. A preoperative interscalene nerve catheter was placed under direct ultrasound-guidance utilizing a posterior in-plane approach. On POD 2, the catheter was removed. Three weeks postoperatively, the patient reported worsening dyspnea with a subsequent chest X-ray demonstrating an elevated right hemidiaphragm. Pulmonary function testing revealed worsening deficit from presurgical values consistent with phrenic nerve palsy. The patient decided to continue conservative management and declined further invasive testing or treatment. He was followed for one year postoperatively with moderate improvement of his exertional dyspnea over that period of time. The close proximity of the phrenic nerve to the brachial plexus in combination with its frequent anatomical variation can lead to unintentional mechanical trauma, intraneural injection, or chemical injury during performance of ISB. The only previously identified risk factor for PPNP is cervical degenerative disc disease. Although PPNP has been reported following TSA in the beach chair position without the presence of a nerve block, it is typically presumed as a complication of the interscalene block. Previously published case reports and case series of PPNP complicating ISBs all describe nerve blocks performed with either paresthesia technique or localization with nerve stimulation. We report a case of a patient experiencing PPNP following an ultrasound-guided placement of an interscalene nerve catheter.