Phrenic nerve palsy is a common interscalene brachial plexus block complication, leading to ipsilateral hemidiaphragmatic paresis. While often transient, it can cause significant respiratory distress in high-risk patients, such as those with chronic obstructive pulmonary disease (COPD) or restrictive lung disease. Symptoms include dyspnoea, hypoxaemia, orthopnoea, and respiratory distress. Diagnosis involves chest X-ray, pulmonary function tests, and diaphragmatic ultrasound, which shows reduced diaphragmatic movement. Management is supportive, including oxygen therapy and close monitoring, with non-invasive ventilation for severe cases. Risk can be minimized by lowering local anaesthetic volume and concentration, altering the injection site, or using alternative nerve blocks like the suprascapular or superior trunk block.