However, not all patients are candidates for peripheral nerve blocks. Pre-existing pulmonary disease, previous neck surgery, cervical arthritis, neurologic disorders and obesity may preclude ISB placement. As well, interscalene blocks are not completely benign procedures. Systemic complications include clinically significant intraoperative hypotension, pneumothorax, vascular injury, cardiac arrest, respiratory failure, seizure and death. Phrenic nerve paralysis is common, although transient. Peripheral nerve injuries related to mechanical injury, medication neurotoxicity, compression or ischemia are infrequent but may be devastating. The experience and number of blocks performed by the anesthesiologist in addition to adjunctive tools, such as ultrasound and/or nerve stimulators, impacts the success of the procedure.