http://www.anes.ccf.org:8080/pilot/ortho/intersca.htm
Our anaesthesiologists used these often, both for surgery and for chronic pain syndromes. The agent they used was usually bupivicaine and delivered through a tiny catheter into the interscalene area to the brachial plexus. We used a small battery operated pump made by Bard. There are many others though.
There are several things that can happen if the catheter is not exactly placed. Read the article I posted. It's long but pretty easy to understand.