As a rule of thumb, Pectus Excavatum patients are capable of having effective CPR done, even with a Nuss bar still implanted. However, the compressions may need to be done with more force. Some Pectus patients wear wrist bands that state they have a Nuss bar and that CPR requires more force, if necessary.
Further resources:
http://www.jpedsurg.org/article/S0022-3468%2805%2900615-9/abstract