This has essentially been addressed in the comments to the question but the issue of halachic death was discussed <a href="http://mi.yodeya.com/questions/376/do-most-leading-orthodox-rabbis-really-hold-that-brain-stem-death-is-death">in this question</a> which is where I think  is the appropriate place to discuss that issue.

With respect to CPR by a Kohen, as Sholom noted this is an issue of [at least safek] Pikuach Nefesh which is certainly permitted. Furthermore, unlike the caricature in "The Good Samaritian", the halacha not only expects a kohen not to stand by his brother's blood, but if a kohen chances upon a dead body which is unattended to he is obligated to attend to it's burial (meis mitzvah is the term as I recall).