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 the warning with respect to pikuach nefesh on Shabbos may apply here as well (One who asks sheds blood). 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).