Subspecialty certification in Critical Care medicine requires completion of a one-year fellowship and passing of the annual examination.
Certification in critical care medicine began in 1986. The success rate on recent exams was:
The ABA has posted the content outline, blueprint, and sample questions.
Alex Yartsev's site, Deranged Physiology, on preparing for New Zealand's CICM Part 2 may be helpful.
Discussion threads at Student Doctor Network: