Based on your previous posts it looks like you are currently in Anesthesiology Residency in Canada.
Unlike in Canada, in the US getting board certified in a subspecialty (e.g. Critical Care) is usually through the primary specialty board (there are some exceptions to this broad rule). The American Board Of Anesthesiology publishes a document which outlines it's rules for access to American Board of Anesthesiology (ABA) Certification Exams:
http://www.theaba.org/PDFs/BOI/2018-Policy-Book. While the ABA allows for credit for the clinical base year in an training program affiliated with LCME school, the final three required years of training are to be completed in an ACGME program. Based on the way the rules are written residency in Canada would not get you access to US Anesthesiology Board exams.
Can a Canadian Trained (FRCPC certified) Critical Care Physician work in the US, the answer is yes, But the answer to the rest of your questions is going to depend a lot on which state you want to work in and what the individual hospital you want to work at would require:
e.g. Massachusetts will grant a graduate of a US or Canadian Medical School who has completed a Canadian Residency and has LMCC a full license on the basis of this training:
Physician Licensing Fees and General Information
e.g. Louisiana does not seem to recognise LMCC but seems to accept Canadian residency, so you might have to take USMLEs if you wanted to work in Louisiana:
Application & Instructions for Initial Licensure - Physicians | State Board of Medical Examiners
These were just examples of two states but the rules differ per state as they do between provinces, however it seems on the whole the combination of US or Canadian Medical School + Canadian Residency + LMCC will get you a full license or license by reciprocity in a lot of different states.
The hospital question comes down to if the individual hospital you want to work at will credential you and is willing to accept Royal College Certification as it is not clear you would have access to ABA certification, this is a much harder question to answer as it's also related to finding a job that wants to hire you, if they want to hire you enough this should not be a big obstacle.
