how to keep the PR of canadian status?

I am doing the Phd study in U.S.A. but I am a PR of Canada, the trouble is I have to study in U.S.A around 4 to 5 years, I am afraid that if I donot go back to Canadan during these year, my PR status will expire. I do not know if anyone of you have a good suggestion for me to keep my PR status and at the same time I can finish my study in U.S.A?
Thank you so much!