Procedure: Code changes while waiting for review

We're having a discussion in my office about what happens when you've submitted a review for a code file, and then you have to modify the code for other reasons before the review is done. How can you start a second review for the new changes to the same code without the second review picking up the previous changes, assuming you haven't checked into the repository (because you are waiting for the first review)? Has anyone else come upon this issue?