Question: my lower back (the lumbar region) tends to round whenever I perform heavy deadlifts. What should I do?
Answer: a few things immediately come to mind. (1) Think "lift my chest up" when executing the deadlift. This cue forces you into a less arched position. Don't overcompensate by hyperextending your back, though. (2) You may have weak erectors. As a result, you should try strengthening your lower back. There as several great exercises that help with this. Good mornings and back extensions are two great options! (3) Maybe you're using too heavy of a weight for your sets. Backing off the intensity until your body is better prepared to handle the load could also be an option.
I would begin with (1) and (3) and see how it goes. If you still deal with the issue, move to (2).