Hello Roberto, This is a really interesting point. I had that question come up to my mind as I was watching the lecture. With some research I found a clear answer to that concern on the FDA website. A device is considered significant risk device if it meets any of the points in this list provided by FDA. Here is the list

“• Is intended as an implant and presents a potential for serious risk to the health, safety, or
welfare of a subject;
• Is purported or represented to be for use supporting or sustaining human life and presents a
potential for serious risk to the health, safety, or welfare of a subject;
• Is for a use of substantial importance in diagnosing, curing, mitigating, or treating disease,
or otherwise preventing impairment of human health and presents a potential for serious risk
to the health, safety, or welfare of a subject; or
• Otherwise presents a potential for serious risk to the health, safety, or welfare of a subject”

If it doesn’t then it is considered a non-significant risk device. They also list who makes the decision. They state that the sponsor first makes the decision unless FDA has already decided that the device is considered significant risk. All the sponsor’s decisions are reviewed by the institutional
review boards (IRBs) to make sure that the decision is made correctly and the FDA is available to aid their judgment.

Reference

http://www.fda.gov/downloads/RegulatoryInformation/Guidances/UCM126418.pdf