How often do Secret Service agents get a polygraph or sodium pentathol?

I was thinking about the level of security they provide, and their employers definitely want loyal employees who will do their jobs right. For a national leader for instance are these guys getting lie-detector tests and such every week? Do they live in bugged houses to constantly monitor their loyalty?

It's a trippy thing to think about I mean these are elite ex-military and such and are entrusted to carry PDW's in close proximity to a national figure.