Evidence not pertinent to whatever legal action 'required' the use of such a device would have to be inadmissable and ineligible for followup - or law-enforcement fishing-trips would become commonplace.
I mentioned that above, rules against fishing trips would have to be strengthened. Lawyers might have to explain to the judge how a question is related to the case at hand before posing it to the witness or suspect.
But that is actually unrelated to the 5th admendment. Fishing questions are already illegal, but it isn't rigidly enforced. And if such a strong lie detector did exist, rules against fishing would have to apply to everybody taking the stand, not just the suspect.
There would be an even bigger problem is such device where cheap enough that the police could use them. An investigation is often a fishing trip by nature, it is unavoidable. Laws that keep police investigations secret would have to be stronger, and police would have to narrow their investigations more sharply. Of course, with such a lie detector, many investigations would end after a few direct questions.
Jay