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To the best of my knowledge, no research has been published on the questions you've raised.
Note that in the most commonly used polygraph technique (the "Control Quetion Test") truth or deception is inferred based on a comparison of a person's reactions to relevant versus "control" questions. A person who reacts more strongly to the relevant questions is deemed deceptive. It seems unlikely that a generalized anxiety would differentially affect reactions to the relevant questions. On the other hand, a rational fear of the consequences of not being believed with regard to the relevant questions, could easily result in a truthful person "failing.")
While it is certainly plausible that an anxious person might react more strongly to the relevant questions than to the "control" questions (even if employing countermeasures) in a particular polygraph examination, it seems doubtful that a person could be generally categorized as "unable to pass the polygraph test" in any and all situations.
Are some people unable to pass the polygraph test regardless of guilt or innocence because of nerves/anxiety? I was just wondering if people that do not perform well in a situation like this would be able to pass even with counter measures.