An isolytic shock was a kind of energy discharge which was possible on a malfunctioning EPS grid. Such shock could certainly be fatal to Humans, as Travis Mayweather was initially believed to have died from exposure to it.
According to Trip Tucker, Mayweather "was trying to tap into the EPS grid [in Enterprise's launch bay] when [a] relay overloaded", resulting in the isolytic shock. Phlox's preliminary autopsy report confirmed that Mayweather's cause of death was "ventricular fibrillation induced by an isolytic shock". In trying to comfort Hoshi Sato about her friend's death, Phlox noted that it would have been a fairly painless death, because isolytic shock "instantly impair[ed]" the central nervous system.
However, it was unlikely that any such exposure to isolytic shock actually occurred, since Phlox was not examining the real Mayweather, but a copy created by the automated repair station at which Enterprise was then docked. Indeed, microbes in the Rigelian fever vaccine administered to the entire crew, including Mayweather, the previous month should have thrived on isolytic energy, but Phlox discovered that these microbes had inexplicably died, alerting him to the facsimile's true nature. (ENT: "Dead Stop")