The correct spelling for the idiomatic adjective meaning causing intense grief, anguish, or distress is heart-rending (also spelled heartrending with near-equal frequency). Heart-wrenching, a contemporary error, makes little sense. The idiomatic metaphor relies on rend, which means to tear or split apart. To wrench the heart would suggest only twisting and turning—painful, but nowhere near the visceral cruelty of tearing apart. Heart-rendering, an occasional misspelling, makes no sense and should be avoided.